Claudio Rabe

Dr Yves Leroy

Dr Manuel Pubellier

Dr Sylvie Leroy

Dr Henrique Penteado

Juliana Maria Gonçalves De Souza

Nadaya Cubas

Jean Letouzey

Renaud Divies

Daniel Brian Praeg

Didier Granjeon

Alberto Machado Cruz

Guizan Silva

Antonio Tadeu Dos Reis

Christian Gorini

Controls on overpressure evolution during the gravitational collapse of the Amazon deep-sea fan

First of all, I want to express my gratitude to God for allowing and sustaining me along of these 4-years, especially in this difficult year that we are living in. More than special thanks and with many saudades to my family Noeme, Alberto, Tatiana and Rogerio, which always supported me and, despite being oceans away, they have always been by my side.

Secondly, I must be thank to Sorbonne

control on gravity tectonics was sediment supply, which differed in magnitude and style between the SE and NW compartments. In both compartments, progradation of the Amazon Fan drove the basinward migration of the deformation front in response to a seaward migration of overpressure along the detachment. In the SE compartment, fault activity was observed only during periods of higher sedimentation, whereas in the NW compartment, continuous fault activity reflected constant high sediment input over the last 8Ma. Disequilibrium compaction (undercompaction) is argued to be the primary mechanism of overpressure in the Amazon Fan, however the secondary role of inflationary overpressures cannot be excluded. The temperature-dependent smectite-illite transition window was present within the fan, mainly above the detachment in the SE compartment, but at the level of the detachment on the inner and outer part of the NW compartment. Thermogenic gas generation (by primary and secondary cracking) did not affect the gravity system in the SE compartment, where most gas was expelled prior to the growth of the fan and thereafter trapped in shale-rich layers beneath iii the detachment, whereas in the NW compartment it has contributed to higher pore pressure on the detachment and some faults. Thus, temperature-driven fluid mechanics played a different role in terms of inflationary overpressure in the two structural compartments. These differences are in part due to differing different crustal types beneath the two compartments, which syn-to post-rift basin modeling shows produced distinct thermal histories, modulated by thermal blanketing during the growth of the Amazon fan. In particular, this led to lower heat flow in the NW compartment over the last 8 Ma, accounting for the greater depth of the smectite-illite transition window relative to the detachment and the later expulsion of thermogenic gases.

The findings of this thesis thus provide new insights into the evolution of pore pressure during the growth and collapse of the Amazon Fan, and distinguish particularities of each structural compartment linked to its long-term history. The results also show that the integration of basin modeling methodologies provides an extremely useful tool to investigate the tectonic and sedimentary dynamics of Late Cenozoic depocentres, even when there is limited data. As a perspective of future work, dedicated studies of the crustal and thermal history of the Amazon margin might be done.

Résumé

Le cône de l'Amazone constitue l'un des prismes sédimentaires silicoclastiques les plus importants en terme de volume. Ce prisme prograde sur la marge et dans le bassin Foz do Amazonas depuis la fin du Miocène tout en subissant un étalement gravitaire au-dessus de niveaux de décollements argileux. Afin d'examiner les mécanismes de surpression en action dans le cône de l'Amazone et leur relation avec la tectonique gravitaire, j'ai développé une approche innovante basée sur l'intégration de méthodes de modélisation couramment utilisées dans l'industrie pétrolière et gazière, appliquées ici pour la première fois à un dépôt-centre de marge passive affectée par de l'étalement gravitaire à l'échelle de toute la marge. Deux coupes sismiques régionales ont été interprétées, converties en profondeur, restaurées puis utilisées pour la modélisation géomécanique du bassin Foz do Amazonas. L'objectif de cette thèse est d'étudier les mécanismes de surpression et la déformation le long du détachement principal localisé dans des argiles du crétacé supérieur et des failles associées à l'étalement gravitaire sur ce détachement. Les résultats de la modélisation fournissent des informations sur l'évolution de la pression interstitielle et de la température et leurs implications pour le fonctionnement du détachement et des failles d'extension aval et de compression amont susjacentes pendant le dépôt de 6-10 km de sédiments au cours des derniers 8 millions d'années.

Le contrôle principal de la tectonique gravitaire est le taux de sédimentation, qui varie en ampleur et en style entre deux compartiments structuraux au sud-est et au nord-ouest du système gravitaire segmenté. Dans les deux compartiments, la progradation du cône de l'Amazone est à l'origine de la migration vers le bassin du front de déformation, en réponse à la migration de la suppression le long du détachement. Dans le compartiment SE, l'activité des failles n'a été observée que pendant les périodes où le taux de sédimentation est important, alors que dans le compartiment NW, l'activité continue des failles reflète un apport sédimentaire toujours important au cours des derniers 8 Ma. Au premier ordre, la souscompaction est considérée comme le principal mécanisme de surpression dans le cône de v l'Amazone. Le rôle secondaire des surpressions lié à la transformation de la smectite en illite et l'augmentation du volume des fluides interstitiels qui en résulte ne peut être exclu pour le compartiment NW. La fenêtre de transition smectite-illite, qui dépend de la température, est localisée principalement au-dessus du détachement dans le compartiment SE. Par contre, cette fenêtre coïncide bien avec l'ensemble du détachement du compartiment NW. En outre, la modélisation a permis de démontrer que la génération de gaz thermogénique (par craquage primaire et secondaire) n'a pas affecté le système gravitaire dans le compartiment SE, dont la majeure partie des gaz a été expulsée avant l'arrivée du cône, puis emprisonnée dans des couches argileuses sous le détachement. Dans le compartiment NW, la génération de gaz thermogénique a contribué à une pression interstitielle plus élevée sur le détachement et sur certaines failles. Ainsi dans les deux compartiments, la mécanique des fluides pilotée par la température a joué un rôle différent en termes de surpression.

Ces différences sont en partie dues à la présence de types de croûtes différentes sous chaque compartiment. D'après la modélisation de bassin synet post-rift, ces différences produiraient des histoires thermiques distinctes, modulées par un 'thermal blanketing' durant la croissance du cône de l'Amazone. En particulier, ceci a engendré de plus faible flux de chaleur dans le compartiment NW sur les 8 derniers Ma, aboutissant à un approfondissement de la fenêtre de transition smectite/illite traversant le détachement ainsi qu'à une expulsion plus tardive des gaz thermogéniques.

Les résultats de cette thèse fournissent donc de nouvelles perspectives sur la croissance et l'étalement gravitaire du cône de l'Amazone, et permettent de mettre en évidence les particularités de chaque compartiment, liées à leur histoire long-terme.

Les résultats montrent également que l'intégration des méthodologies issues de la modélisation de bassin sont un outil précieux pour étudier la tectonique et la dynamique sédimentaire des dépôt-centres du Cénozoïque supérieur, même lorsque les données sont limitées. Des études géophysiques dédiées à la caractérisation de la croûte de la marge amazonienne et de son histoire thermique seraient maintenant nécessaires pour confirmer ces résultats.

vii

TABLE OF CONTENTS

GEOPRESSURE: PROBLEMATICS AND OPEN QUESTIONS

Excess pore fluid pressure plays a key role in the stability of continental margins, through its influence on the strength of sedimentary successions during their deposition. Burial and consolidation generally lead to an increase in sediment strength, but this first-order process can be temporarily reversed by variations in permeability and/or loading leading to the development of overpressure (e.g. [START_REF] Maltman | How sediments become mobilized[END_REF]. Overpressure, defined as any pressure that exceeds the hydrostatic pressure of a column of water or formation brine, leads to variations in sediment strength that may lead to formation failure [START_REF] Dickinson | Geological aspects of abnormal reservoir pressures in Gulf Coast, Louisiana[END_REF].

Overpressured fluids are particularly important in the mobilization of fine-grained sediments and play a fundamental role in gravity-driven processes of sediment failure and deformation on continental slopes [START_REF] Bolton | The importance of overpressure timing and permeability evolution in fine-grained sediments undergoing shear[END_REF], and in the operation of syn-sedimentary faults as well as the stability of wellbores [START_REF] Moos | Comprehensive wellbore stability analysis utilizing Quantitative Risk Assessment[END_REF]. Hence, pore pressure is an issue of major concern for both our understanding of continental margin evolution and hydrocarbon exploitation [START_REF] Morley | Deepwater fold and thrust belt classification, tectonics, structure and hydrocarbon prospectively: A review[END_REF].

Mechanisms generating overpressures in sedimentary successions can be divided into those related to external stresses or those that increase internal fluid content. Stress-related processes include (1) regional tectonics, and (2) disequilibrium compaction (undercompaction), the latter being most common in deltaic depositional environments with high sedimentation rates [START_REF] Osborne | Mechanisms for generating overpressure in sedimentary basins: A reevaluation[END_REF][START_REF] Burrus | Overpressure models for clastic rocks, their relation to hydrocarbon expulsion: a critical reevaluation[END_REF][START_REF] Law | Abnormal pressures in hydrocarbon environments[END_REF][START_REF] Nashaat | Abnormally high fluid pressure and seal impacts on hydrocarbon accumulations in the Nile Delta and North Sinai Basins, Egypt[END_REF][START_REF] Swarbrick | Mechanisms that generate abnormal pressures: an overview[END_REF][START_REF] Grauls | Overpressure: Causal Mechanisms, Conventional and Hydromechanical Approaches[END_REF][START_REF] Zoback | Reservoir Geomechanics[END_REF]. Increased fluid content can be generated by processes that depend on pressure and temperature, either (3) clay diagenesis, such as the smectite-illite transition [START_REF] Hower | Mechanism of burial and metamorphism of argillaceous sediment: mineralogical and chemical evidence[END_REF][START_REF] Law | Geological implications of coal dewartering[END_REF][START_REF] Bruce | Smectite dehydrationits relation to structural development and hydrocarbon accumulation in northern Gulf of Mexico basin[END_REF][START_REF] Hall | Mechanisms of overpressuringan overview[END_REF][START_REF] Burrus | Overpressure models for clastic rocks, their relation to hydrocarbon expulsion: a critical reevaluation[END_REF][START_REF] Swarbrick | Mechanisms that generate abnormal pressures: an overview[END_REF][START_REF] Bowers | Determining an appropriate pore-pressure estimation strategy[END_REF], or (4) petroleum generation. In the latter case, volume changes occur when kerogen transforms to oil and gas (primary cracking) and oil cracks to gas (secondary cracking), but only gas is able to generate significant overpressures [START_REF] Law | Abnormal pressures in hydrocarbon environments[END_REF][START_REF] Swarbrick | Mechanisms that generate abnormal pressures: an overview[END_REF][START_REF] Grauls | Overpressure: Causal Mechanisms, Conventional and Hydromechanical Approaches[END_REF].

The post-rift successions of many passive continental margins are affected by gravity tectonics, in which a sediment volume moves downslope above one or more zones of decollement containing detachment surfaces. The gravitational potential is directly proportional to the slope of the seafloor and the properties of the detachment surface [START_REF] Vendeville | Glissements gravitaires synsedimentaires et failles normales listriques: modèles expérimentaux[END_REF][START_REF] Cobbold | Some experiments on interacting sedimentation and deformation above salt horizons[END_REF]. Deformation of the succession above the detachment surface typically results in an upslope extensional domain and a downslope compressional domain similar to a fold-and-thrust belt [START_REF] Rowan | Gravity-driven fold belts on passive margins[END_REF]. Gravitational collapse can be related to the presence beneath a sedimentary succession of ductile evaporites (e.g. [START_REF] Morley | Comparison of gravity -driven deformations styles and behavior associated with mobile shales and salt tectonics[END_REF], but on many margins it has been shown to be linked to the presence of overpressured shale levels, such as the Niger Delta [START_REF] Morley | Comparison of gravity -driven deformations styles and behavior associated with mobile shales and salt tectonics[END_REF][START_REF] Hooper | The role of deformation in controlling depositional patterns in the south-central Niger Delta, West Africa[END_REF][START_REF] Morley | Deepwater fold and thrust belt classification, tectonics, structure and hydrocarbon prospectively: A review[END_REF][START_REF] Wu | Niger Delta gravity-driven deformation above the relict Chain and Charcot oceanic fracture zones, Gulf of Guinea: Insights from analogue models[END_REF], the northwestern Gulf of Mexico [START_REF] Rowan | Gravity-driven fold belts on passive margins[END_REF], the Brunei margin [START_REF] Berthelon | [END_REF] or the Amazon Fan [START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF]Oliveira, 2005;[START_REF] Morley | Deepwater fold and thrust belt classification, tectonics, structure and hydrocarbon prospectively: A review[END_REF][START_REF] Perovano | Modelagem fisica experimental de mecanismos de dedormação gravitacional simulando multiplos intervalos superpressurizados : Apliacação à Bacia da Foz do Amazonas[END_REF].

Shale detachments form in response to pore fluid overpressures, which depend on factors including sediment loading, burial and deformation history, and the production and migration of thermogenic fluids [START_REF] Morley | Comparison of gravity -driven deformations styles and behavior associated with mobile shales and salt tectonics[END_REF][START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF][START_REF] Rowan | Gravity-driven fold belts on passive margins[END_REF][START_REF] Mourgues | An experimental investigation of gravity-driven shale tectonics in progradational delta[END_REF][START_REF] Ings | Continental margin shale tectonics: preliminary results from coupled fluid-mechanical models of basin scale delta instability[END_REF][START_REF] Oliveira | Linked extensional-compressional tectonics in gravitational systems in the Equatorial Margin of Brazil[END_REF]. The origin of shale detachments was reviewed by [START_REF] Morley | Deepwater fold and thrust belt classification, tectonics, structure and hydrocarbon prospectively: A review[END_REF]. According to these authors, the main cause of overpressures within weak shale successions is disequilibrium compaction induced by high sedimentation rates and deep burial, enhanced by possible inflationary overpressure mechanisms resulting from diagenesis during burial. These results were mainly based on analogue mechanical models, which assumed the origin and type of overpressured fluids.

While such studies have clearly identified the need to consider overpressure along detachment surfaces (e.g. Shaw and Bilotti, 2005;[START_REF] Yuan | Reappraisal of gravity instability conditions for offshore wedges: consequences for fluid overpressures in the Niger Delta[END_REF][START_REF] Yuan | Control of fluid pressures on the formation of listric normal faults[END_REF][START_REF] Berthelon | [END_REF], none have integrated basin pore pressure evolution or examined the possible origins of fluid pressures within the basin and/or along the detachment.

I.2 THE AMAZON MARGIN AS A STUDY AREA

The Amazon River has one of the world's largest discharges of suspended sediment [START_REF] Meade | Suspended sediments of the modern Amazon and Orinoco rivers: Quaternary International[END_REF] and culminates in one of its largest deep-sea fans (Fig. 1.1), comparable in size to those of the Bengal, Indus and Mississippi rivers [START_REF] Rimington | Amazon Fan sands: Implications for provenance[END_REF]. Onshore to offshore studies indicate that the trans-continental Amazon River has developed since the mid-Miocene in response to Andean uplift [START_REF] Hoorn | Andean tectonics as a cause or changing drainage patterns in Miocene northern South America[END_REF][START_REF] Hoorn | Amazonia through time: Andean uplift, climate change, landscape evolution and biodiversity[END_REF][START_REF] Hoorn | The Amazon at sea: Onset and stages of the Amazon River from a marine record, with special reference to Neogene plant turnover in the drainage basin[END_REF], resulting in the progradation of a shelf-slope depocentre up to 10 km thick [START_REF] Silva | Cenozoic tectonics of Amazon Mouth Basin[END_REF][START_REF] Watts | Seismic structure, gravity anomalies and flexure of the Amazon continental margin, NE Brazil[END_REF]. Stratigraphic studies of the Amazon fan based on seismic and well data indicate that sediment flux to the Amazon offshore increased through time to reach a maximum during the Quaternary, i.e. the last 2.6 Ma [START_REF] Figueiredo | Late Miocene onset of the Amazon River and the Amazon deep-sea fan: Evidence from the Foz do Amazonas Basin[END_REF][START_REF] Cruz | Neogene evolution and demise of the Amapá carbonate platform, Amazon continental margin, Brazil[END_REF]. Seismic data also

show that the growth of the fan has been accompanied by its gravitational collapse, expressed as linked extensional-compressional fault systems rooted on detachment surfaces up to hundreds of kilometers in extent at subsurface depths of up to 10km [START_REF] Silva | Cenozoic tectonics of Amazon Mouth Basin[END_REF][START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF]Oliveira, 2005;[START_REF] Perovano | O processo de colapso gravitacional da secao marinha da Bacia da Foz do Amazonas -Margem Equatorial Brasileira[END_REF][START_REF] Perovano | Modelagem fisica experimental de mecanismos de dedormação gravitacional simulando multiplos intervalos superpressurizados : Apliacação à Bacia da Foz do Amazonas[END_REF]. Three main detachment surfaces have been recognized within the Upper Cretaceous to upper Miocene interval, recording successive phases of collapse [START_REF] Silva | Cenozoic tectonics of Amazon Mouth Basin[END_REF][START_REF] Silva | O processode colapso gravitacional e a estruturação da seção marinha da bacia da Foz doAmazonas -Margem equatorial brasileira[END_REF][START_REF] Reis | Two-scale gravitational collapse in the Amazon Fan: a coupled system of gravity tectonics and mass-transport processes[END_REF][START_REF] Reis | Effects of a regional décollement level for gravity tectonics on late Neogene-Quaternary large -scale slope instabilities in the Foz do Amazonas Basin, Brazil[END_REF].

Above the upper detachment level, the on-going collapse of the Amazon fan is expressed as paired belts of extensional and compressional faults across the shelf and upper slope above water depths of 2100 m, including thrust-folds with up to 500 m of seafloor relief [START_REF] Silva | Cenozoic tectonics of Amazon Mouth Basin[END_REF][START_REF] Silva | O processode colapso gravitacional e a estruturação da seção marinha da bacia da Foz doAmazonas -Margem equatorial brasileira[END_REF][START_REF] Reis | Two-scale gravitational collapse in the Amazon Fan: a coupled system of gravity tectonics and mass-transport processes[END_REF][START_REF] Reis | Effects of a regional décollement level for gravity tectonics on late Neogene-Quaternary large -scale slope instabilities in the Foz do Amazonas Basin, Brazil[END_REF]. and available seismic profiles and wells. The Amazon Fan is composed of NW and SE structural compartments, as defined by [START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF] and Oliveira et al (2005). Gravitational collapse has resulted in paired domains of extensional and compressive faults across the shelf and upper slope. The available regional dataset comprises multi-channel seismic reflection profiles and drilling results from industry and ODP/DSDP wells. For this study, we use a composite seismic profile across the SE compartment referred to as A-B, shown in red, calibrated with reference to two wells: 1-BP-3-APS and 1-APS-31A-AP (also shown in red), and a profile across the NW compartment referred to as C-D (show in red).

Detachment surfaces within the Amazon Fan have been inferred to correspond to weak levels composed of overpressured clays (Oliveira, 2005;[START_REF] Zalán | Origin and structural evolution of the Cenozoic Rift System of Southeastern Brasil [Origem e evolução estrutural do Sistema de Rittes Cenozóicos do Sudeste do Brasil[END_REF][START_REF] Perovano | O processo de colapso gravitacional da secao marinha da Bacia da Foz do Amazonas -Margem Equatorial Brasileira[END_REF][START_REF] Perovano | Modelagem fisica experimental de mecanismos de dedormação gravitacional simulando multiplos intervalos superpressurizados : Apliacação à Bacia da Foz do Amazonas[END_REF]. Based on sandbox modeling, [START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF] proposed that the structural patterns of the Amazon fan could be reproduced on the assumption that fluid overpressures existed over a triangular area beneath the detachment. They noted that deep wells had encountered overpressures within the upper several kilometers of the fan, and stated that numerical modeling (which was not presented) supported the origin of the fluids from thermogenic gas generation from Cenomanian-Turonian source rocks beneath the detachment. Evidence for an active hydrocarbon system within the Foz do Amazonas Basin is available from results from published wells reports (Brazilian Petroleum and Gas Agency -ANP) that indicate thermogenic hydrocarbons, mainly gas. [START_REF] Miranda | Application of Unsupervised Semivariogram Textural Classification of RADARSAT-1 data for the detection of natural oil seeps offshore the Amazon Mouth: proceedings of the Radarsat Adro Symposium[END_REF] used satellite data to identify natural oil seeps at the sea surface above growth faults and thrust-folds across the shelf break and upper slope, mainly in the northwest part of the Amazon fan. In the same area, [START_REF] Ketzer | Gas seeps and gas hydrates in the Amazon deep-sea fan[END_REF] presented multibeam sonar evidence of gas flares rising up to 900 m into the water column from seafloor vents in water depths of 650-2600 m, while samples of gas hydrates were recovered at two sites and found to contain methane isotopically consistent with biogenic sources, with possible thermogenic contributions at a third site northwest of the fan.

The Amazon deep-fan provides an opportunity to address outstanding questions about gravity tectonics and pore pressure evolution on passive continental margins. Most of these questions are related to: (1) how and when the detachment surface formed;

(2) what are the main mechanisms acting on the pore fluid pressure evolution; and (3) what is the state of pore fluid pressure today and the risks associated with drilling wells.

I.3 OBJECTIVES OF THE THESIS

The overall objective of this thesis is to investigate the evolution of pore fluid pressure during the growth and gravitational collapse of the Amazon deep-fan. This is achieved through an innovative application of methods of basin analysis that are widely used in hydrocarbon exploration, but have not previously been used to examine the dynamics of a collapsing passive margin depocentre. The study is based on exploration seismic and well data, used to investigate two regional sections (A-B and C-D, see Fig. 1.1), located in southeastern and northwestern compartments of the Amazon fan. Careful analysis of the stratigraphic and structural record of the Amazon deep-sea fan along these two sections allows a reconstruction of its history of sedimentary growth and gravity tectonics, and an assessment of associated overpressure mechanisms.

To undertake this analysis, we developed a new and innovative approach, involving the integration of state-of-the-art methodologies, to determine how pore fluid pressure originates and evolves over time. This method incorporates several modeling techniques commonly used in the oil and gas industry, including: (1) interpretation of commercial 2D seismic reflection data across the shelf and slope, stratigraphically and geomechanically constrained by well data;

(2) a 2D kinematic restoration of depth-converted seismic sections, followed by (3) forward modeling of the evolution of pore pressure and temperature along the sections; and (4) the integration of these results to reproduce the fault movements observed on the seismic sections and constrain the behavior of the detachment surface.

The findings provide new perspectives on the relative roles of different overpressure mechanisms during the growth and collapse of the Amazon deep-sea fan, including the origin of fluids from clay mineral transitions and from hydrocarbon gas generation within the Foz de Amazonas Basin. The results have implications for the current risks associated with gravity tectonics in the Amazon fan as well as for the drilling of wells, and provide general insights applicable to other rapidly deposited continental margin depocentres.

The thesis is subdivided into 8 chapters:

Chapter I introduces the subject, study area and objectives of the thesis.

Chapter II provides a detailed review of the main overpressure mechanisms (disequilibrium compaction, tectonic stress and temperature-driven mechanism), and the process associated with them, including gravity tectonics.

Chapter III describes the tectono-sedimentary framework of the Foz do Amazonas Basin and in particular, the late Cenozoic growth of the Amazon Fan. In addition, section III.3

presents the thermal history and hydrocarbon potential of the basin.

Chapter IV presents the dataset used in this thesis as well as the innovative methodology applied to achieve the objectives outlined in chapter I.

Chapter V consists of a recent publication in the journal Marine and Petroleum Geology [START_REF] Souza | Controls on overpressure evolution during the gravitational collapse of the Amazon deep-sea fan[END_REF], entitled: Controls on overpressure evolution during the gravitational collapse of the Amazon deep-sea fan, which presents results from the southwestern compartment of the Amazon fan.

Chapter VI presents results from the northwestern compartment of the Amazon Fan, based on the same approach proposed Chapter V. Chapter VI represents an article in preparation that will also be submitted to Marine and Petroleum Geology.

Chapter VII discusses and compares the results obtained in the southeastern and northwestern compartments of the Amazon Fan in terms of pore pressure evolution, overpressure mechanisms and implications of crustal history on overpressure mechanisms.

Chapter VIII concludes the thesis by presenting its main findings and some perspectives on further studies.

Chapter II

Overpressure products and process

__________________________________________________________________________

II.1 OVERPRESSURE MECHANISMS

Abnormally high pore pressures within sedimentary successions are the result of fluid retention within a formation. In general, fluid is retained in the pores, but when it does not find a path for expulsion and pressure dissipation, it generates a pressure imbalance. This  Stresses related to disequilibrium compaction (undercompaction) and tectonics stress (lateral compressive stress) [START_REF] Osborne | Mechanisms for generating overpressure in sedimentary basins: A reevaluation[END_REF][START_REF] Burrus | Overpressure models for clastic rocks, their relation to hydrocarbon expulsion: a critical reevaluation[END_REF][START_REF] Law | Abnormal pressures in hydrocarbon environments[END_REF][START_REF] Nashaat | Abnormally high fluid pressure and seal impacts on hydrocarbon accumulations in the Nile Delta and North Sinai Basins, Egypt[END_REF][START_REF] Swarbrick | Mechanisms that generate abnormal pressures: an overview[END_REF][START_REF] Grauls | Overpressure: Causal Mechanisms, Conventional and Hydromechanical Approaches[END_REF][START_REF] Zoback | Reservoir Geomechanics[END_REF];

 Fluid volume increase during temperature increases (aquathermal), mineral transformation often release water [START_REF] Hower | Mechanism of burial and metamorphism of argillaceous sediment: mineralogical and chemical evidence[END_REF][START_REF] Law | Geological implications of coal dewartering[END_REF][START_REF] Bruce | Smectite dehydrationits relation to structural development and hydrocarbon accumulation in northern Gulf of Mexico basin[END_REF][START_REF] Hall | Mechanisms of overpressuringan overview[END_REF][START_REF] Burrus | Overpressure models for clastic rocks, their relation to hydrocarbon expulsion: a critical reevaluation[END_REF][START_REF] Swarbrick | Mechanisms that generate abnormal pressures: an overview[END_REF]Osborne, 1998, Bowers, 2001), hydrocarbon fluids can be released due to primary or secondary cracking [START_REF] Law | Abnormal pressures in hydrocarbon environments[END_REF][START_REF] Swarbrick | Mechanisms that generate abnormal pressures: an overview[END_REF][START_REF] Grauls | Overpressure: Causal Mechanisms, Conventional and Hydromechanical Approaches[END_REF].

 Fluid movement and buoyancy [START_REF] Osborne | Mechanisms for generating overpressure in sedimentary basins: A reevaluation[END_REF][START_REF] Swarbrick | Mechanisms that generate abnormal pressures: an overview[END_REF]. Among the mechanisms mentioned above, the principal ones resulting in large overpressures are disequilibrium compaction and fluid volume expansion during gas generation [START_REF] Swarbrick | Mechanisms that generate abnormal pressures: an overview[END_REF]. It is emphasized that disequilibrium compaction is often associated in basins with a high sedimentation rate, including Tertiary deltas and some intracratonic basins.

For studies on the pressure evolution (normal or abnormal) of sedimentary basins, it is important to identify the main generating mechanism, or set of mechanisms. These mechanisms can be tested from hypothetical scenarios, in numerical or analog models.

II.1.1 Disequilibrium compaction

This mechanism, also defined as vertical loading stress, is most common in deltaic depositional environments. The sediments are initially unconsolidated and remain in a suspended load with the carrying fluid until they are deposited at the base of a water body to be further compacted as the pressure exerted by younger sediments progressively increases.

Overpressure generated by undercompaction is common in basins with a high sedimentation rate, including Tertiary deltas and some intracratonic basins [START_REF] Swarbrick | Mechanisms that generate abnormal pressures: an overview[END_REF].

This process occurs as high sedimentation rates causes increasing burial of low-permeability and hence undercompacted sediments. As burial continues, the pore fluids start to support part of the weight of overlying sediments, instead of the grain-to-grain contacts, and the fluid becomes overpressured.

Undercompaction can preserve porosity, when sediments with low permeability inhibit the escape of pore fluid (fluid retention) as rapidly as the pore space can compact. Fluid retention is controlled by rapid sedimentation and by the low intrinsic permeability of rocks such as shales or well-cemented carbonates, formations are also referred to as 'seals'.

Measuring the permeability of seals is difficult and prone to large inaccuracies.

According to McBride et al (1991), sandstones compact at a relatively slow rate, from a starting point of approximately 40-50% porosity to as little as 5-10% porosity, mainly due to the rearrangement of sand grains, as well as due to dissolution at grain contacts. Low rates of compaction together with high rates of fluid expulsion (due to high permeability) lead to reduced porosity with depth. In contrast, clays have a typical porosity of approximately 65-80% at the point of deposition, and compact more quickly than sands. In addition, clay compaction is largely controlled by mudstone lithology, and in certain conditions, the sediments are not fully compacted relative to their overburden.

II.1.2 Tectonic stress

Tectonic stress may increase the total stress state and cause overpressures. This situation is common in strike-slip and reverse-fault stress regimes known to be under high lateral stresses, such as parts of California (Berry, 1969). According to [START_REF] Swarbrick | Mechanisms that generate abnormal pressures: an overview[END_REF], lateral compression can increase pore pressures in the same way as vertical stress causing over pressuring through disequilibrium compaction. The stress variation can result from variations of intraplate stress due to plate tectonic processes or from lithospheric flexure during deglaciation [START_REF] Swarbrick | Mechanisms that generate abnormal pressures: an overview[END_REF][START_REF] Zoback | Reservoir Geomechanics[END_REF]. Recent rapid sedimentation and upsurges in horizontal compression that drive compaction can increase pore pressure.

II.1.3 Clay diagenesis

Diagenesis consists of physical, chemical, and/or biological changes that alter sediments during lithification or post-depositional compaction. Diagenetic processes are driven by increasing pressure and temperature with depth, and may result in the formation of new minerals, recrystallization and lithification. Several diagenetic mineral transformations common in sedimentary successions lead to dehydration, or the release of bound water.

During diagenesis, bound water becomes free water. If the increased amount of free water cannot escape, then the pore pressure becomes abnormally pressured. Dehydration reactions include gypsum-to-anhydrite in evaporitic sediments, and coalification [START_REF] Law | Geological implications of coal dewartering[END_REF]. The most common transformation involves the dehydration of smectite, a multi-layered, mixed-layer clay commonly found in mud-rocks. Smectite transforms to a new mineral, illite, along with the release of water.

In many mud-dominated basins, a gradual and systematic downward change from smectite to illite is observed in stratigraphic successions, broadly coincident with a transition to high amounts of overpressure. The transition occurs over a temperature range of 70-150°C [START_REF] Bruce | Smectite dehydrationits relation to structural development and hydrocarbon accumulation in northern Gulf of Mexico basin[END_REF]Schneider et al., 2003) and appears to be independent of sediment age and burial depth.

II.1.4 Hydrocarbon generation

This mechanism is driven by temperature increase with depth, and depends on time, the presence and richness of source rocks, facies distributions and permeability. Even though hydrocarbon generation may promote the development of overpressure, and affect compaction disequilibrium, it is difficult to quantify its effects on pore pressure distribution [START_REF] Grauls | Overpressure: Causal Mechanisms, Conventional and Hydromechanical Approaches[END_REF] Meisser [START_REF] Mckenzie | Some remarks on the development of sedimentary basins[END_REF]Du Rochet (1978) observed primary migration in source rocks and interpreted the transformation from kerogen to oil, or primary cracking, to occur at temperatures from 100-120°C.

Progressively, after hydrocarbon expulsion from source rocks, a pressure transference occurs due to secondary hydrocarbon migration, also called differential fluid density. In this overpressure mechanism, the formation contains a fluid with a different density than water.

This dynamic transfer occurs along structures with significant dip, such as active faults, leakage of seal and through the permeability of rocks with a significant dip.

Secondary oil cracking can also occur during this oil-to-gas transformation, at temperatures exceeding 175-180°C (Hedberg, 1974, Mackenzie andQuigley, 1988). In some cases, at depths of burial typically between 3-5 km, the compressibility of gas must be considered as well as its solubility in brine. Large volume increases are still likely, and can potentially generate overpressures.

II.2 Gravity Tectonics

The study of gravity tectonics or thin-skinned extension has received special attention in the last decades since this form of deformation has structural consequences in important petroleum producing areas. These studies were based on case studies and analogue modeling [START_REF] Vendeville | How normal faulting and sedimentation interact to produced listric fault profiles and stratigraphy wedges[END_REF][START_REF] Vendeville | Mechanisms generating normal fault curvature: a review illustrated by physical models[END_REF][START_REF] Nelson | Salt tectonics and listric-normal faulting. The Gulf of Mexico Basin[END_REF]Vendeville andJackson, 1992a, 1992b;Jackson, 1994;Mohriak et al., 1995;[START_REF] Morley | Comparison of gravity -driven deformations styles and behavior associated with mobile shales and salt tectonics[END_REF].

Thin-skinned extension consists of the development of structural systems within a sedimentary cover due to its horizontal translation above a basal detachment surface. It may take place in mobile layers consisting of evaporites or clays, which can act as one or multiple detachment levels. On continental margins, the presence of siliciclastic and/or carbonate sedimentary successions overlying rheologically more ductile stratigraphic levels may allow a mechanism of thin-skinned extension to develop [START_REF] Vendeville | Glissements gravitaires synsedimentaires et failles normales listriques: modèles expérimentaux[END_REF][START_REF] Cobbold | Some experiments on interacting sedimentation and deformation above salt horizons[END_REF].

According to [START_REF] Mourgues | Surpressions de fluide et décollementsmodélisations analogique et numérique[END_REF] based on analogue and numerical models, extensional deformation begins when the gravity potential is sufficient to overcome the internal resistance (friction) of the sediment load and the resistance to gliding along potential basal surfaces. The detachment levels can be of two types:

 Ductile behaviordécollement -that accommodates the movement of the sediment cover through a continuous deformation. The shear strength in this case is a function of viscosity and shear rate. Mobile levels with rheological behavior similar to evaporites can deform with practically no resistance under conditions of low deformation rates.

 Detachment surface, which corresponds to a discontinuity between two fragile levels, on which the translation or sliding of the sedimentary cover occurs. The development of such as a surface requires overpressures generated by interstitial fluids, which are responsible for reducing friction (resistance to movement) between the interfaces (for example, overpressured clays).

The gravity potential is directly proportional to the slope of the detachment surface and gradient slope. Active sedimentation may play an important role as a triggering agent, acting as the motor of gravity tectonics [START_REF] Vendeville | Glissements gravitaires synsedimentaires et failles normales listriques: modèles expérimentaux[END_REF]. As a result, gravity tectonics can be generated by combination of two inducing mechanisms:

 Gravity gliding: involves the translation of a sedimentary cover on a sloping detachment surface towards the basin under the action of gravity, and is generally associated with the beginning of thermal subsidence of the oceanic lithosphere (Fig. 2.2) [START_REF] Schultz-Ela | Excursus on gravity gliding and gravity spreading[END_REF][START_REF] Rowan | Gravity-driven fold belts on passive margins[END_REF]. The intensity of the sliding process is directly proportional to the slope of the bottom surface and to the slope of the mobile substrate, and can occur even on surfaces with soft slopes (less than 2°).

 Gravity spreading: movement on ductile levels is induced by variations in the lithostatic pressure gradient generated by differential sediment load, for example due to the construction of prograding sedimentary prisms such as deltas, carbonate platforms and submarine fans (Ramberg, 1981;Gaullier and Vendeville, 2005). According to [START_REF] Rowan | Gravity-driven fold belts on passive margins[END_REF], the erosion of the continental shelf and slope during lowered relative sea level, as well as bypass events leading to the deposition of large volumes in the slope and in the ocean basin, attenuate the gravity potential and, consequently, the sedimentary cover (Fig. 2.3). Several authors have proposed that gravity tectonics is associated with distinct structural domains, based on the interaction between sedimentation and structuring, and on local morphological characteristics of the substrate. These analyses were obtained based on a combination of improvements in seismic data processing and analogue modeling. The mechanism of extensive surface deformation develops a structural system composed of a proximal extensive domain, mechanically connected to a distal compressive domain (Fig. 2.4;

Vendeville [START_REF] Vendeville | How normal faulting and sedimentation interact to produced listric fault profiles and stratigraphy wedges[END_REF][START_REF] Cobbold | Some experiments on interacting sedimentation and deformation above salt horizons[END_REF][START_REF] Nelson | Salt tectonics and listric-normal faulting. The Gulf of Mexico Basin[END_REF]Cobbold & Szatmari, 1991;[START_REF] Vendeville | Mechanisms generating normal fault curvature: a review illustrated by physical models[END_REF]Vendeville e Jackson, 1992ae 1992b;Jackson et al.,1994;Mohriak et al., 1995;[START_REF] Morley | Comparison of gravity -driven deformations styles and behavior associated with mobile shales and salt tectonics[END_REF]. However, other studies recognize an intermediate structural domain, mechanically interconnecting the extensive and compressive domains, characterized by the rigid translation of the sedimentary cover (Fig. 2.4; Damuth, 1994;[START_REF] Hooper | The role of deformation in controlling depositional patterns in the south-central Niger Delta, West Africa[END_REF]Oliveira et al., 2005;[START_REF] Lecomte | Analogue modelling and numerical simulations of gravitational spreading along passive margin under fluid overpressure[END_REF]. The extensive domain is structurally composed of synthetic antithetic faults, rollover anticlines and, in the case of an evaporite layer, salt rollers formed behind the fault plane. The listric faults are predominantly perpendicular to the direction of translation and horizontally in depth, anchored to levels of decollement, accommodating translation and tilting of the sedimentary blocks [START_REF] Vendeville | Glissements gravitaires synsedimentaires et failles normales listriques: modèles expérimentaux[END_REF][START_REF] Cobbold | Some experiments on interacting sedimentation and deformation above salt horizons[END_REF].

The compressive domain accommodates the extensional and the translation deformation of the sedimentary cover. The shortening of the sediment cover in this domain can take place in the form of different compressive structures, such as, fold belts, thrust faults, diapirs or walls of salt or shale. The compressive structures usually form at the base of the slope, close to the limit of effective influence of the levels of detachment. In the case of deformation associated with overpressured levels, this limit is defined by the decrease of the interstitial pressurization and consequent loss of mobility of the mobile level [START_REF] Thomas | Mushward: Ductile in the Appalachian thrust belt in Alabama[END_REF][START_REF] Rowan | Gravity-driven fold belts on passive margins[END_REF].

The tectonic evolution of passive continental margins related to occurrence of gravity tectonics with overpressured shales is directly controlled by the balance of the generation and dissipation rates of the interstitial fluid overpressure condition. According to [START_REF] Mourgues | Surpressions de fluide et décollementsmodélisations analogique et numérique[END_REF], the maintenance of the overpressure state depends on the permeability (porosity) of the rocks and the velocity of generation of interstitial pressure, i.e. it depends on the ratio between the rate of fluid release and loss of pressurization, controlled by porosity and rock fracture, and the rate of pressure generation through physical, chemical and mineralogical mechanisms.

Chapter III Geological Setting

__________________________________________________________________________

III.1 The Foz do Amazonas Basin

The Foz do Amazonas Basin represents the northernmost basin on the Brazilian Equatorial Margin (Fig. 3.1). It is located off the coasts of the State of Amapá and the northwest coast of the State of Pará, and extends over an area of 268,000 km 2 [START_REF] Brandão | Bacia da Foz do Amazonas[END_REF]. The basin includes a major seafloor depositional feature, the Amazon Fan, one of the world's largest deep-sea fans, which extends from the continental shelf to abyssal depths of 4800 meters [START_REF] Silva | Cenozoic tectonics of Amazon Mouth Basin[END_REF].

The basins of the Brazilian Equatorial Margin developed as a consequence of the separation of the South American and African plates in a transform tectonic context characterized by multiple phases of subsidence and complex structural styles [START_REF] Matos | Tectonic evolution of the equatorial South Atlantic[END_REF].

According to Szatimari et al. (1987) and [START_REF] Matos | Tectonic evolution of the equatorial South Atlantic[END_REF], rifting of the Brazilian Equatorial Margin occurred in several evolutionary stages between the Aptian and Cenomanian. [START_REF] Matos | Tectonic evolution of the equatorial South Atlantic[END_REF] recognized three stages in the tectonic evolution of the Equatorial Atlantic: pre-transform, syntransform and post-transform.

The pre-transform stage is divided into two sedimentary cycles:

 Pre-pull apart, corresponding to sediments of Jurassic and Lower

Cretaceous age that precede the main opening phase of the Equatorial Atlantic, preserved as rift deposits in the Marajó basin and in the Potiguar onshore basin.

 Syn-pull apart, corresponding to sediments of end Barremian to Albian age, marking the beginning of fragmentation and stretching of the lithosphere in the Equatorial Atlantic in response to transtensional tectonism.

During the Aptian, high rates of lithospheric stretching resulted in the formation of en echelón basins, of NW-SE orientation, with no evidence of syn-rift basins typical of orthogonal extension or volcanic margins [START_REF] Azevedo | Tectonic Evolution of Brazilian Equatorial Continental Margin Basins[END_REF]Matos, 1999a;[START_REF] Matos | Tectonic evolution of the equatorial South Atlantic[END_REF].

The syn-transform stage is characterized by distinct tectonic events along the margin between the Albian and Cenomanian, involving transform faulting and the formation of an oceanic crust, creating the lithospheric boundary between Africa and South America. The divergent oblique movement between the lithospheric plates produced complex transcurrent fault systems, associated with normal and reverse movements, distributed as a function of transpressional and transtensional domains along the equatorial margin (Matos, 1999a;[START_REF] Matos | Tectonic evolution of the equatorial South Atlantic[END_REF].

The post-transform stage, corresponding to a passive margin, began in the Late Albian and is interpreted to mark a time when the tectonic control exerted by fracture zones and proximal spreading centers had less influence than the creation of space due to cooling of the lithosphere and relative sea level variations [START_REF] Matos | Tectonic evolution of the equatorial South Atlantic[END_REF]. The stratigraphic evolution of the Foz do Amazonas basin can be divided into three main sedimentary megasequences, based on 2D interpretation seismic data and set of wells:

 Pre-rift: the first megasequence, aged between 222-186 Ma, is characterized by reddish, clean, fine to medium sandstones, interbedded with basaltic lavas, of Triassic age (Calçoene Formation), interpreted to record a stable tectonic environment [START_REF] Brandão | Bacia da Foz do Amazonas[END_REF][START_REF] Figueiredo | Bacia da Foz do Amazonas[END_REF].

 Syn-rift: the Lower Cretaceous to Albo-Aptian megasequence associated with South Atlantic opening consists of the siliciclastic Cassiporé formation, composed of shales interspersed with fine sandstones recording fluvio-deltaic deposition in lake systems [START_REF] Brandão | Bacia da Foz do Amazonas[END_REF][START_REF] Figueiredo | Bacia da Foz do Amazonas[END_REF]. o Second phase: dominated by Middle Paleocene to Middle Miocene carbonates of the Marajó and Amapá Formations, intercalated with siliciclastic intervals [START_REF] Wolff | Microfacies, depositional environments, and diagenesis of the Amapá carbonates (Paleocene-Middle Miocene), Foz do Amazonas basin, offshore NE Brazil[END_REF][START_REF] Gorini | Late Neogene sequence stratigraphic evolution of the Foz do Amazonas Basin, Brazil[END_REF], together forming a mixed platform up to 4000 meters thick. This sequence is laterally equivalent to deep water calcilutites and mudstones/claystones of the Travosas Formation (Fig. 3.2).

o Third phase: the Miocene to middle Pleistocene Pará Group (Tucunaré, Pirarucu and Orange Formations) records siliciclastic progradation, covering the carbonate platform and resulting in the construction of the Amazon Fan (Fig. 3.2). This phase has been related to the onset of Amazon River drainage to the South Atlantic, in response to rapid uplift of the Andes between Tortonian and Messinian (Brandão e Feijó, 1994;[START_REF] Hoorn | Andean tectonics as a cause or changing drainage patterns in Miocene northern South America[END_REF][START_REF] Silva | Cenozoic tectonics of Amazon Mouth Basin[END_REF][START_REF] Figueiredo | Bacia da Foz do Amazonas[END_REF].

III.2 The Amazon Fan gravity tectonics

The Amazon deep-sea fan is a wedge of siliciclastic sediment up to 10km thick [START_REF] Silva | Cenozoic tectonics of Amazon Mouth Basin[END_REF], the majority of which has been deposited in the last 8 Ma [START_REF] Cruz | Neogene evolution and demise of the Amapá carbonate platform, Amazon continental margin, Brazil[END_REF]. The greatest sediment thicknesses underlie the Upper Fan, corresponding to the outer continental shelf and upper slope in water depths of 100-500m, thinning seaward across the Lower Fan which reaches water depths of more than 4000 m [START_REF] Damuth | Anatomy and growth pattern of Amazon deep-sea fan as revealed by long-range side-scan sonar (GLORIA) and high-resolution seismic studies[END_REF]. The rapid supply of clastic sediment, associated with fast burial, is inferred to have driven the gravitational collapse of the fan, resulting in paired belts of extensional and compressional structures across the outer shelf and upper slope (Figs 3.3-3.6;[START_REF] Reis | Effects of a regional décollement level for gravity tectonics on late Neogene-Quaternary large -scale slope instabilities in the Foz do Amazonas Basin, Brazil[END_REF]. According to [START_REF] Silva | Cenozoic tectonics of Amazon Mouth Basin[END_REF] and [START_REF] Perovano | O processo de colapso gravitacional da secao marinha da Bacia da Foz do Amazonas -Margem Equatorial Brasileira[END_REF], gravity tectonics has taken place above basal décollement levels constituted by overpressured shales. [START_REF] Vendeville | Glissements gravitaires synsedimentaires et failles normales listriques: modèles expérimentaux[END_REF] were among the first to study gravity tectonics in the Amazon Fan. Their study was focused on analogue modeling of 'mud diapirs' at a large scale, based on their character as observed on seismic profiles.

Subsequently, improved seismic data quality led these diapirs being reinterpreted as reverse faults, grouped in compressive fronts [START_REF] Silva | Cenozoic tectonics of Amazon Mouth Basin[END_REF][START_REF] Oliveira | Shale Tectonics and Structural Deformation on the Amazon Submarine Fan: Preliminaries Results[END_REF]Oliveira, 2005;[START_REF] Reis | Gravity-driven processes at the offshore Amazon Mouth Basin -Brazilian Equatorial Atlantic margin[END_REF].

The extensional belt was studied by Silva and Rodarte (1989), who observed a system of large normal listric faults, of NW-SE orientation, anchored on a detachment surface corresponding to the base Tertiary, forming large anticlines associated with the rollover process. These authors suggested movement in the sediment cover was triggered by the onset of thermal subsidence at the end of the Cretaceous. [START_REF] Silva | Cenozoic tectonics of Amazon Mouth Basin[END_REF] correlated the development of gravity tectonics in the Foz do Amazonas Basin to the beginning of the deposition of large sedimentary volumes in the continental slope region, favored during periods of relative sea level decrease and the increased rates of sediment supply during Miocene. Slope instability would have triggered a gravitational gliding process induced by a mobile basal level composed of overpressured clays, causing extensive thin-skinned extension in the sedimentary cover.

These authors also observed for the first time that the gravity structures were rooted on two different levels of detachment, which seem to be conditioned by a surface corresponding to the top Cretaceous (Fig. 3.5). The tectonic framework results in a structural system characterized by an extensional proximal domain, mechanically connected to a distal compressional domain. The extensional domain consists of normal listric faults covering the upper slope region; while the compressional domain consists of distal reverse faults, accommodating the translation of the sedimentary cover ). [START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF] (Fig. 3.3) and Oliveira (2005;et al., 2005), based on the variation of the structural style along the slope and the continental rise, divided the gravitational system into two main structural domains, which Oliveira (2005;et al., 2005) defines as structural compartments:

 Southeast (SE) compartment: characterized by a narrower structural system, restricted to the slope (Fig. 3.4)

 Northwest (NW) compartment: characterized by a more extended system, covering part of the continental rise (Fig. 3.4) [START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF] attributed the gravity tectonics of the sedimentary cover in the NW compartment to the presence of a detachment surface induced by the generation of interstitial fluids (hydrocarbons, based on the analysis of geochemical data from wells).

According to the authors, the presence of overpressured shale layers would be unlikely, because these layers would be old and lithified when it occurred. Oliveira (2005;et al., 2005) attributed the structural compartmentalization to a mechanism of differential overloading of the sedimentary cover with the mobilization of the basal levels of detachment. The authors associated the structural evolution of the system to the development of two main depocentres in the basin, so-called D1 and D2, depending on the translation of the cover under the action of gravity (Fig. 3.4). According to [START_REF] Perovano | O processo de colapso gravitacional da secao marinha da Bacia da Foz do Amazonas -Margem Equatorial Brasileira[END_REF], the stratigraphic recurrence of overpressured sedimentary intervals within the Amazon fan has resulted in multiple detachment levels over which the gliding of the sedimentary section takes place. These authors identified at least three main stratigraphic levels that have acted as detachment surfaces, at either regional or local scale (Figs.

III.3 THERMAL HISTORY AND HYDROCARBON POTENTIAL

The Amazon margin lies in a complex plate tectonic context of basin formation, from the syn-rift opening of the equatorial Atlantic Ocean in the Late Aptian through phases of postrift sedimentation. Knowledge of these stages is important to characterize the thermal history in the basin, which is linked to the evolution of crustal geometry. The characterization of crustal geometry and associated basement type along the Brazilian equatorial margin remains a subject of discussion, due to limited available data and published works.

The character of the crust in the area of the Amazon fan was studied by [START_REF] Rodger | A Marine Geophysical Study of the Amazon Continental Margin, North Eastern Brazil[END_REF] and [START_REF] Watts | Seismic structure, gravity anomalies and flexure of the Amazon continental margin, NE Brazil[END_REF], who presented results from refraction and reflection seismic data across the central-northwestern compartment of the fan (Amazon Cone Experiment Survey -ACE), as well as from regional wells and modelling of gravimetric and magnetometric data. The thermal history of the basin has direct implications for the petroleum system and hydrocarbon generation. As previously mentioned, the presence of hydrocarbons is considered as one possible mechanism of overpressure generation. [START_REF] Mello | Foz do Amazonas Area: the last frontier for elephant hydrocarbon accumulation in the South Atlantic realm[END_REF] reviewed the main generating rocks present in the Foz do Amazonas basin, based on geochemical analyses of samples (oil type) and comparison to source rocks in the Niger Delta (Fig. 3.8). The objective was to assess the depositional paleoenviroments and the age of source rocks in the Foz do Amazonas basin from biological marker data. The results of the biomarker investigation revealed similarities and differences among the oil samples studied. These authors identified two major types of hypoxic-anoxic environments:

Upper Cretaceous marine carbonates and Tertiary marine deltaics.

The first group of source rocks is derived from the Upper Albian-Cenomanian marls and calcareous black shale deposited in a marine environment [START_REF] Mello | Geochemical and molecular studies of the depositional environments of source rocks and their derived oils from the Brazilian marginal basins[END_REF][START_REF] Requejo | Hidrocarbon source facies in the Niger Delta petroleum province as indicated by the molecular characteristics of oil[END_REF]. This source rock was identified in the Limoeiro Formation comprising around 50-150m of well-laminated shales, with a total organic carbon (TOC) average from 2 to 5%. The kerogen sterane content for Niger Delta and Foz do Amazonas oil types. [START_REF] Mello | Foz do Amazonas Area: the last frontier for elephant hydrocarbon accumulation in the South Atlantic realm[END_REF].

More recent work during IODP Leg 207 (2004, Fig. 3.10) brought new information on potential source rocks in the Equatorial South Atlantic, in particular due to oceanic anoxic events. Ocean anoxic events represent major disruptions to the ocean system defined by massive deposition of organic carbon in marine environments. Several authors [START_REF] Jenkyns | Cretaceous anoxic events: from continents to oceans[END_REF][START_REF] Arthur | Stratigraphy, geochemistry, and paleoceanography of organic carbon-rich Cretaceous sequences[END_REF]Erbacher et al., 1996) have suggested that up to six OAEs in the mid-Late Cretaceous (OAEs 1a-1d, 2, and 3) to be of particular importance, as they left records not only in shallow seas but also in the deep oceans [START_REF] Erbacher | [END_REF]. OAE 2 is dated c.93.5 Ma Cenomanian/Turonian boundary ("Bonarelli Event") [START_REF] Arthur | Stratigraphy, geochemistry, and paleoceanography of organic carbon-rich Cretaceous sequences[END_REF][START_REF] Thurow | The Cenomanian/Turonian boundary event in the Indian Ocean-a key to understand the global picture[END_REF][START_REF] Bralower | Timing and paleoceanography of oceanic dysoxia/anoxia in the late Barremian to early Aptian[END_REF]. OAE 3 is dated c. 89-83.6 Ma (Coniacian-Santonian) [START_REF] Friedrich | Evolution of middle to Late Cretaceous oceans -A 55 m.y. record of Earth's temperature and carbon cycle[END_REF].

The principal objective of Leg 207 was to recover relatively expanded, shallowly buried Cretaceous and Paleogene sediments from Demerara Rise off Suriname, South America, that could be used for paleoceanographic study of the tropical Atlantic [START_REF] Erbacher | [END_REF]. The Demerara Rise is a prominent submarine plateau located at c.5°N off the coasts of Suriname and French Guyana (Fig. A review of all industry wells available for this thesis (Fig. 3.15 and Table 3.1) and the results published by [START_REF] Miranda | Application of Unsupervised Semivariogram Textural Classification of RADARSAT-1 data for the detection of natural oil seeps offshore the Amazon Mouth: proceedings of the Radarsat Adro Symposium[END_REF] allows the identification of locations where hydrocarbons are present, indicating that the petroleum system remains active, mainly on the northwestern compartment (Fig. 3.16 and Table 3.1). In addition, important parameters for model calibration, such as pressure, temperature, vitrinite reflectance, and source rock characteristics e.g. total organic carbon (TOC), were reviewed (Tables 3.1 and3 .2,Fig. 3.15).

However, almost all wells are located on the continental shelf, making it difficult to characterize possible source rock levels in the Amazon Fan depocentres (basinward). Source rock levels were sampled in the Limoeiro Formation (post-rift sedimentary recover). In general, the average of residual TOC values present in the source rocks levels are low, except for 1APS18 and 1APS29 (Fig. 3.15), which may represent possible ranges of original TOC.

In addition, some industry wells have revealed the presence of biogenic gas in the Amazon fan series (Middle-Late Miocene, represented by Tucunaré, Pirarucu and Orange Formations, see Fig. 3.2, table 3.2). Most of them are located in the SE compartment (6 wells), only 1 well is located in the NW compartment (Table 3. 2,Fig. 3.15). [START_REF] Ketzer | Gas seeps and gas hydrates in the Amazon deep-sea fan[END_REF] presented evidence of widespread gas venting from the gas hydrate stability zone (GHSZ) on the upper Amazon deep-sea fan (Fig. 3.17), using multibeam imagery of the seafloor and water column and geochemical analyses of gas hydrates sampled in seafloor vents. Most gas vents are located along seafloor faults that seismic profiles show are related to gravitational collapse of the Amazon fan (Fig. 3.18). The composition of gas hydrates recovered at two locations on the Amazon fan, was dominated by methane of biogenic origin, whereas samples from a vent north of the fan proper included possible thermogenic contributions. Table 3.1: Summary of wells available for the study within the southeastern and northwestern compartments (see Fig. 3.15). Some wells show the presence of gas or oil, indicating that petroleum system is active and may contribute to the generation of overpressures.

Wells Last level archieved Classification

Southeastern Compartment

III.4 Summary

This thesis aims to characterize relationships between sedimentation, thermal structure, hydrocarbon generation and the evolution of pore fluid pressure and gravitydriven tectonism. This chapter reviewed the state-of-art of each of these processes in the Foz do Amazonas basin and the highlights are as follows:

 The Amazon deep-see fan has been subject to gravity tectonics over the last c. 8Ma, resulting in linked extensional and compressional belts rooted on deep shale detachments, divided into SE and NW compartments, the latter more deformed;

 The gravity tectonic system has been proposed to be driven by overpressures due to burial and fast sedimentation (undercompaction) as well as by deep hydrocarbon generation;

 Spatial variation in the character and geometry of the crust beneath the Amazon margin has been identified and has implications for the thermal history of the study area;

 At least three potential source rock levels are present on the Brazilian equatorial margin, based on oceanic anoxic events identified on the Demerara rise (OAE2 and 3) and in the NW compartment of the Foz do Amazonas Basin (OAE1d and OAE2/Albian-Cenomanian);

 There is weak evidence for the presence of thermogenic hydrocarbons (oil and gas) in the SE compartment, from literature and wells;

 There is strong evidence of biogenic gas based on industry wells (at least 6 located on southeastern compartment) and on studies made by [START_REF] Ketzer | Gas seeps and gas hydrates in the Amazon deep-sea fan[END_REF].

IV.1 DATASET

The dataset used in this project (Fig. 4.1) consists of seismic reflection and well data, field tests and direct measurements acquired in the course of hydrocarbon exploration. The 2D seismic data comprises a set of PSTM (Post-Stack Time Migrated) seismic profiles. The lines are tied to a set of 25 wells, for which are available lithological logs, downhole geophysical profiles (sonic, resistivity, density and gamma-ray), as well as data on direct pressure measurements, temperature, total organic carbon (TOC) and vitrinite reflectance (Table 4.1).

Together, this dataset provides the necessary basis to interpret the stratigraphic and structural succession of the Amazon deep-sea fan along sections across the SE and NW structural compartments, as well as downhole calibration data for the proposed numerical modeling. Southeastern Compartment

1-APS-01 x 1-APS-03 1-APS-07 x x x 1-APS-09 x x x 1-APS-12 x x 1-APS-14 x x x 1-APS-20B x 1-APS-24 1-APS-32 x 1-APS-35 x 1-APS-38 1-APS-41 x x x 1-APS-47B 1-APS-51A x x 1-BP-3-APS x x x
Northwestern compartment 1-APS-04

x x 1-APS-05 x x x x x 1-APS-06A x x x 1-APS-11 x x 1-APS-18 x x x 1-APS-22 x 1-APS-23 1-APS-25 x x 1-APS-27A x x x x 1-APS-28 1-APS-29 x x x x 1-APS-31A x x x x 1-APS-36 x x x x 1-APS-44A x x x x 1-APS-45B x x x x 1-BP-2-APS x x x

IV.2 METHODS

In order to understand the origin of pore fluid overpressure and its effects on the evolution of the Amazon Fan, this study proposes an integration of a series of methodologies, involving both 2D seismic interpretation and downhole geophysical data from wells, which are in turn used to support a combination of geomechanical and petroleum system modeling. This integrated approach involves nine main steps, outlined invidually below. b) Review of regional stratigraphic frameworks to constrain interpretations of seismic profiles across the Amazon Fan, based [START_REF] Cruz | Integrated geological and geophysical studies applied to understanding the evolution of the Offshore Amazon Basin[END_REF][START_REF] Cruz | Neogene evolution and demise of the Amapá carbonate platform, Amazon continental margin, Brazil[END_REF], and their application to the interpretation of sections across the SE and NW compartments using available well data (see details in Chapter V, section VI. and in Chapter VI, section VI.). f) Pore pressure estimation and mechanicals parameters from 1D geomechanical models :

The 1D pore pressure estimation, in which rock strength and stress field modeling are also performed to build the geomechanical model, is done using geophysical logs available from offset wells. The input data consist of geophysical profiles (gamma-ray, density, resistivity and compressional transit time), direct formation pressure measurements associated with pore pressure, rock strength and stress (overburden, minimum horizontal stress and maximum horizontal stress). For the studied wells, the proposed geomechanical model allows constraining cohesion, friction and pore fluid pressure at present day, which were used as calibration data for the 2D pore pressure and mechanical models. For more details, see chapter V, section V.3.5. For this study, top basement and the limit of oceanic crust were interpreted from the seismic sections.

The second input data for thermal modeling is the surface temperature.

Paleotemperatures at level 0 m (top of seawater) were based on studies by Friedrich et al According to [START_REF] Cardoso | Caracterizacao do campo térmico atual e evoluçao téermica da Plataforma Continental do Estado do Rio de Janeiro[END_REF] and Gutierrez ad Hamza (2009), there is a relation between Earth surface paleotemperature on and paleobathymetry. This is an empirical formula developed for the calculation of seafloor temperatures, in a specific group of Eastern Brazilian basins (Camamu, Almada, Jequitinhonha, Cumuruxatiba). It is valid for water depths up to 2,950 m. The formula is defined as: Tp𝒆 -𝟎,𝟎𝟎𝟏𝟒𝒁 +7,64x𝟏𝟎 -𝟏𝟒 𝒁 + 𝟏, 𝟗𝟎, where Tp is the temperature at surface=0m, Z as bathymetry (Fig. 4.5). h) 2D basin modeling of pressure and temperature evolution:

Basin modeling involves dynamic forward modeling of multiple geological processes in order to evaluate the pore pressure evolution and temperature history of sedimentary basins [START_REF] Hantschel | Fundamentals of Basin and Petroleum Systems Modeling[END_REF]. We used TemisFlow Petroleum System Software (IFPen; version 2019), which integrates geological, geochemical (source rocks) and mechanical data.

The input data are the geometries and absolute ages of horizons obtained from the 2D restoration performed on sections A-B and C-D using KronosFlow, as well as lithologies, paleobathymetries, boundary conditions, timing of faults and source rocks (Table 4.2). Erbacher et al., 2000) 50m each

Type II Kerogen [START_REF] Mello | Geochemical and molecular studies of the depositional environments of source rocks and their derived oils from the Brazilian marginal basins[END_REF][START_REF] Requejo | Hidrocarbon source facies in the Niger Delta petroleum province as indicated by the molecular characteristics of oil[END_REF][START_REF] Mello | Foz do Amazonas Area: the last frontier for elephant hydrocarbon accumulation in the South Atlantic realm[END_REF] Type To perform the mechanical modeling, I used Optum G2 (2013) to deduce the evolution of pore fluid pressure along the detachment. The software is based on limit analysis theory, using the Mohr-Coulomb criterion and the weak formulation of the force equilibrium [START_REF] Salençon | De l'Elasto-Plasticité au Calcul à la Rupture[END_REF][START_REF] Chandra & Sekharaiah | Continuum Mechanics[END_REF][START_REF] Maillot | Kink-fold onset and development based on the maximum strength theorem[END_REF]. This method allows predicting the location of deformation, from the mechanical parameters (cohesion and internal friction) and the pore fluid pressure [START_REF] Krabbenhoft | OptumG2 Theory[END_REF]. Therefore, if the deformation is known, the approach can be applied to infer effective frictions of faults [START_REF] Kuncoro | Tsunamigenic potential due to frontal rupturing in the Sumatra locked zone[END_REF][START_REF] Cubas | Relationship link between landward vergence in accretionary prism and tsunami generation[END_REF][START_REF] Berthelon | [END_REF]. The pore fluid pressure is then deduced from the effective frictions. In this study, for each age, I seek the effective friction along the faults and the detachment that best reproduce the deformation predicted from the 2D restoration. To do so, I used a safety factor here defined as the ratio between the load necessary to generate a collapse and the gravitational force (g = 9.8m/s²). Collapse is thus predicted when the safety factor is lower than 1.

This chapter consists of a research paper recently published in the journal "Marine and Petroleum Geology". The study addresses the controls on overpressure evolution along section (A-B) across the SE compartment of the Amazon fan. The study applies an innovative methodology based on techniques common in the oil and gas industry and here applied for the first time to a collapsing passive margin depocenter, involving structural restoration of section A-B and different forms of numerical modelling to reconstruct the basin history. The main results are to show that during the growth and collapse of the fan over the last 8 Ma: (1) disequilibrium compaction was the main overpressure mechanism acting on gravity tectonics;

(2) temperature-controlled clay mineral (smectite-to-illite) transformation mainly took place above the detachment and may have made a secondary contribution to pore pressures; and

(3) thermogenic gas was trapped in successions beneath the detachment and did not play a role on pore pressure along the detachment and faults. The paper is based entirely on the work undertaken for my doctoral degree; I wrote the paper, and coordinating a research collaboration with my supervisors and colleagues with both expertise in the relevant methologies and interests in the processes active within the study area.

V.1 INTRODUCTION

Excess pore fluid pressure plays a key role in the stability of continental slopes, through its influence on the strength of sedimentary successions during their deposition. Burial and consolidation generally lead to an increase in sediment strength, but this first-order process can be temporarily reversed by variations in permeability and/or loading leading to the development of overpressures (e.g. [START_REF] Maltman | How sediments become mobilized[END_REF]. Overpressure, defined as any pressure that exceeds the hydrostatic pressure of a column of water or formation brine, leads to variations in sediment strength that may lead to formation failure [START_REF] Dickinson | Geological aspects of abnormal reservoir pressures in Gulf Coast, Louisiana[END_REF].

Overpressured fluids are particularly important in the mobilization of fine-grained sediments and play a fundamental role in gravity-driven processes of sediment failure and deformation on continental slopes [START_REF] Bolton | The importance of overpressure timing and permeability evolution in fine-grained sediments undergoing shear[END_REF], and in the operation of syn-sedimentary faults as well as the stability of wellbores [START_REF] Moos | Comprehensive wellbore stability analysis utilizing Quantitative Risk Assessment[END_REF]. Hence, pore pressure is an issue of major concern for both our understanding of continental margin evolution and hydrocarbon exploitation [START_REF] Morley | Deepwater fold and thrust belt classification, tectonics, structure and hydrocarbon prospectively: A review[END_REF].

Mechanisms generating overpressures in sedimentary successions can be divided into those related to external stresses or those that increase internal fluid content. Stress-related processes include (1) regional tectonics, and (2) disequilibrium compaction (undercompaction), the latter being most common in deltaic depositional environments with high sedimentation rates [START_REF] Osborne | Mechanisms for generating overpressure in sedimentary basins: A reevaluation[END_REF][START_REF] Burrus | Overpressure models for clastic rocks, their relation to hydrocarbon expulsion: a critical reevaluation[END_REF][START_REF] Law | Abnormal pressures in hydrocarbon environments[END_REF][START_REF] Nashaat | Abnormally high fluid pressure and seal impacts on hydrocarbon accumulations in the Nile Delta and North Sinai Basins, Egypt[END_REF][START_REF] Swarbrick | Mechanisms that generate abnormal pressures: an overview[END_REF][START_REF] Grauls | Overpressure: Causal Mechanisms, Conventional and Hydromechanical Approaches[END_REF][START_REF] Zoback | Reservoir Geomechanics[END_REF]. Increased fluid content can be generated by processes that depend on pressure and temperature, either (3) clay diagenesis, such as the smectite-illite transition [START_REF] Hower | Mechanism of burial and metamorphism of argillaceous sediment: mineralogical and chemical evidence[END_REF][START_REF] Law | Geological implications of coal dewartering[END_REF][START_REF] Bruce | Smectite dehydrationits relation to structural development and hydrocarbon accumulation in northern Gulf of Mexico basin[END_REF][START_REF] Hall | Mechanisms of overpressuringan overview[END_REF][START_REF] Burrus | Overpressure models for clastic rocks, their relation to hydrocarbon expulsion: a critical reevaluation[END_REF][START_REF] Swarbrick | Mechanisms that generate abnormal pressures: an overview[END_REF][START_REF] Bowers | Determining an appropriate pore-pressure estimation strategy[END_REF], or (4) hydrocarbon generation. In the latter case, volume changes occur when kerogen transforms to oil and gas (primary migration) and oil cracks to gas (secondary migration), but only gas is able to generate significant overpressures [START_REF] Law | Abnormal pressures in hydrocarbon environments[END_REF][START_REF] Swarbrick | Mechanisms that generate abnormal pressures: an overview[END_REF][START_REF] Grauls | Overpressure: Causal Mechanisms, Conventional and Hydromechanical Approaches[END_REF].

The post-rift successions of many passive continental margins are affected by gravity tectonics, in which a sediment volume moves downslope above one or more zones of decollement containing detachment surfaces. The gravitational potential is directly proportional to the slope of the seafloor and the properties of the detachment surface [START_REF] Vendeville | Glissements gravitaires synsedimentaires et failles normales listriques: modèles expérimentaux[END_REF][START_REF] Cobbold | Some experiments on interacting sedimentation and deformation above salt horizons[END_REF]. Deformation of the succession above the detachment surface typically results in an upslope extensional domain and a downslope compressional domain similar to a fold-and-thrust belt [START_REF] Rowan | Gravity-driven fold belts on passive margins[END_REF]. Gravitational collapse can be related to the presence beneath a sedimentary succession of ductile evaporites (e.g. [START_REF] Morley | Comparison of gravity -driven deformations styles and behavior associated with mobile shales and salt tectonics[END_REF], but on many margins it has been shown to be linked to the presence of overpressured shale levels, such as the Niger Delta [START_REF] Morley | Comparison of gravity -driven deformations styles and behavior associated with mobile shales and salt tectonics[END_REF][START_REF] Hooper | The role of deformation in controlling depositional patterns in the south-central Niger Delta, West Africa[END_REF][START_REF] Morley | Deepwater fold and thrust belt classification, tectonics, structure and hydrocarbon prospectively: A review[END_REF][START_REF] Wu | Niger Delta gravity-driven deformation above the relict Chain and Charcot oceanic fracture zones, Gulf of Guinea: Insights from analogue models[END_REF], the northwestern Gulf of Mexico [START_REF] Rowan | Gravity-driven fold belts on passive margins[END_REF], the Brunei margin [START_REF] Berthelon | [END_REF] or the Amazon Fan [START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF]Oliveira, 2005;[START_REF] Morley | Deepwater fold and thrust belt classification, tectonics, structure and hydrocarbon prospectively: A review[END_REF][START_REF] Perovano | Modelagem fisica experimental de mecanismos de dedormação gravitacional simulando multiplos intervalos superpressurizados : Apliacação à Bacia da Foz do Amazonas[END_REF].

Shale detachments form in response to pore fluid overpressures, which depend on factors including sediment loading, burial and deformation history, and the production and migration of thermogenic fluids [START_REF] Morley | Comparison of gravity -driven deformations styles and behavior associated with mobile shales and salt tectonics[END_REF][START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF][START_REF] Rowan | Gravity-driven fold belts on passive margins[END_REF][START_REF] Mourgues | An experimental investigation of gravity-driven shale tectonics in progradational delta[END_REF][START_REF] Ings | Continental margin shale tectonics: preliminary results from coupled fluid-mechanical models of basin scale delta instability[END_REF][START_REF] Oliveira | Linked extensional-compressional tectonics in gravitational systems in the Equatorial Margin of Brazil[END_REF]. The origin of shale detachments was reviewed by [START_REF] Morley | Deepwater fold and thrust belt classification, tectonics, structure and hydrocarbon prospectively: A review[END_REF]. According to these authors, the main cause of overpressures within weak shale successions is disequilibrium compaction induced by high sedimentation rates and deep burial, enhanced by possible inflationary overpressure mechanisms resulting from diagenesis during burial. These results were mainly based on analogue mechanical models, which assumed the origin and type of overpressured fluids.

While such studies have clearly identified the need to consider overpressure along detachment surfaces (e.g. Shaw and Bilotti, 2005;[START_REF] Yuan | Reappraisal of gravity instability conditions for offshore wedges: consequences for fluid overpressures in the Niger Delta[END_REF][START_REF] Yuan | Control of fluid pressures on the formation of listric normal faults[END_REF][START_REF] Berthelon | [END_REF], none have integrated basin pore pressure evolution or modelled the possible origins of fluid pressures within the basin and along the detachment.

The Amazon River has one of the world's largest discharges of suspended sediment [START_REF] Meade | Suspended sediments of the modern Amazon and Orinoco rivers: Quaternary International[END_REF] and culminates in one of its largest deep-sea fans (Fig. 5.1), comparable in size to those of the Bengal, Indus and Mississippi rivers [START_REF] Rimington | Amazon Fan sands: Implications for provenance[END_REF]. Onshore to offshore studies indicate that the trans-continental Amazon River has developed since the mid-Miocene in response to Andean uplift [START_REF] Hoorn | Andean tectonics as a cause or changing drainage patterns in Miocene northern South America[END_REF][START_REF] Hoorn | Amazonia through time: Andean uplift, climate change, landscape evolution and biodiversity[END_REF][START_REF] Hoorn | The Amazon at sea: Onset and stages of the Amazon River from a marine record, with special reference to Neogene plant turnover in the drainage basin[END_REF], resulting in the progradation of a shelf-slope depocentre up to 10 km thick [START_REF] Silva | Cenozoic tectonics of Amazon Mouth Basin[END_REF][START_REF] Watts | Seismic structure, gravity anomalies and flexure of the Amazon continental margin, NE Brazil[END_REF]. Stratigraphic studies of the Amazon fan based on seismic and well data indicate that sediment flux to the Amazon offshore increased through time to reach a maximum during the Quaternary, i.e. the last 2.6 Ma [START_REF] Figueiredo | Late Miocene onset of the Amazon River and the Amazon deep-sea fan: Evidence from the Foz do Amazonas Basin[END_REF][START_REF] Cruz | Neogene evolution and demise of the Amapá carbonate platform, Amazon continental margin, Brazil[END_REF]. Seismic data also

show that the growth of the fan has been accompanied by its gravitational collapse, expressed as linked extensional-compressional fault systems rooted on detachment surfaces up to hundreds of kilometers in extent at subsurface depths of up to 10km [START_REF] Silva | Cenozoic tectonics of Amazon Mouth Basin[END_REF][START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF]Oliveira, 2005;[START_REF] Perovano | O processo de colapso gravitacional da secao marinha da Bacia da Foz do Amazonas -Margem Equatorial Brasileira[END_REF][START_REF] Perovano | Modelagem fisica experimental de mecanismos de dedormação gravitacional simulando multiplos intervalos superpressurizados : Apliacação à Bacia da Foz do Amazonas[END_REF]. Three main detachment surfaces have been recognized within the Upper Cretaceous to upper Miocene interval, recording successive phases of collapse [START_REF] Silva | Cenozoic tectonics of Amazon Mouth Basin[END_REF][START_REF] Silva | O processode colapso gravitacional e a estruturação da seção marinha da bacia da Foz doAmazonas -Margem equatorial brasileira[END_REF][START_REF] Reis | Two-scale gravitational collapse in the Amazon Fan: a coupled system of gravity tectonics and mass-transport processes[END_REF][START_REF] Reis | Effects of a regional décollement level for gravity tectonics on late Neogene-Quaternary large -scale slope instabilities in the Foz do Amazonas Basin, Brazil[END_REF].

Above the upper detachment level, the on-going collapse of the Amazon fan is expressed as paired belts of extensional and compressional faults across the shelf and upper slope above water depths of 2100 m, including thrust-folds with up to 500 m of seafloor relief [START_REF] Silva | Cenozoic tectonics of Amazon Mouth Basin[END_REF][START_REF] Silva | O processode colapso gravitacional e a estruturação da seção marinha da bacia da Foz doAmazonas -Margem equatorial brasileira[END_REF][START_REF] Reis | Two-scale gravitational collapse in the Amazon Fan: a coupled system of gravity tectonics and mass-transport processes[END_REF][START_REF] Reis | Effects of a regional décollement level for gravity tectonics on late Neogene-Quaternary large -scale slope instabilities in the Foz do Amazonas Basin, Brazil[END_REF]. Detachment surfaces within the Amazon Fan have been inferred to correspond to weak levels composed of overpressured clays (Oliveira, 2005;[START_REF] Zalán | Origin and structural evolution of the Cenozoic Rift System of Southeastern Brasil [Origem e evolução estrutural do Sistema de Rittes Cenozóicos do Sudeste do Brasil[END_REF][START_REF] Perovano | O processo de colapso gravitacional da secao marinha da Bacia da Foz do Amazonas -Margem Equatorial Brasileira[END_REF][START_REF] Perovano | Modelagem fisica experimental de mecanismos de dedormação gravitacional simulando multiplos intervalos superpressurizados : Apliacação à Bacia da Foz do Amazonas[END_REF]. Based on sandbox modeling, [START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF] proposed that the structural patterns of the Amazon fan could be reproduced on the assumption that fluid overpressures existed over a triangular area beneath the detachment. They noted that deep wells had encountered overpressures within the upper several kilometers of the fan, and suggested that numerical modeling (not presented) supported the origin of the fluids from thermogenic gas generation from Cenomanian-Turonian source rocks beneath the detachment. Evidence for an active hydrocarbon system within the Foz do Amazonas Basin is available from results from published wells reports (Brazilian Petroleum and Gas Agency -ANP) that indicate thermogenic hydrocarbons, mainly gas. [START_REF] Miranda | Application of Unsupervised Semivariogram Textural Classification of RADARSAT-1 data for the detection of natural oil seeps offshore the Amazon Mouth: proceedings of the Radarsat Adro Symposium[END_REF] used satellite data to identify natural oil seeps at the sea surface above growth faults and thrust-folds across the shelf break and upper slope, mainly in the Northwest part of the Amazon fan. In the same area, [START_REF] Ketzer | Gas seeps and gas hydrates in the Amazon deep-sea fan[END_REF] presented multibeam sonar evidence of gas flares rising up to 900 m into the water column from seafloor vents in water depths of 650-2600 m, while samples of gas hydrates were recovered at three sites and found to contain methane isotopically consistent with biogenic sources and with possible thermogenic contributions at a site north-western of the fan.

The objective of this paper is to investigate the evolution of pore fluid pressure during growth and gravitational collapse of the Amazon Fan. This is approached through the application of modeling techniques commonly used for petroleum systems, here applied for what we believe to be the first time to a late Cenozoic continental margin depocentre. The study is based on the interpretation of commercial 2D seismic reflection data across the shelf and slope, stratigraphically and geomechanically constrained by well data; these data are used to undertake a 2D kinematic restoration, followed by forward modeling of the evolution of pore pressure and temperature along the section. The results allow us to investigate potential overpressure mechanisms and discuss their implications for the operation of the detachment surface. Our findings provide new constraints on the relative roles of fluid generation versus compaction in driving gravity tectonics within the Amazon fan, and provide insights applicable to other rapidly deposited continental margin depocentres.

V.2 GEOLOGICAL SETTING

The Foz do Amazonas Basin was formed as a consequence of rifting of the South American and African plates from the Aptian to the Cenomanian. The basin is characterized by multiple phases of subsidence and a complex structural style influenced by transtensional tectonism [START_REF] Matos | Tectonic evolution of the equatorial South Atlantic[END_REF]. According to [START_REF] Brandão | Bacia da Foz do Amazonas[END_REF] Formations - [START_REF] Wolff | Microfacies, depositional environments, and diagenesis of the Amapá carbonates (Paleocene-Middle Miocene), Foz do Amazonas basin, offshore NE Brazil[END_REF][START_REF] Gorini | Late Neogene sequence stratigraphic evolution of the Foz do Amazonas Basin, Brazil[END_REF][START_REF] Cruz | Neogene evolution and demise of the Amapá carbonate platform, Amazon continental margin, Brazil[END_REF]. The final phase of post-rift deposition took place from c. 8 Ma, and involved a massive influx of siliciclastic sediments that suppressed carbonate production over most of the shelf [START_REF] Figueiredo | Late Miocene onset of the Amazon River and the Amazon deep-sea fan: Evidence from the Foz do Amazonas Basin[END_REF][START_REF] Gorini | Late Neogene sequence stratigraphic evolution of the Foz do Amazonas Basin, Brazil[END_REF]. This final phase corresponds to the establishment of a transcontinental Amazon River from the late Miocene [START_REF] Hoorn | The Amazon at sea: Onset and stages of the Amazon River from a marine record, with special reference to Neogene plant turnover in the drainage basin[END_REF]) and the growth of the Amazon deep-sea fan as a prograding sediment wedge up to 10 km thick beneath the continental shelf and slope [START_REF] Figueiredo | Late Miocene onset of the Amazon River and the Amazon deep-sea fan: Evidence from the Foz do Amazonas Basin[END_REF][START_REF] Cruz | Neogene evolution and demise of the Amapá carbonate platform, Amazon continental margin, Brazil[END_REF]. The Upper Miocene to Quaternary succession of the Amazon fan contains allochthonous masses corresponding to stacked megaslide complexes sourced from the upper slope, argued to record triggering by gravity tectonics during three main collapse phases in the Late Miocene-Upper Pliocene, the mid-Pliocene-Pleistocene, and from the mid-Pleistocene [START_REF] Reis | Effects of a regional décollement level for gravity tectonics on late Neogene-Quaternary large -scale slope instabilities in the Foz do Amazonas Basin, Brazil[END_REF]. Collapse of the upper Amazon fan has resulted in paired extensional and compressional belts (Fig. 5.1) that affect the sedimentary sequence from Cretaceous to recent, consisting of listric normal faults across the shelf and upper slope, and reverse faults and foldand-thrust structures to depths of 2100 m [START_REF] Silva | Cenozoic tectonics of Amazon Mouth Basin[END_REF][START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF]Oliveira, 2005;Oliveira et al., 2005;[START_REF] Reis | Two-scale gravitational collapse in the Amazon Fan: a coupled system of gravity tectonics and mass-transport processes[END_REF][START_REF] Reis | Effects of a regional décollement level for gravity tectonics on late Neogene-Quaternary large -scale slope instabilities in the Foz do Amazonas Basin, Brazil[END_REF][START_REF] Perovano | Modelagem fisica experimental de mecanismos de dedormação gravitacional simulando multiplos intervalos superpressurizados : Apliacação à Bacia da Foz do Amazonas[END_REF]). Three regional detachment surfaces have been identified from seismic interpretation of fault geometries and linked to shale successions (Fig. 5.2) : lower and intermediate detachments at the top of the Albian (c. 100 Ma) and at Upper Cretaceous (c. 69.7 Ma) [START_REF] Silva | Cenozoic tectonics of Amazon Mouth Basin[END_REF][START_REF] Perovano | O processo de colapso gravitacional da secao marinha da Bacia da Foz do Amazonas -Margem Equatorial Brasileira[END_REF][START_REF] Perovano | Modelagem fisica experimental de mecanismos de dedormação gravitacional simulando multiplos intervalos superpressurizados : Apliacação à Bacia da Foz do Amazonas[END_REF] and an upper detachment level within the upper Miocene at the base of the Amazon fan [START_REF] Silva | O processode colapso gravitacional e a estruturação da seção marinha da bacia da Foz doAmazonas -Margem equatorial brasileira[END_REF][START_REF] Reis | Two-scale gravitational collapse in the Amazon Fan: a coupled system of gravity tectonics and mass-transport processes[END_REF][START_REF] Reis | Effects of a regional décollement level for gravity tectonics on late Neogene-Quaternary large -scale slope instabilities in the Foz do Amazonas Basin, Brazil[END_REF].

The Amazon Fan is characterized by two asymmetric structural compartments, referred to as the Northwest and the Southeast, separated by the Amazon canyon (Fig. 5.1), each associated with thin-skinned extensional and compressional structures rooted on detachment surfaces [START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF]Oliveira et al., 2005). The NW compartment is the larger (>200 km in seaward extent) and thicker (up to 10 km), and is structurally more complex, with evidence of long-lasting compressional deformation along multiple overlapping fronts that have caused up to 500 m of seafloor relief [START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF]Oliveira et al., 2005) . The SE compartment is smaller (<100 km wide) and the compressional system is restricted to a pair of active reverse faults that cause no major seafloor relief. Our study focuses on the SE compartment of the Amazon Fan deformation related to the base Tertiary detachment surface in the SE compartment.

V.3 DATA AND METHODS

The study is based on exploration data obtained from the Brazilian Petroleum and Gas Agency (ANP), comprising a composite multichannel seismic reflection profile calibrated with data from exploration wells (Fig. 5.1). These data were analyzed following the methodological steps shown in Fig. 5.3 and described below, involving an interpretation and kinematic restoration of the seismic profile, 1D geomechanical modeling to obtain a present-day pore pressure profile, 2D basin modeling of pore pressure and temperature evolution and finally 2D mechanical modeling of pore pressure evolution along the detachment.

V.3.2 Composite seismic reflection profile

The profile used for this study crosses the SE structural compartment of the Amazon fan (Fig. 5.1). The 192 km long multichannel seismic profile A-B is a composite of two shorter profiles, each of which have a record length of 12 seconds two-way travel time (TWTT), with vertical resolution of 10-50 m (depending on depth and lithology). The two profiles were subject to standard processing techniques including post-stack time migration (PSTM). The composite profile was interpreted using standard techniques of seismic sequence stratigraphy, and correlation of horizons to regional wells (Fig. 5.1) with reference to the stratigraphic framework detailed by [START_REF] Cruz | Integrated geological and geophysical studies applied to understanding the evolution of the Offshore Amazon Basin[END_REF][START_REF] Cruz | Neogene evolution and demise of the Amapá carbonate platform, Amazon continental margin, Brazil[END_REF].

V.3.3 Time-depth conversion of interpreted section

The stratigraphic and structural interpretation along section A-B was converted from TWTT to depth using a layer cake method [START_REF] Marsden | Layer cake depth conversion in Seismic Interpretation 13 series[END_REF]. The technique takes into account geological layers, bounded by time horizons, and laterally varying interval velocities for each layer [START_REF] Etris | True depth conversion: more than a pretty picture[END_REF]. For the conversion, we used: 1) gridded maps of horizons picked along the composite profile A-B in TWTT and along other profiles of the regional seismic grid (Fig. 5.1), including the seafloor, 2) well markers to identify the ages of the intervals bound by the horizons (see Table 5.2), 3) for each layer younger than 69.7 Ma, a map of interval velocities from industry wells based on sonic logs and check shots (Fig. 5.1, Table 5.1) and 4)

for older/deeper layers, refraction velocities from Watts et al (2009) (Table 5.2).

scheme of [START_REF] Cruz | Integrated geological and geophysical studies applied to understanding the evolution of the Offshore Amazon Basin[END_REF][START_REF] Cruz | Neogene evolution and demise of the Amapá carbonate platform, Amazon continental margin, Brazil[END_REF], introducing intermediate horizons to improve the resolution of geometry and lithofacies distribution. In particular, we added horizons in the intervals between the present and 2 Ma and between 3.7 Ma and 5.5 Ma as isopach layer ages.

Some hypothetical constraints were imposed on the restoration:

 mass balance from one step to another was applied with a 5% tolerance  a paleobathymetric profile was imposed for each restoration age  undeformed surfaces were assumed for the two detachment levels at 100 Ma and 69.7

Ma and for the basement below those levels  left and right limits were preserved through time Thus the restoration was strictly constrained to the borders of the study, by the total volume of material, fault displacements, and the amount of deformation.

V.3.5 Pore pressure estimation from 1D geomechanical models

The present-day pore fluid pressure in well 1-BP-3 -APS was estimated using both the method of [START_REF] Eaton | The Equation for Geopressure Prediction from Well Logs[END_REF] and that of [START_REF] Bowers | Pore Pressure Estimation from Velocity Data; Accounting for Overpressure Mechanisms Besides Undercompaction[END_REF][START_REF] Bowers | Determining an appropriate pore-pressure estimation strategy[END_REF]) (see Appendix A for method descriptions). For both methods, we used the JewelSuite Geomechanics Software (Baker Hughes). Eaton's method considers that excess of pore pressure results from undercompaction, i.e. trapped pore fluid has been squeezed by the weight of more recently deposited sediments [START_REF] Bowers | Pore Pressure Estimation from Velocity Data; Accounting for Overpressure Mechanisms Besides Undercompaction[END_REF]. The pore pressure calculated by Eaton's method was used as a calibration data for the 2D pressure modeling, presented in section 5.2.1. Bowers' method allows the presence and depth of fluid expansion mechanisms to be captured from reductions of velocity and effective stress. Quantification of unloading requires downhole information (e.g. mineralogy, gas content, caving analyses) that were not available for this study. Nonetheless, it was possible to use the results from well 1-BP-3 -APS to identify the presence of unloading mechanisms (see section 5.3.3).

V.3.6 2D basin modeling of pore pressure and temperature

Basin modeling involves dynamic forward modeling of multiple geological processes in order to evaluate the pore pressure evolution and temperature history of sedimentary basins [START_REF] Hantschel | Fundamentals of Basin and Petroleum Systems Modeling[END_REF]. We used the TemisFlow Petroleum System Software (IFPen; version 2019), which integrates geological, geochemical and mechanical data. The input data are the geometries and absolute ages of horizons obtained from the 2D restoration performed by KronosFlow, as well as lithologies, paleobathymetries, boundary conditions, and timing of faults.

V.3.7 2D mechanical modeling

We used Optum G2 (2013) to infer the evolution of pore fluid pressure along the detachment. The software relies on limit analysis theory, which is based on the Mohr-Coulomb criterion and the weak formulation of the force equilibrium (Salençon, 2002[START_REF] Chandra & Sekharaiah | Continuum Mechanics[END_REF][START_REF] Maillot | Kink-fold onset and development based on the maximum strength theorem[END_REF], and allows prediction of the deformation of a structure submitted to a load from the distribution of its frictional properties [START_REF] Krabbenhoft | OptumG2 Theory[END_REF]. Hence, if the deformation is known, the approach can be applied to infer effective frictions of faults [START_REF] Kuncoro | Tsunamigenic potential due to frontal rupturing in the Sumatra locked zone[END_REF][START_REF] Cubas | Relationship link between landward vergence in accretionary prism and tsunami generation[END_REF][START_REF] Berthelon | [END_REF]. The pore fluid pressure is then deduced from the effective frictions. In this study, for each age, we seek the effective friction along the faults and the detachment to reproduce the deformation predicted from the 2D restoration. To do so, we used a safety factor here defined as the ratio between the load necessary to generate a collapse and the gravitational force (𝑔 = 9.8𝑚/𝑠²). Collapse is thus predicted when the safety factor is lower than 1.

V.4 MODEL SET-UP AND CALIBRATION FROM WELL DATA

In this section, we address the setting-up and calibration of the models used to analyze section A-B based on information obtained from wells (see Fig. 5.3). This includes the interpretation of the composite seismic profile (Fig. 5.4), the lithofacies distribution (Fig. 5.5)

and the conversion to depth (table 5.2, Fig. 5.5), as well as the estimation of 1D pore pressures to build 2D basin and mechanical models ). The latter modeling steps were calibrated using data from two wells: 1-BP-3-APS and 1-APS-31A-AP (Fig. 5.1).

V.4.1 2D seismic interpretation (TWTT)

The stratigraphic and structural interpretation of composite seismic profile A-B (Fig. 5.1) is presented in Fig. 5.4. We identify a total of 15 horizons, together with the seafloor horizon defining 16 stratigraphic intervals younger than 120 Ma; the stratigraphic units are offset by extensional and compressional faults that root on two main detachment surfaces (Fig. 5.4).

The interpreted succession is simplified to accommodate limitations in well control, seismic data quality and structural complexity (Fig. 5.4).

The chronostratigraphic framework in Fig. 5.4 is based in part on that presented for the Offshore Amazon Basin by [START_REF] Cruz | Integrated geological and geophysical studies applied to understanding the evolution of the Offshore Amazon Basin[END_REF][START_REF] Cruz | Neogene evolution and demise of the Amapá carbonate platform, Amazon continental margin, Brazil[END_REF], constrained by biostratigraphic data from 40 wells located on the continental shelf, upper fan and Ceara Rise (see Fig. 5.1). The framework includes nine seismic horizons, dated by [START_REF] Cruz | Neogene evolution and demise of the Amapá carbonate platform, Amazon continental margin, Brazil[END_REF] to 2Ma, 3.7Ma, 5.5Ma, 8Ma, 24Ma, 28Ma, 33.5Ma, 38Ma and 66 Ma. To extend this framework downward to older horizons observed along section A-B (Fig. 5.4), we used data from [START_REF] Erbacher | [END_REF] on the age of Oceanic Anoxic Events (OAEs) in the Demerara Rise, and from Boulila et al (2020) on their correlation to well data in the adjacent Foz do Amazonas Basin. [START_REF] Erbacher | [END_REF] used results from IODP Leg 207, which drilled five sites and recovered multiple samples of OAE 2, dated at c. 93.5 Ma, the Cenomanian/Turonian boundary ("Bonarelli Event") [START_REF] Arthur | Stratigraphy, geochemistry, and paleoceanography of organic carbon-rich Cretaceous sequences[END_REF][START_REF] Thurow | The Cenomanian/Turonian boundary event in the Indian Ocean-a key to understand the global picture[END_REF][START_REF] Bralower | Timing and paleoceanography of oceanic dysoxia/anoxia in the late Barremian to early Aptian[END_REF], and of OAE 3, dated at c. 89-83.6 Ma (Conician-Santonian) [START_REF] Friedrich | Evolution of middle to Late Cretaceous oceans -A 55 m.y. record of Earth's temperature and carbon cycle[END_REF]. We applied these ages to results from Boulila et detachment to correspond to the Upper Albian at c. 100 Ma [START_REF] Perovano | Modelagem fisica experimental de mecanismos de dedormação gravitacional simulando multiplos intervalos superpressurizados : Apliacação à Bacia da Foz do Amazonas[END_REF]. We also added the top of the syn-rift succession (Cacipore Formation c. 102 Ma) and a basal horizon at c. 120 Ma [START_REF] Brandão | Bacia da Foz do Amazonas[END_REF][START_REF] Figueiredo | Bacia da Foz do Amazonas[END_REF].

The structural interpretation In ). These structures affect the overlying layers up to the seabed without reaching it, although the seabed is slightly deformed above Cp2 (Fig. 5.4). We note that the seaward extent of compressional features along the profile coincides with that of the lower detachment surface (c. 100Ma, dark blue horizon), which may therefore have played a role in deformation prior to Amazon Fan deposition. Hence, this surface was not taken into account in our study.

V.4.2 2D lithofacies distribution in depth

Section A-B extends from the outer shelf to the upper fan across water depths of c.75-2150 m. A simplified lithofacies model was created, honoring information regarding the shelf and upper slope domains available from well 1-BP-3-APS (Fig. 5.1) and from the literature.

Three facies groups were created to represent these domains from 66 Ma to present (Fig. 5.5):

(1) outer shelf, initially dominated by a carbonate platform later mixed with sandstones and mudstones;

(2) transitional domain, dominated by marl, with minor mudstone and carbonate;

and

(3) upper fan, dominated by mudstones (over 70%), with carbonates and silt fractions (Fig.

5.5).

V.4.3 2D Basin modeling set-up

V.4.3.1 Porosity-permeability relation

The porosity-permeability relation is essential to calibrate 2D pressure modeling.

Porosity corresponds to the volume of rock containing fluids. The total porosity includes all space occupied by fluids, including water in clays, which is part of the clay structure and cannot be produced. The total porosity is usually denoted by 𝜙𝑇 and defined by:

𝜙 𝑇 = 𝑉𝑝 𝑉𝑡 (Eq. 1)
with 𝑉𝑝 the volume of the rock occupied by fluids and 𝑉𝑡 the total volume of the rock.

Two sets of downhole data were used to build a porosity-depth profile (Fig. 5.6). From seafloor to c. 350 m TVD we used porosity data from ODP site 931, derived from neutron logs and core samples (Flood et al. 1995). From c. 998-5600m TVD we used data from well 1-BP-3-APS, based on petrophysical analysis of acoustic data.

2D pore pressure modeling requires definition of mechanical properties to simulate fluid flow and pore pressure. The Kozeny-Carman equation determines the fluid flow rate that crosses a porous material media, and establishes a relation between porosity and permeability [START_REF] Kozeny | Ueber kapillare Leitung des Wassers im Boden[END_REF][START_REF] Carman | Fluid flow through granular beds[END_REF][START_REF] Carman | Flow of gases through porous media[END_REF]. This law is commonly applied to sandstones and carbonates, but may underestimate permeability when applied to shales and mudstones [START_REF] Chenevert | Permeability and effective pore pressure of shales: Society of Petroleum Enginners[END_REF]. According to the Kozeny-Carman law, at 3-5 km depth, shales/mudstones with normal compaction are expected to reach 5-9% of porosity for permeabilities of 10 -4 -10 -5 mD. No permeability data are available from the Amazon fan, but at such depths in well 1-BP-3-APS porosities, of 30-40% are observed, consistent with the presence of an undercompacted and overpressured interval. A Kozeny-Carman relation would imply, permeabilities ranging from 10 -2 -10 -3 mD, making it impossible to reproduce present-day pore pressures in the well. Therefore, we established a user-defined porosity-permeability relation for our 2D pressure model, based on the observed porosities and pore pressure of 1-BP-3-APS well (Fig. 5.7). The resulting range of permeability values (10 -1 -10 -7 mD) is consistent with those in the literature for shales, which may reach values of up 10 -7 mD [START_REF] Dickey | Comment on Bayou Field[END_REF][START_REF] Mesri | Mechanisms controlling the permeability of clays[END_REF].

V.4.3.2 Boundary conditions

As a first step, we built a crustal model for the study area to extract heat flow at the base of the basin for all ages (Fig. 5.8). To do so, a stretching factor was calculated [START_REF] Mckenzie | Some remarks on the development of sedimentary basins[END_REF], considering (1) a Moho profile inspired by [START_REF] Watts | Seismic structure, gravity anomalies and flexure of the Amazon continental margin, NE Brazil[END_REF], ( 2) the top basement surface interpreted from seismic line A-B in depth and (3) an initial crustal thickness of 32 km, divided into upper (20 km) and lower crust (12 km). As a second step, we defined paleobathymetries, based on off-lap breaks observed along the 2D seismic line and literature sources [START_REF] Brandão | Bacia da Foz do Amazonas[END_REF][START_REF] Figueiredo | Bacia da Foz do Amazonas[END_REF][START_REF] Cruz | Integrated geological and geophysical studies applied to understanding the evolution of the Offshore Amazon Basin[END_REF]. The third step consisted of defining paleosurface temperatures. For the paleosurface temperature at sea level (0 m), we applied a function derived from paleobathymetry developed by [START_REF] Cardoso | Caracterizacao do campo térmico atual e evoluçao téermica da Plataforma Continental do Estado do Rio de Janeiro[END_REF] and [START_REF] Gutierrez | Campo Termico da Crosta na Regiao Sedimentar Bahia Sul e na Area Pre-Cambriana adjacente: Reavaliaçao com base em dados corrigidos da Margem Continental[END_REF]. Paleosurface temperatures at depth were based on results

from [START_REF] Friedrich | Evolution of middle to Late Cretaceous oceans -A 55 m.y. record of Earth's temperature and carbon cycle[END_REF] and [START_REF] Tremblin | Eocene-Oligocene sea surface temperature and pCO2 estimates[END_REF]. [START_REF] Friedrich | Evolution of middle to Late Cretaceous oceans -A 55 m.y. record of Earth's temperature and carbon cycle[END_REF] Finally, to model 2D hydrocarbon generation, we assumed three potential source rock levels corresponding to Ocean Anoxic Events (Fig. 2): (1) OAE1d, dated c. 101.1Ma, (2) OAE2, dated c. 93.6Ma and (3) OAE3, dated c. 83.6Ma. The kerogen is type II (marine carbonate environment [START_REF] Mello | Geochemical and molecular studies of the depositional environments of source rocks and their derived oils from the Brazilian marginal basins[END_REF][START_REF] Requejo | Hidrocarbon source facies in the Niger Delta petroleum province as indicated by the molecular characteristics of oil[END_REF][START_REF] Mello | Foz do Amazonas Area: the last frontier for elephant hydrocarbon accumulation in the South Atlantic realm[END_REF]. The kinetics was based on PT-beds in Aptian gas-prone source rocks in Brazil [START_REF] Penteado | Study of biodegradation process along the Carnaubais trend, Potiguar basin (Brazil) -Part 1[END_REF]. The original total organic carbon (TOC) was defined as 8% for all levels based on [START_REF] Erbacher | [END_REF], with an average thickness of 50 meters. Residual TOC values range from 1-5%.

V.4.4 2D mechanical model set-up

The model requires values of cohesion, and pore fluid pressure for each layer.

Internal pore fluid pressures were obtained from the 2D basin model. Values for friction and cohesion were deduced from data from wells 1-BP-3-APS and 1-APS-31A-AP. The calculation of internal friction coefficient, cohesion and pore pressure are detailed in Appendix B.

V.5 MODELING RESULTS ALONG PROFILE A-B

In this section, we present the results of the different types of modeling applied to section A-B over the last 8 Ma, i.e. from the onset of Amazon Fan siliciclastic deposition to the present day. The 2D models were performed from 120Ma (base of the model) to the presentday, except for 2D mechanical model which was performed from 8Ma to the present-day. We first present the structural restoration of section A-B, followed by results for the spatial and temporal evolution of temperature and pore pressure within the basin, and their respective implications for fluid and gas generation and fault movements.

V.5.1 2D restoration of SE Amazon Fan growth and deformation

The 2D restoration of the Amazon fan sequential record obtained using KronosFlow are presented in Figure 5.9. Extensional activity (faults F1 and F2, shown in red) and basinward propagation of compressional deformation both start at 8 Ma and persist until 5.5 Ma (Fig. 5.9).

During this phase, siliciclastic sedimentation predominates on the shelf [START_REF] Cruz | Neogene evolution and demise of the Amapá carbonate platform, Amazon continental margin, Brazil[END_REF].

From 5.5 Ma to 3.7 Ma, no deformation takes place (Fig. 5.9). During this phase, sediment supply to the SE compartment decreases in favor of the NW one, resulting in the progressive burial of carbonates on the inner NW shelf [START_REF] Cruz | Neogene evolution and demise of the Amapá carbonate platform, Amazon continental margin, Brazil[END_REF]. The total quantity of hydrocarbon (oil and gas) generated was trapped on thickness against faults, where the gas fraction might act as an overpressure generating mechanism.

V.5.2.1 Present-day pore pressure

Pore pressures in well 1-BP-3-APS calculated using Eaton's method are hydrostatic down to 3.2km depth, followed by a pore pressure ramp significantly increasing at 3.8 km depth (Fig. 5.12). The maximum pore pressure reaches 82 MPa at approximately 5.3 km depth (c.

38 Ma).

To calibrate our model, we extracted a 1D pore pressure profile at the same position of 1-BP-3-APS well from the 2D pressure modeling at the present-day and compared it with the results from Eaton's method (Fig. 5.11a). The two profiles are comparable, with a pore overpressure ramp from 3.2 to 5.5 km depth.

Figure 5.12: Pressure-depth plots for well 1-BP-3-APS well obtained using the methods of [START_REF] Bowers | Pore Pressure Estimation from Velocity Data; Accounting for Overpressure Mechanisms Besides Undercompaction[END_REF][START_REF] Bowers | Determining an appropriate pore-pressure estimation strategy[END_REF] and [START_REF] Eaton | The Equation for Geopressure Prediction from Well Logs[END_REF]. The two methods result in comparable trends of pore pressure and indicate the existence of present-day fluid expansion mechanisms from at least 3200-5500 m.

V.5.3 Temperature-driven fluid expansion mechanisms

Results from the 2D temperature modeling were used to analyze two temperaturedependent processes that represent potential overpressure mechanisms, smectite-illite transition and hydrocarbon gas generation. The predicted distribution of fluid expansion mechanisms are checked against present-day pore pressure conditions in well 1-BP-3-APS using Bowers' method.

V.5.3.1 Clay mineral transformation

The diagenesis of clay minerals during lithification or post-depositional compaction of sediments may generate overpressure due to the release of bound water, notably during the smectite-illite transition. In mud-dominated basins, a gradual and systematic change from smectite to illite is commonly observed with increasing depth. [START_REF] Bruce | Smectite dehydrationits relation to structural development and hydrocarbon accumulation in northern Gulf of Mexico basin[END_REF] observed a correspondence with high values of overpressure. [START_REF] Hower | Mechanism of burial and metamorphism of argillaceous sediment: mineralogical and chemical evidence[END_REF] proposed that the smectite-to-illite transition occurs over a temperature range of 100-175°C, whereas Bruce

V.5.3.2 Hydrocarbon gas generation

The modeling results along section A-B indicate that the three OAE source rock levels used as input are efficient in terms of the total amount of hydrocarbons expelled (Fig. 5.10).

Figure 5.13 presents an average plot of the mass of gas expelled from the source rocks. The expelled gas window for OAE1d lies between c. 100-80 Ma and 33.5 Ma, for OAE2 between c. 80-66 Ma and 24 Ma, and for OAE3 between c. 66 Ma and 24 Ma (Fig. 5.13). We find that almost all gas generated form the three OAE source rock levels was expelled long before the onset of deposition of the Amazon Fan at around 8 Ma. Fig. 5.10 shows that the hydrocarbons generated migrated preferably along faults to be trapped in older successions at depths of hundreds of meters to a few kilometers beneath the detachment below the detachment. The fraction of gas that migrated and accumulated in these locations, including any gas generated by secondary cracking, represents a potential overpressure generating mechanism. However, from 8 Ma to the present-day, there is no evidence of hydrocarbon migration upward to or along the detachment (Fig. 5.10). Instead, the sequence beneath the detachment (decollement) has acted as a natural barrier (or seal) to upward fluid flow. Hydrocarbons are able to migrate upward through successions beneath the detachment (up-dip migration) but would only be able to migrate into overlying successions if there are structures that breach the detachment.

V.5.3.3 Present-day fluid expansion (unloading)

Present-day pore pressures in well 1-BP-3-APS calculated using Eaton's and Bower's methods are compared in Figure 5.12. Both methods show a similar behavior at shallow depth and a ramp at 3.2 km depth. However, at greater depths, Bower's method yields higher pore pressures, reaching 100 MPa at the base of the well. This is because the method of [START_REF] Bowers | Pore Pressure Estimation from Velocity Data; Accounting for Overpressure Mechanisms Besides Undercompaction[END_REF][START_REF] Bowers | Determining an appropriate pore-pressure estimation strategy[END_REF] considers unloading as a secondary pressure mechanism. The method considers that an elastic deformation of rocks under high pore pressure is a reversible process and can be identified in the cross-plot of density versus compressional velocity (Fig. 5.14). The plot shows elastic reloading (trend line in violet) when the velocity drops from 3300 m/s to 1800 m/s without comparable changes in bulk density (c. 2.6 g/cm 3 ), at 3.2-5.5km depth. The results thus provide evidence of unloading in this depth range, consistent with a fluid expansion mechanism.

V.5.4 Evolution of deformation along the detachment and faults

To constrain the pore pressure evolution along the detachment and faults in section A-B, we used the OptumG2 software to run a series of trial and error tests, and selected the effective friction along the detachment that best reproduced the observed deformation. When deformation was observed, we searched for the properties producing a factor of safety lower than 1. When no deformation was observed, we checked that the predicted parameters led to a factor of safety larger than 1.

The effective frictions were then converted in pore pressure ratio (𝜆) assuming a fixed friction for the faults and detachment of 0.4, typical of clays [START_REF] Saffer | Comparison of smectite-and illite-rich gouge frictional properties: application to the updip limit of the seismogenic zone along subduction megathrusts[END_REF]. We checked the effect of frictional uncertainties for the sedimentary layers. Slightly different frictions or cohesions led to slightly different pore pressure ratios, with very limited impact on the safety factor, and without producing any significant change on the general trend of the pore pressure evolution.

The results from 8 Ma to present are synthesized in Fig. 5.15, while the vertical and horizontal movements for each model step are given in Figs 5.16 and 5.17. At 8 Ma, lithostatic pore pressure is required along the detachment and all faults in order to generate extension across F1 and F2 and compression along C1, C2 and C3 (Figs 5.15 and 5.16). From 5.5-3.7 Ma, deformation is not possible, even considering a lithostatic pore pressure along the detachment and faults (Fig. 5.16). A stable structure during this period is consistent with our seismic interpretation and restoration, which shows no evidence of deformation. At 1.6 Ma, deformation resumes. In order to lock F1 and F2, a decrease of pore fluid pressure is required. In order to activate F3 and F4, the faults need to reach lithostatic pore pressure, with higher values along F4 than F3. Therefore, a migration of pore fluid pressure is required, from F1-F2 to F3, and then to F4 is (Figs 5.15 and 5.16). The same scenario holds at 1.2 Ma. At 1.6 and 1.2 Ma, we could not find properties allowing for the activation of C2 and C3 (Figs 515 and 5.16). At 0.8 Ma, to abandon F3, a decrease of its pore fluid pressure is necessary, as for F1 and F2 (Figs 5.15 and 5.16). To propagate deformation from C1 to C2-C3, we need to decrease the pore fluid pressure along C1, and increase the pore fluid pressure upward along C2 and C3.

On seismic profile A-B, faults F4 and F5 reach the seabed and form stepped scarps, attesting that the faults are or have recently been active. The compressional domain is also active, since overlying layers are deformed up to the seafloor. However, we could not find a set of properties allowing for the coeval activation of F4 and F5, neither at 0.4 Ma nor at the present-day (Fig. 5.16). We thus considered two alternatives, assuming that deformation may have jumped from F4 to F5 during the last 0.4 Ma (Figs 5.15 and 5.17). To activate F4, the pore fluid pressure along the fault needs to reach lithostatic values. The same applies for F5, however its activation requires a strong decrease of pore fluid pressure along F4. In both cases, a lithostatic pore fluid pressure along C1, C2 and C3 led only to the activation of C1.

To summarize, we are able to reproduce the deformation required by the structural restoration. To do so, a lithostatic pore pressure along the detachment is required at all ages.

Moreover, to simulate the migration of the fault activity from F1 to F5, a migration of the pore fluid pressure is required.

V.6. DISCUSSION

The results presented above provide information on the evolution of pressure and temperature within the SE compartment of the Amazon Fan during the accumulation and deformation of up to 6 km of sediment over the last 8 Ma, and their consequences for fluid generation and sediment deformation. The results are synthesized in Fig. 5.18 as four main stages, in order to discuss the controls on the growth and gravitational collapse of this passive margin depocentre. V.6.1 Pore pressure evolution in the southeast Amazon Fan At 8 Ma, the onset of Amazon fan growth was marked by an increased influx of siliciclastic sediments (Fig. 18a). In response, the SE compartment of the Amazon fan underwent a phase of gravitational collapse, leading to the activation of two listric normal faults (F1, F2) and generating compression at the thrust front. This deformation was generated by stress field variation, caused both by the rapid deposition of sediments and by changes in effective stress due to increasing pore pressures along the detachment. Our basin modeling results indicate that overpressure starts to increase through time (Fig. 5.12, 5.18). Modelled temperatures indicate that the lower part of the smectite-illite transition window intersected the detachment below the continental shelf (landward of F1) and to seaward beneath the compressional domain (Fig. 5.18a), and so could have contributed to increased overpressure in these areas. The modeling results further indicate that the three OAE source rock levels identified beneath the central part of the fan reached maturity by 8 Ma and almost generated thermogenic gas were expulsed and migrated over basin (Fig. 5.14). Gas trapped in successions at depths of hundreds to a few thousand meters beneath the detachment might account for overpressure at these depths, but the values are not significant (< 5MPa). At this stage of fan growth, the slope break is located landward of the first active normal fault F1.

From 5.5-3.7 Ma (Fig. 5.18b), sediment supply from the Amazon River was deviated to the larger NW compartment, as testified by progressive burial of the Amapá carbonates on the shelf [START_REF] Cruz | Neogene evolution and demise of the Amapá carbonate platform, Amazon continental margin, Brazil[END_REF]. Decreased sediment supply to the SE compartment can account for the lack of deformation observed along section A-B, consistent with the results of 2D mechanical modeling (Figs 5.15,5.16). Even considering a high pore pressure along the detachment, collapse structures within the Amazon fan are inactive at this stage. Nonetheless, overpressure continues to build within the fan (grey on Fig. 5.18b). This may reflect fluid contributions from smectite-illite transition, the temperature window for which remains similar to that at 8 Ma (Fig. 5.18a). Gas migration is no longer significant at this stage.

According to [START_REF] Cruz | Integrated geological and geophysical studies applied to understanding the evolution of the Offshore Amazon Basin[END_REF], during the Quaternary (the last c. 2.6 Ma), increasing sediment supply across the NW shelf drove gravity-driven syn-sedimentary deformation in response to higher sedimentation rates. This is consistent with our results along section A-B, where increased sediment flux from c. 2 Ma is recorded by a basinward shift of the slope break (Fig. 5.18c). Gravitational deformation resumed and intensified from 1. 5.17,5.18c). Faults F1 and F2 were abandoned for F3 and F4, which requires a migration of high pore pressure along the detachment. At the same time, overpressure on the overlying layers increases, and starts to be compartmentalized between normal faults F2-F4 (grey on Fig. 5.18c). The smectite-to-illite transformation window remains active along the sequences between detachment and the Amazon Fan series.

From 0.8 Ma to the present, continued sediment supply leads to further progradation of the slope break, now located between F3 and F4 (Fig. 5.18d). Normal faults F4 and F5 reach the seabed and form stepped scarps, showing that these faults remain active together with the frontal thrusts. To activate F4, pore fluid pressure along the fault needs to increase substantially. However, to activate F5, a strong decrease of the pore fluid pressure along F4 is required, attesting to a seaward migration of pore pressure. Within the sedimentary pile, overpressure continues to increase and remains compartmentalized by active faults. In terms of overpressure mechanisms, smectite-to-illite transition continues to represent an important secondary pore pressure mechanism within the fan.

Our modeling results suggest extremely high pore pressure along the detachment and the faults for all steps modeled of Amazon Fan. However, we cannot rule out that they are biased by the triggering of viscous deformation once a certain pore pressure threshold is reached [START_REF] Ings | Continental margin shale tectonics: preliminary results from coupled fluid-mechanical models of basin scale delta instability[END_REF]. In any case, the pore pressure along the detachment remains close to lithostatic and taking into account viscous deformation will not change the spatial and temporal evolution of the pore pressure in the Amazon fan.

V.6.2 Overpressure mechanisms in the Amazon Fan

Our results allow us to consider the roles of three potential pore pressure generation mechanisms within the SE compartment of the Amazon Fan: clay mineral transformation (smectite-to-illite); hydrocarbon generation from gas-prone source rocks ; and disequilibrium compaction (undercompaction) controlled by sedimentation rates.

From 8 Ma to the present-day, smectite-to-illite transition is found to take place within the sedimentary succession of the Amazon Fan mainly above the detachment (c. 69.7Ma) (Fig.

5.18

). This result is consistent with an analysis of present-day pore pressure in well 1-BP-3-APS using the method of Bowers, which attests to the presence of a fluid expansion mechanism below depths of 3500m. The lower part of the transition window intersects the detachment in certain places (beneath the continental shelf and to seaward beneath the compressional domain), but not along the most active part of the detachment (between F1 and compressional domain), However, we cannot rule out that the temperature transition may have been located at the detachment level before 8 Ma, and that fluids generated were trapped near the detachment. Therefore, our results provide evidence for the smectite-to-illite transition at best being a possible early contributor to the generation of overpressures along the detachment.

The maturation of OAE source rocks and migration of thermogenic gas took place at least 16 Ma and up to 92 Ma before the onset of deposition of the Amazon Fan at 8 Ma (Fig.

5.14).

In contrast to a previous study [START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF], a recent gas expulsion from Cenomanian-Turonian source rocks (equivalent to OAE2) at c. 6 Ma does not seem possible.

Hydrocarbons migrated upward to be trapped in successions that in places lie at depths of hundreds meters beneath the detachment (Fig. 5.12, 5.18). The accumulation of thermogenic gas beneath the detachment might have acted as an overpressure generating mechanism before the arrival of Amazon Fan (c. 8Ma), but there is no evidence of up-dip migration to the level of the detachment after that time. It appears that hydrocarbons would only be able to are sufficient to drive in situ methanogenesis and lead to gas hydrate formation [START_REF] Arning | The quantification of methane formation in Amazon Fan sediments (ODP Leg 155, Site 938) by hydrogeochemical modeling solid -Aqueous solution -Gas interactions[END_REF]. On the NW fan, gas hydrates sampled from seafloor vents within the upper slope compressional belt are composed of methane isotopically consistent with a biogenic origin [START_REF] Ketzer | Gas seeps and gas hydrates in the Amazon deep-sea fan[END_REF]. In the subsurface, biogenic gas is reported on geochemical analysis of some wells in SE compartment (1APS-09-AP, 1-APS-14-AP, 1-APS-41-AP, 1-APS-51A-AP).

Of the smaller number of wells drilled in the NW compartment, only one (1-APS-36-AP) shows evidence of biogenic gas. Results from well 1-BP-3-APS using the method of Bowers indicates fluid expansion mechanism at the present-day. If this fluid is gas, its origin might be biogenic.

In any case, the present-day overpressure conditions show that the SE compartment of the Amazon Fan is not at equilibrium and gravity driven deformation could occur at any time.

V.6.3 Implications for other deep-sea fans

The gravitational collapse of sedimentary successions above salt or shale detachments to generate syn-sedimentary tectonic belts is increasingly recognized as an aspect of continental margin evolution [START_REF] Rowan | Gravity-driven fold belts on passive margins[END_REF][START_REF] Morley | Deepwater fold and thrust belt classification, tectonics, structure and hydrocarbon prospectively: A review[END_REF]. Most of the world's major rivers culminate in deep-sea fans of kilometric thickness that contain evidence of collapse systems developed above overpressured shale detachments [START_REF] Morley | Deepwater fold and thrust belt classification, tectonics, structure and hydrocarbon prospectively: A review[END_REF]. The dynamics of these late Cenozoic depocentres have not previously been investigated using a modeling methodology as developed for our study, and we suggest that results from the Amazon fan provide possible insights into the functioning of other collapse systems.

The rapid deposition of deep-sea fans is thought to favor the development of overpressures within shale detachments by disequilibrium compaction, which may be supplemented by inflationary overpressures due to smectite-illite transition or hydrocarbon generation [START_REF] Swarbrick | Mechanisms that generate abnormal pressures: an overview[END_REF][START_REF] Morley | Deepwater fold and thrust belt classification, tectonics, structure and hydrocarbon prospectively: A review[END_REF]. Our modeling results from the Amazon fan provide strong support for the role of disequilibrium compaction, both in generating overpressures within the detachment zone and in driving fault movements over time.

Moreover, our results suggest that changes in sediment supply over time, and in space during the seaward progradation of the shelf-slope wedge, make the gravitational collapse of deepsea fans a fundamentally episodic process, at the scale of the activity of the extensionalcompressional system.

Our modeling results also support the role of clay mineral transformations in the development of overpressure within the thickening depocentre. However, the smectite-illite transition window remains mainly above the detachment during the growth of the fan, notably beneath all but the outermost part of the compressional system (Fig. 5.18), indicating it has not significantly influenced the dynamics of collapse. The position of the smectite-illite transition window is a consequence of rapid burial of the detachment, despite relatively low temperature gradients within the fan (Fig. 5.13). In general, geothermal gradients within deepsea fans are relatively low due to thermal blanketing by rapid deposition of cold sediments (e.g. [START_REF] Wangen | The blanketing effect in sedimentary basins[END_REF], but also depend on the underlying crustal heat flow. Thus slower sedimentation rates will raise thermal gradients (and the smectite-illite transition window), as will higher crustal heat flow. We infer that the possible role of clay mineral transformations on overpressures at detachment level will depend on the interplay of sedimentation rate with regional geothermal gradients. It would be of interest to test the role of clay mineral transformations within other deep-sea fans using our modeling technique.

Our results from the Amazon fan also indicate that gas does not influence overpressures at or above the level of the detachment, as thermogenically generated hydrocarbons remain trapped in the successions beneath the detachment (Fig. 5.18). This is broadly consistent with an understanding of shale detachments as shear zones that act as barriers to the vertical migration of fluids [START_REF] Brown | Heterogeneous hydrofracture development and accretionary fault dynamics[END_REF]. Nonetheless, fluids may be able to cross such barriers during tectonic movements, e.g. due to the formation of duplexes that incorporate underlying material [START_REF] Moore | Fluids in accretionary prisms[END_REF]. Many deep-sea fans overlie deeper hydrocarbon systems and include evidence of thermogenic gas venting at seafloor [START_REF] Loncke | Mud volcanoes, gas chimneys, pockmarks and mounds in the Nile deep-sea fan (Estern Mediterranean): geophysical evidences[END_REF][START_REF] Ketzer | Gas seeps and gas hydrates in the Amazon deep-sea fan[END_REF], suggesting either that thermogenesis takes place above the detachment or that fault movements have breached the detachment. In the SE compartment of the Amazon fan, our results suggest it would be of interest to seek evidence of deep hydrocarbon leakage from seafloor vents above the duplexes at the leading edge of the compressional system (Fig. 5.18d).

V.7 CONCLUSIONS

In this study, we have investigated the main mechanisms of overpressure that contributed to the gravitational collapse of the Amazon fan during its growth over the last 8 Ma.

The study is based on the structural restoration of a seismic profile across the Southwestern Amazon Fan, used to constrain numerical modeling of the evolution of pore pressure and temperature.

The modeling results indicate that disequilibrium compaction has been the predominant overpressure generation mechanism along the Amazon Fan detachment. Phases of gravity spreading and deformation at 8 Ma and after 2 Ma correlate to increase sediment flux to the deep-sea fan, whereas decreased flux to the SW Amazon fan from 5.5-3.7 Ma was accompanied by cessation of gravity spreading/deformation. This observation, coupled with our numerical modeling suggests that disequilibrium compaction is the dominant driving mechanism of overpressure in the Amazon Fan. Furthermore, from 2 Ma, progradation of the Amazon Fan drove the basinward migration of the deformation front, which modeling results

show was matched by a downslope migration of overpressure along the detachment.

The modeling results also show that clay mineral transformations and hydrocarbon generation played secondary role in fluid overpressure generation. Fluids were generated by smectite-illite transition within the sedimentary succession of the Amazon Fan over the last 8

Ma. However, fluid release took place above the detachment surface throughout the growth and collapse of the fan. Modeling of hydrocarbon generation from oceanic anoxic events (source rocks) shows that hydrocarbon gas expulsion took place before the onset of deposition of the Amazon Fan and that the migrating fluids were trapped in strata beneath the detachment level.

altered for Amazon fan, due to absence of direct pressure measurements to account for overpressure mechanisms other than undercompaction.

We thus use Eaton's equation (Fig. A1) :

𝑃 𝑃 = 𝜎 𝑣 -(𝜎 𝑣 -𝑃 ℎ𝑦𝑑 ) ( ∆𝑡 𝑛 ∆𝑡 𝑙𝑜𝑔 ) 3.0 (Eq. A1)
where 𝑃 is the pore pressure, 𝜎 𝑣 is the overburden or vertical stress, 𝑃 ℎ𝑦𝑑 the hydrostatic pressure, ∆𝑡 𝑛 is the interval transit time of shale considering normal compaction trend and ∆𝑡 𝑙𝑜𝑔 is the interval transit time obtained from well logging.

A.2 -Bowers' Method

According to [START_REF] Bowers | Pore Pressure Estimation from Velocity Data; Accounting for Overpressure Mechanisms Besides Undercompaction[END_REF], the depth at which excess pressure is generated by fluid expansion mechanisms can be captured from strain parameters such as velocity and density profiles. The effective stress can be retrieved from seismic interval velocities and pore fluid pressure deduced from overburden stress using Terzaghi's relationships [START_REF] Terzaghi | [END_REF]. This method captures mechanisms responsible for fluid pressure generation and provides pore pressure profiles from effective stress accounting for these mechanisms.

To capture the depth at which fluid expansion mechanisms might start unloading sediments, we cross-plot compressional velocity (from sonic log) and density to seek a possible velocity reversal. Only densities and compressional velocities of mudstones and shales intervals were used to minimize the effect of lithology changes [START_REF] Bowers | Pore Pressure Estimation from Velocity Data; Accounting for Overpressure Mechanisms Besides Undercompaction[END_REF][START_REF] Bowers | Determining an appropriate pore-pressure estimation strategy[END_REF].

To do this, we applied a cut-off used the gamma ray log, assuming 65gAPI values for shales, 25gAPI for carbonates and 35gAPI for sandstones. Visual characterization of cuttings in the shakers are then compared with the gamma ray log to execute the cut-off procedure.

We apply Terzaghi's equation to calculate the effective stress [START_REF] Terzaghi | [END_REF] and deduce the pore pressure. To do so, we first need to estimate the overburden stress. The magnitude of the total vertical stress (𝜎 𝑣 ) is the weight of the overlying formations assuming relatively flat topography. 𝜎 𝑣 is thus calculated from the integration of the density log:

𝜎 𝑣 =∫ 𝜌𝑔𝑑𝑧 (Eq. A2)
with 𝑔 is the gravitational acceleration in m.s -2 , 𝜌 the rock density (kg.m -3 ) and 𝑧 depth (m).

Since logs are not run at shallow depths, density data were extrapolated to the surface for the upper sections using correlations of [START_REF] Gardner | Formation velocity and density -the diagnostic basics for stratigraphic traps[END_REF]. Gardner's equation is an empirical equation that relates P-wave velocity to bulk density. This pseudo-velocity relationship is commonly used in estimating acoustic or density logs when only one of them is available. Gardner showed that:

𝜌 = 𝛼𝑉^𝛽 (Eq. A3)
where 𝜌 is bulk density, 𝑉 the compressional P velocity, and 𝛼 and 𝛽 empirically derived constant geology dependent. [START_REF] Gardner | Formation velocity and density -the diagnostic basics for stratigraphic traps[END_REF] proposed that one can obtain a good estimate of density in g.cm -3 , given velocity in 𝑓𝑡/𝑠, by taking 𝛼 = 0.23 and 𝛽 = 0.25.

Assuming this, and using units of g.cm -3 , the equation is reduced to the following:

𝜌 = 0.23𝑉 𝑝 ^(0.25) kg/m 3 (Eq. A4)
To calculate the pore pressure, we then used velocity/effective stress relations established by [START_REF] Bowers | Pore Pressure Estimation from Velocity Data; Accounting for Overpressure Mechanisms Besides Undercompaction[END_REF]. Effective stresses are calculated from compressional transit time and Terzaghi's equation is then used to derive pore pressure. In compaction disequilibrium conditions, outside the velocity reversal zone, [START_REF] Bowers | Pore Pressure Estimation from Velocity Data; Accounting for Overpressure Mechanisms Besides Undercompaction[END_REF] proposed an empirical method to calculate the effective stress as follows:

𝑉 𝑝 = 𝑉 0 + 𝐴𝜎 𝑣 ′ 𝐵 (Eq. A5)
where, 𝑉 𝑝 is the compressional velocity at a given depth and 𝑉 0 stands for the surface velocity (mudline velocity); 𝜎 𝑣 ′ represents the vertical effective stress; 𝐴 and 𝐵 are the parameters which are calibrated with offset velocity and the effective stress data. 𝑉 0 is generally taken as 1520m/s that is the velocity near the sea floor.

The vertical effective stress can be expressed as:

𝜎 𝑣 ′ = 𝜎 𝑣 -𝑃 𝑃 (Eq. A6)
Where, 𝜎' is the effective stress, 𝜎 𝑣 is total vertical stress.

𝑃 𝑃 = 𝜎 𝑣 -((𝑉 𝑝 -𝑉 𝑜 )/𝐴) ( 1 𝐵 ) (Eq. A7)
The values 𝐴 = 10 -20 and 𝐵 = 0.7 -0.75 are taken from Gulf of Mexico (considering pressure in psi) and transit time 𝑓𝑡𝑠 -1 (1ft s -1 = 0.3048m s -1 SI). Transit time is the inverse of velocity.

To address the effect of unloading effects, [START_REF] Bowers | Pore Pressure Estimation from Velocity Data; Accounting for Overpressure Mechanisms Besides Undercompaction[END_REF] introduced an empirical correlation:

V = V 0 + A [σ max ( 𝜎 𝑣 ′ σ max ) 1 U ] B (Eq. A8)
Where, 𝑈 is the unloading parameter, which is a measure of how plastic the sediment is. U represents the uplift of the sediment. 1 implies no permanent deformation and 𝑈 = ∞ corresponds to a completely irreversible deformation.

𝜎′ 𝑚𝑎𝑥 = ( 𝑉𝑚𝑎𝑥-𝑉𝑜 𝐴 ) 1 𝐵 (Eq. A9)
𝜎′ 𝑚𝑎𝑥 and 𝑉 𝑚𝑎𝑥 represent the values of vertical effective stress and velocity at the starting point, which are the maximal possible values, respectively [START_REF] Bowers | Pore Pressure Estimation from Velocity Data; Accounting for Overpressure Mechanisms Besides Undercompaction[END_REF]. The parameters 𝐴, 𝐵 and 𝑈 were obtained from Gulf of Mexico and Gulf Coast areas and works proposed by Bowers was developed using their well in the study area. The pore pressure for the unloading case can be estimated from:

𝑃 𝑢𝑛𝑙 = 𝜎 𝑣 -( 𝑉 𝑝 -V 0 𝐴 ) 𝑈 𝐵 (𝜎 𝑚𝑎𝑥 ) 1-𝑈 (Eq. A10)
Where 𝑃 𝑢𝑛𝑙 represents the pore pressure due to unloading behavior.

𝜆 =

𝑃 +𝜌𝜔𝑔𝐷 |𝜎𝑧|+𝜌𝜔𝑔𝐷 (B.4) with 𝜌 and 𝜌𝜔 being the wedge material and water densities, 𝐷 is the water depth, 𝜎𝑧 is the stress perpendicular to the detachment and 𝑃 is the pore pressure.

For each age, we took the maximum pore fluid pressure of each layer obtained during the basin modeling stage. Effective friction was then calculated from this pore fluid pressure and values for friction retrieved from the well data

B.3 -Cohesion

Cohesion (𝐶𝑜) was retrieved from the unconfined strength (𝑈𝐶𝑆) and the internal friction coefficient (𝜇). The 𝑈𝐶𝑆 is the maximum axial compressive stress that a right-cylindrical sample of material can withstand under unconfined conditions. To determine the 𝑈𝐶𝑆, we used correlations established as a function of 𝑉 𝑝 and lithologies. For limestones/carbonates, we used the Militzer equation [START_REF] Militzer | Einige Beitrageder geophysics zur primadatenerfassung im Bergbau[END_REF], defined as: 𝑈𝐶𝑆 = 𝑝𝑜𝑤((7682/∆𝑡),1.82), 𝑈𝐶𝑆 in psi. (B.7)

Cohesion was estimated using equation 9 [START_REF] Jaeger | Fundamentals of rock mechanics[END_REF]:

𝐶𝑜 = 𝑈𝐶𝑆 (2 * 𝜇 2 +1)+1 (B.8)
This chapter consists of a research paper in preparation for submission to the journal "Marine and Petroleum Geology". It presents the results of an investigation of the evolution of fluid pressure along a section (C-D) across the NW compartment of the Amazon deep-sea fan, using the same methodology proposed for the SE compartment by Souza et al (2020, Chapter V). Sections on Regional Setting and Data and Methods do not appear here as they have been presented in Chapters III and IV, but will form part of the final manuscript. Chapter VI first presents the set-up and calibration of the modelling along section C-D with reference to well data, then results of the 2D restoration of the growth and deformation of the Amazon fan, of basin modeling of the evolution of pressure and temperature, and of the evolution of deformation along the detachment and faults. These results will be compared to those from the SE compartment in Chapter VII, which forms part of the paper to be submitted. The paper will again be based on my doctoral research, in collaboration with my supervisors and colleagues with relevant expertise and interests in the area.

VI.1 INTRODUCTION

The trans-continental Amazon River developed since the mid-Miocene in response to Andean uplift [START_REF] Hoorn | Andean tectonics as a cause or changing drainage patterns in Miocene northern South America[END_REF][START_REF] Hoorn | Amazonia through time: Andean uplift, climate change, landscape evolution and biodiversity[END_REF][START_REF] Hoorn | The Amazon at sea: Onset and stages of the Amazon River from a marine record, with special reference to Neogene plant turnover in the drainage basin[END_REF], culminates in a prograding shelf-slope depocentre up to 10 km thick corresponding to the Amazon deep-sea fan [START_REF] Silva | Cenozoic tectonics of Amazon Mouth Basin[END_REF][START_REF] Watts | Seismic structure, gravity anomalies and flexure of the Amazon continental margin, NE Brazil[END_REF]. Sediment supply to the Atlantic continental margin increased through time to reach a maximum during the last 2.6 Ma [START_REF] Figueiredo | Late Miocene onset of the Amazon River and the Amazon deep-sea fan: Evidence from the Foz do Amazonas Basin[END_REF][START_REF] Cruz | Neogene evolution and demise of the Amapá carbonate platform, Amazon continental margin, Brazil[END_REF] and included lateral variations in offshore distribution due to a NW migration of sediment flux from 5.5 to 3.7 Ma [START_REF] Cruz | Neogene evolution and demise of the Amapá carbonate platform, Amazon continental margin, Brazil[END_REF]. The rapid growth of the fan was accompanied by its gravitational collapse, expressed as thin-skinned structures comprising linked extensionalcompressional fault belts rooted on detachment surfaces in overpressured shales [START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF]Oliveira, 2005;[START_REF] Perovano | O processo de colapso gravitacional da secao marinha da Bacia da Foz do Amazonas -Margem Equatorial Brasileira[END_REF][START_REF] Perovano | Modelagem fisica experimental de mecanismos de dedormação gravitacional simulando multiplos intervalos superpressurizados : Apliacação à Bacia da Foz do Amazonas[END_REF]. Collapse tectonics have resulted in an asymmetrical structural configuration, consisting of compartments in the southeast and northwest, separated by the Amazon canyon ). The thickest sediment depocentres are located between the extensional and contractional domains and reach up to 10 km in the northwest [START_REF] Silva | Cenozoic tectonics of Amazon Mouth Basin[END_REF] and 5-6 km in the southeast [START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF][START_REF] Souza | Controls on overpressure evolution during the gravitational collapse of the Amazon deep-sea fan[END_REF]. The two compartments differ in extent and character : in the NW, thin-skinned structures are up to 200 km in extent and exhibit evidence of long-lasting deformation from multiple partially-overlapping compressional fronts associated with up to 500 m of seafloor relief [START_REF] Reis | Effects of a regional décollement level for gravity tectonics on late Neogene-Quaternary large -scale slope instabilities in the Foz do Amazonas Basin, Brazil[END_REF], whereas in the SE they are up to 50 km wide and restricted to a pair of active reverse faults with no seafloor expression [START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF][START_REF] Reis | Two-scale gravitational collapse in the Amazon Fan: a coupled system of gravity tectonics and mass-transport processes[END_REF][START_REF] Reis | Effects of a regional décollement level for gravity tectonics on late Neogene-Quaternary large -scale slope instabilities in the Foz do Amazonas Basin, Brazil[END_REF].

A pioneering study of the crustal structure of the Amazon margin was presented by [START_REF] Watts | Seismic structure, gravity anomalies and flexure of the Amazon continental margin, NE Brazil[END_REF], based on seismic refraction profiles and gravity and magnetic data. The results indicate that the Amazon Fan was deposited above thin and cold crust with an unusually high flexural strength. According to the authors, the thin crust may be attributed to either slow seafloor spreading or a limited magma supply during the initial rifting of South America and Africa, in the Early Cretaceous. In addition, [START_REF] Watts | Seismic structure, gravity anomalies and flexure of the Amazon continental margin, NE Brazil[END_REF] showed that the Upper Amazon fan overlies continental crust, whereas the lower fan overlies deeper transitional or oceanic crust.

The two compartments also differ in their hydrocarbon potential, especially for gas from thermogenic sources. According to industry wells drilling reports (Brazilian Petroleum and Gas Agency -ANP), at least 6 wells (1-APS-18, 1-APS27A, 1-APS29, 1-APS-31A, 1-APS45B, 1-BP-2, see Fig. 6.1) found gas in the NW compartment, but only one, in the SE (1-APS-51A, see Fig. 6.1).

As further implications, we then compare results obtained by [START_REF] Souza | Controls on overpressure evolution during the gravitational collapse of the Amazon deep-sea fan[END_REF] along profile A-B located in the SE compartment, and present it on the chapter VII, with a discussion of the origins of the NW-SE variations. a maximum flooding surface (MFS 6, Boulila et al., 2020). The lower of the two detachments seems to be a local feature (Fig. 6.3, orange horizon, c. 81 Ma), and might be another maximum flooding surface (MFS 4, Boulila et al., 2020). In addition, we identified a third, deeper detachment within the Upper Albian (Fig. 6.3, brown horizon, c. 100 Ma, Perovano et al., 2011).

The ages of the units in Fig. 6.3 are based on the stratigraphic framework presented for the Offshore Amazon Basin by [START_REF] Cruz | Integrated geological and geophysical studies applied to understanding the evolution of the Offshore Amazon Basin[END_REF][START_REF] Cruz | Neogene evolution and demise of the Amapá carbonate platform, Amazon continental margin, Brazil[END_REF] and applied to the interpretation of section A-B in the SE compartment in Chapter VI [START_REF] Souza | Controls on overpressure evolution during the gravitational collapse of the Amazon deep-sea fan[END_REF]. The stratigraphic framework of [START_REF] Cruz | Integrated geological and geophysical studies applied to understanding the evolution of the Offshore Amazon Basin[END_REF][START_REF] Cruz | Neogene evolution and demise of the Amapá carbonate platform, Amazon continental margin, Brazil[END_REF] was constrained by biostratigraphic data from 40 wells located on the shelf, upper fan and Ceara Rise (see Fig. 6.1). The framework includes nine seismic horizons, dated by [START_REF] Cruz | Neogene evolution and demise of the Amapá carbonate platform, Amazon continental margin, Brazil[END_REF] to 2 Ma, 3.7 Ma, 5.5 Ma, 8 Ma, 24 Ma, 28 Ma, 33.5 Ma, 38 Ma and 66 Ma. To extend this framework downward to older horizons observed along section C-D (Fig. 6.3), we used data from [START_REF] Erbacher | [END_REF] for the ages of Oceanic Anoxic Events (OAEs) in the Demerara Rise, and their correlation to well data in the adjacent Foz do Amazonas Basin, dated OAE 3 at c.89-83.6 Ma and OAE2 at c. 93.5 Ma (see Chapters IV and V). In addition, we added OAE1d representing potential source rock levels at c. 100.35 Ma (Fig. 6.3, Boulila et al., 2020). To finalize our stratigraphic model, we added the top of the syn-rift succession (Cacipore Formation c. 102 Ma) and a basal horizon at c. 120 Ma (Brandão and Feijo, 1994;[START_REF] Figueiredo | Bacia da Foz do Amazonas[END_REF].

The structural interpretation of section C-D in Fig. 6.3 includes extensional faults across the outer shelf and upper slope and compressional structures to seaward (cf. Fig. 6.1) The main extensional structures are 4 normal faults (F1 to F4), three of which (F1-F2-F4) are rooted on the upper of two Upper Cretaceous detachments (dark blue horizon on Fig. 6.3) while the third (F3) is also rooted on the lower detachment (orange horizon on Fig. 6.3). Two other normal faults (F5-F6) located in the transitional zone are rooted on the upper detachment (Fig. 6.3). The compressional structures are thrust-folds expressed as duplexes at the detachment front, with synthetic and antithetic fault sets (C1-C4 on Fig. 6.3). These structures affect the overlying layers up to the seabed without reaching it, although the seabed is slightly deformed above both thrust-folds. Along section C-D, we do not observe any link between the lowermost detachment surface (c. 100 Ma, dark blue horizon) and the overlying succession, including within the Amazon Fan. The lower of the two Upper Cretaceous detachments (c. 81Ma (?), orange surface in Fig. 6.3), deformation is inferred to have been active prior to the onset of deposition of the Amazon fan, between 38-8 Ma (see Fig. 6.3). Hence, neither of these two detachment surfaces was not taken into account in modelling.

VI.2.2 lithofacies distribution in depth

Section C-D extends from the outer shelf to the upper fan across water depths of c.130-4600 m. A simplified lithofacies model was generated, based on information regarding the shelf and upper slope domains available from wells 1-APS-22, 1-APS31A-AP, 1-APS-36-AP, 1-APS-45B-AP, 1APS-52-AP, 1-BP-2-APS (Fig. 6.4) and from the literature (Figuiredo et al., 2007). Four facies groups were created to represent these domains from 120 Ma to present (Fig. 6.4) from top to bottom: (1) Amazon fan series (seabed -5.5Ma), dominated by a mixed of mudstone and sandstones (outershelf and upper fan), and by mudstones, with minor silt and sands (Fig. 6.4); (2) carbonate platform and distal equivalent (5.5Ma -66Ma), dominated by carbonates (outershelf), marls, with minor mudstone and carbonate (paleoslope) and mudstones (over 70%), with carbonates and silt fractions (Fig. 6.4): (3) post-rift (Limoerio Fm., 66Ma -c.102M), dominated by a mixed of shale (75%) and sandstone (25%), and source rock's levels (shale and limestone) (Fig. 6.4); (4) Syn-rift (Casipore Fm., c.102Ma -120Ma), composed by a mixed of sandstone and shales (Fig. 6.4).

VI.3 MODELING RESULTS ALONG PROFILE C-D

In this section, we present the results of different types of numerical modeling applied to section C-D. The 2D models were performed from 120 Ma (base of the model) to the present-day, except for the 2D mechanical model which was performed from 8 Ma to present.

We begin by presenting the structural restoration of section C-D, followed by results for the spatial and temporal evolution of temperature and pore pressure within the basin, and their respective implications for fluid and gas generation and fault movements during the growth and collapse of the fan.

VI.3.2 2D basin modeling of pressure and temperature evolution

The results of the modeling of the evolution of pore pressure and temperature along section C-D over the last 8 Ma are presented together in Fig. 6.7.

The pore pressure evolution is shown on the top left of Fig. 6.7 and in every second panel beneath (with active faults again shown in red). At 8 Ma, we do not observe significant overpressure, with most values lower than 10 MPa. Normal fault F1 and compressive faults C3/C4 are active (Fig. 6.7, shown in red). Overpressure increases at 5.5 Ma with the onset of growth of the Amazon fan. Values of about 20 MPa are observed on faults F1 (not active, Fig. 6.7), F2 (active, in red on Fig. 6.7), and along the detachment (in red on Fig. 6.7) and value of about 15 MPa within the compressional domain (C1 to C4, Fig. 6.8 in red). From 3.7 Ma to 2Ma, overpressures intensify significantly (to include values higher than 30 MPa), and propagate within the sedimentary column and along the detachment. During this phase, both F2 and the compressional faults remain active (Fig. 6.7, in red) and overpressure tends to be compartmentalize between these structures. This tendency continues until the present, when the values of overpressure reach more than 50 MPa, near active faults (F3 and F4, C1 to C4, Fig. 6.7, in red) and along the detachment.

The results from 2D thermal modeling are shown on the top right of Fig. 6.7 and in two adjacent panels beneath, in order to highlight two temperature-dependent processes, clay mineral diagenesis and hydrocarbon gas generation (both potential mechanisms of overpressure generation, see next section). From 8 Ma to the present-day, both temperaturedependent processes are active over depth ranges that affect the upper detachment (c. 69.7 Ma, Fig 6.7,in red). At top right the color scale is calibrated to the smectite-to-illite temperature window [START_REF] Bruce | Smectite dehydrationits relation to structural development and hydrocarbon accumulation in northern Gulf of Mexico basin[END_REF][START_REF] Hower | Mechanism of burial and metamorphism of argillaceous sediment: mineralogical and chemical evidence[END_REF][START_REF] Souza | Controls on overpressure evolution during the gravitational collapse of the Amazon deep-sea fan[END_REF], see chapter V), which is seen to bracket the detachment at 8 Ma, but to rise over time in response to the growth of the fan from 5.5 Ma ; the inner and outer parts of the detachment remains within the window to the presentday, whereas from 3.7 Ma the central part lies just beneath the window (Fig. 6.7). The modelled temperature history clearly supports smectite-illite transition within the Amazon Fan throughout its growth and at the present-day (Fig. 6.7).

The modeling results along section C-D indicate that the three OAE source rock levels used as input are efficient in terms of amount of hydrocarbons gas generated (Fig. 6.7). The adjacent panels beneath show the total mass of gas that has accumulated over time as a result of primary and secondary cracking of three OAE source rocks (see Fig. 2). Gas migration has taken place over timescales of up to 100 Ma, but is seen to have accelerated in the last 8 Ma in response to growth of the Amazon fan (Fig. 6.7). The mass of gas is mainly trapped beneath the detachment and from 5.5 Ma against fault F1 along which it is able to migrate upwards towards seafloor (Fig. 6.7).

However, comparison of a 1D output from the 2D temperature simulation for the present-day with bottom-hole temperatures measured in projected wells 1-APS-31-AP, 1-APS-45B-AP, 1-APS-52-AP and 1-BP-2-APS at around 4 km depth yields a reasonable correspondence, suggesting a good regional calibration (Fig. 6.8). Fig. 6.9 presents an average plot of the mass of gas expelled from the source rocks.

The expelled gas window for OAE1d lies between c. 100-70 Ma, for OAE2 between c. 70-0 Ma, and for OAE3 between c. 65 Ma and present-day (Fig. 6.8. We find that almost all gas generated form the OAE1d was expelled long before the onset of deposition of the Amazon Fan at 5.5Ma. However, for OAE2 and OAE3, the gas generated has been expelled since 70-65Ma and these source rocks levels seem to be still expelling after the deposition of the Amazon Fan. Fig. 6.7 shows that hydrocarbon mass of gas was generated, migrated and accumulated preferably along thickness below, and along the detachment, and near fault F1.

In terms of kinetics of the gas, the primary and secondary cracking are present all the time, before and during the onset of the Amazon Fan. In addition, the quantity of secondary cracking gas (8E5 -1.2E6 kg) is predominant in relation to primary cracking gas (2 -4E5 kg), being a strong contribution as overpressure mechanism. Overpressure tends to compartmentalize between faults, notably from 5.5 Ma with the arrival of Amazon Fan. The clay diagenesis window (70-175 °C), lies along the detachment surface at 8 Ma but rises upwards from 5.5 Ma to the present. Gas expulsion from three OAE source rock levels takes places over long timescale (OAE1 from 100 Ma, OAE2 and 3 from 60Ma) but intensifies over last 8Ma (see Fig. 6.8). The total mass of migrated gas (by primary and secondary cracking) was trapped on thicknesses below the detachment from 5.5 Ma migrates upward along fault F1. At 8 Ma, lithostatic pore pressure is required along the detachment and faults in order to generate extension across F1 and compression along C3/C4 (Figs 6.10 and 6.11). From 5.5Ma, pore pressure starts to migrate basinward and F2 is activated as well as C3/C4 (Figs 6.10 and 6.11). According to the 2D seismic interpretation and restoration, F1 is no longer active at this stage. In order to lock F1, a decrease of pore fluid pressure is required. There is no evidence of deformation for C1/C2. At 3.7Ma, F2 remains active and its activation can produce two possible deformations, in different steps: first step, lithostatic pore pressure is required for F2, along the detachment and C1/C2 to active F2 and C1/C2 (Figs 6.10 and 6.11). As a consequence, pore pressure must decrease between C1/C2-C3/C4 and C3/C4 faults. To propagate the deformation to C3/C4, a lower pore pressure is necessary along F2 and the detachment, and a larger one along C1/C2, C3/C4 and the detachment between these compressional features. In this case, only F2 and C3/C4 are active (Figs 6.10 and 6.11).

The same scenario holds at 2Ma, where the deformation can be reproduced in two different steps. Lithostatic pore pressure is required to activate F2 in both steps. We could not find properties allowing for the activation of C1/C2 and C3/C4 at the same time. To activate C1/C2, a decrease of pore pressure on C3/C4 is necessary (Figs 6.10 and 6.12). To activate C3/C4, pore pressure remains lithostatic up to the front (Figs 6.10 and 6.12).

The deformation observed on the seismic profile C-D and restoration between 1.6Ma to 0.8Ma can be synthetized at 0.4Ma. At this stage, a migration of pore fluid pressure is required, from F1-F2 to F3, and then to F4 is (Figs 6.10 and 12). F1 and F2 are no longer active. In order to lock F2, pore fluid pressure must decrease. F3 is very efficient to propagate the deformation to C1/C2 and C3/C4 (under lithostatic pore pressure conditions). Activation of F4 and C3/C4 is possible with a lower pore fluid pressure.

At the present-day (Figs. 6.10 and 6.13), we observe along section C-D that extensional (F3, F4) and compressional domains (C1/C2 and C3/C4) are still active. We can reproduce this deformation, with a lithostatic pore pressure along the detachment and a slight decrease basinward.

To summarize, we are able to reproduce the deformation deduced from the structural restoration. To do so, a lithostatic pore pressure along the detachment is required at all ages.

Moreover, to simulate the migration of the fault activity from F1 to F4, a migration of the pore fluid pressure is necessary. Figure 6.11: 2D Normalized virtual velocities in horizontal and vertical directions obtained from OptumG2, from 8 Ma to 3.7 Ma with g factor of safety, defined as the ratio between the load necessary to generate a collapse and the gravitational force (𝑔 = 9.8𝑚/²): (1) 8 Ma, F1 is active and propagate up to C3/C4. For the simulation here shown 𝑔 = 0.8. (2) 5.5 Ma, F2 is active and propagate up to C3/C4 (𝑔 = 0.98). (3) 3.7 Ma, F2 is active and propagate only C1/C2 (𝑔 = 0.97).

(4) 3.7Ma, F2 active and propagate up to C3/C4 (𝑔 = 0.94).

VII.1 Comparison of results from SE and NW compartments

In this chapter, I compare and contrast results from SE compartment, presented in Chapter V [START_REF] Souza | Controls on overpressure evolution during the gravitational collapse of the Amazon deep-sea fan[END_REF] and those from the NW compartment, described in Chapter VI.

VII.1.1 Pore pressure evolution

The onset of growth of the Amazon Fan started at ca. 8 Ma (Fig. 7.1a), with the first sedimentary expression in the SE compartment, where a transition between a carbonate platform and siliciclastic sedimentation is recorded. The arrival of siliciclastic sediment is directly reflected in the deformation observed in the SE compartment, in the activation of extensional faults (F1 and F2, Fig. 7.1a) coeval with basinward compression (C1,C2 and C3,Fig. 7.1a). In contrast (?), in the NW compartment, deformation is still incipient, with a unique extensional fault F1, and compression only on C3. In both compartments, pore pressure must be high to explain the observed gravitational deformation (Fig. 7.1a).

From 5.5 Ma (Fig. 7.1b), there is increased sedimentary input to the NW compartment. At this stage, there is no record of activity in the SE compartment and deformation becomes stronger in the NW, as reflected in the migration of the extensional domain from normal faults F1 to F2 (Fig. 7.1b), reflecting a basinward migration of the pore pressure.

The same conditions persist in both compartments at 3.7 Ma, when the NW compartment is active while the SE remains quiet.

From 2 Ma, sedimentation rates increase in the Amazon fan, resulting in resumed deformation in both the SE and NW compartments (Fig. 7.1c). In the SE compartment, activity on both extensional and compressional faults (F3, F4, C1, C2 and C3, Fig. 7.1c) reflects a basinward migration of the pore pressure. In the NW compartment, pore pressure remains high, to the extent that activity on normal fault F2 is accompanied by activity within the entire compressive domain (Fig. 7.1c), with a slight deformation on C1/C2 and a stronger one along C3/C4 (Fig. 7.1c).

The Amazon fan has remained fully active in terms of gravity tectonics from 2 Ma until the present-day, with activity in both the SE and NW compartments (Fig. 7.1d). In both compartments, both pore pressure and deformation migrate basinward, represented by F4 and F5 and the activation of C1, C2 and C3 for the SE compartment and F3 and F4 activating C1, C2, C3 and C4 for the NW (Fig. 7.1d).

VII.1.2 Overpressure mechanisms

The results presented in chapters V and VI indicate that sedimentation is the major factor controlling the evolution of pore pressure during the growth and collapse of the Amazon Fan. This is observed through episodic gravity tectonism in the SE compartment, where fault activity was limited to episodes of greater sedimentary input, versus continuous activity in the NW compartment, where sedimentation never ceased. In the Amazon fan, high rates of deposition accompanied by a low compaction state of sediments point to disequilibrium compaction (undercompaction) being the primary overpressure mechanism in the system. According to [START_REF] Cruz | Integrated geological and geophysical studies applied to understanding the evolution of the Offshore Amazon Basin[END_REF], based on gravity data, the Araguari sub-basin is characterized by N-S trending grabens and/or half-grabens. The landward limit of this subbasin corresponds to a series of E-W trending (aligned with the Saint-Paul Fracture Zone) that separate it from the Marajo basin. [START_REF] Cruz | Integrated geological and geophysical studies applied to understanding the evolution of the Offshore Amazon Basin[END_REF] argued that the northward intensification of crustal extensional might have affected the Offshore Amazon Basin, resulting in thinner crust beneath the Araguari sub-basin. His interpretation is in agreement with studies developed by Lara (1994), who showed that central and northern portions of Marajo basin are underlain by transitional crust with higher amounts of extension, while the southern region is underlain by continental crust. In addition, thinner crust may be more susceptible to flexural subsidence due to sedimentary load and thus favor the location of post-rift depocenters along the axis of the Araguari sub-basin.

Section X-Y shows that the continental crust thins rapidly seaward, over a distance of less than 100 km, although [START_REF] Rodger | A Marine Geophysical Study of the Amazon Continental Margin, North Eastern Brazil[END_REF] could not resolve the precise location of the oceancontinent transition (OCT) due to limitations of the seismic data (Fig. compartment it continued until 70Ma (see Fig. 6.9). The NW compartment was hotter than the SE, but the latter underwent higher rates of burial (ca. 0.13 km/Ma in the SE versus ca. 0.09 km/Ma in the NW). Thus, differing rates of burial can explain the differences of expulsion from OAE-1d source rocks, with higher rates resulting in earlier complete expulsion in the SE.

During the rest of the post-rift phase, heat flow began to decrease (values between 50-60mW/m², Fig. 7.3B), reflecting the onset of crustal cooling. In the SE compartment, the Amazon fan gravity tectonic system is mainly underlain by continental and OCT crust with thin oceanic crust at its distal limit (see SB as reference, Fig. 7.2). In the NW compartment, the gravity tectonic system in mainly underlain by oceanic crust that thins to meet OCT crust is beneath its proximal parts (before SB, Fig.  The smectite-illite transition has been present in both compartments over time, but has not significantly contributed to overpressure along the detachment in the SE compartment. In contrast, it may have played an important role along parts of the detachment in the NW compartment. Here, I cannot quantify how this mechanism contributes as an overpressure mechanism due the lack of calibration data.

However, globally, I consider the smectite-illite transition as a secondary mechanism because this mechanism is not predominant in the whole fan, except in the NW compartment.

 Thermogenic gas generation (by primary and secondary cracking) acted as an overpressure mechanism in the NW compartment, contributing to higher pore pressure on the detachment and some faults. The presence of hydrocarbon at the present-day is confirmed by the industry wells drilled nearby the section C-D (radius up to 90km), proving the potential of this fluid on the basin. On the contrary, thermogenic gas did not contribute to overpressures in the SE compartment, where most gas was expelled prior to the growth of the Amazon fan and trapped by shalerich layers beneath the detachment;

 The difference of gas generation contribution as an overpressure mechanism on the southeastern and northwestern compartments might be associated with the thermal history of the Foz do Amazonas basin, which in turn is linked to the crustal framework and its evolution over time;

 Several authors [START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF], Oliveira et al., 2005[START_REF] Rodger | A Marine Geophysical Study of the Amazon Continental Margin, North Eastern Brazil[END_REF][START_REF] Watts | Seismic structure, gravity anomalies and flexure of the Amazon continental margin, NE Brazil[END_REF][START_REF] Cruz | Integrated geological and geophysical studies applied to understanding the evolution of the Offshore Amazon Basin[END_REF] suggested that the formation of the SE and NW depocentres are conditioned by crustal tectonic structures, and some authors [START_REF] Rodger | A Marine Geophysical Study of the Amazon Continental Margin, North Eastern Brazil[END_REF][START_REF] Watts | Seismic structure, gravity anomalies and flexure of the Amazon continental margin, NE Brazil[END_REF] pointed out that the Amazon Fan was deposited under a thinner crust. Afterwards, the importance of further study on the evolution of the crust, the crustal types and the influence of this crustal framework on the thermal history of the basin is evident;

 The modeling results show that the Amazon Fan is active today, and the overpressure conditions is inducing gravity tectonics. Drilling through zones with high pore pressure, weak and reactive sediments (i.e. shales, mudstones) represent a great hazard for wellbore operations. A dedicated study like this research investigation, combined with the geomechanical pore pressure prediction, can strongly contribute to analyze the risks of overpressure and failure.

 As perspectives:

a. Consider the relevance of findings from the Amazon margin for such processes on other passive margin depocentres (Delta of Niger, Nile) b. Go further on investigating biogenic gas as potential overpressure mechanism acting on the recent sedimentary records of Amazon Fan. The biogenic gas was founded on five wells drilled on the basin -1-APS-9, 1-APS-14, 1-APS-41, 1-APS-51A and 1-APS-36).

c. Studies dedicated to the origin and evolution of the crust framework.

  : Problematics and open questions…………………………………………..13 I.2 The Amazon Margin as a study area ………………………………………………………15 I.3 Objectives of the thesis ………………………………………………………………………18 II. OVERPRESSURE PROCESSES AND PRODUCTS .......................................................20 II.1 Overpressure mechanims…………………………………………………………………21 II.1.1 Desequilibrium compaction………………………………………………………….23 II.1.2 Tectonic stress………………………………………………………………………..24 II.1.3 Clay Diagenesis……………………………………………………………………….24 II.1.4 Hydrocarbon generation……………………………………………………………..25 II.2 Gravity Tectonics…………………………………………………………………………...26 III. GEOLOGICAL SETTING .............................................................................................31 III.1 The Foz do Amazonas Basin ....................................................................................32 III.2 The Amazon Fan gravity tectonics ............................................................................37 III.3 Thermal History and Hydrocarbon Potential .............................................................44 III.4 Summary ..................................................................................................................61

Figure 1 . 1 :

 11 Figure 1.1 : Bathymetry of the Equatorial Margin of Brazil showing the location of the Amazon Fan

  imbalance is a consequence of different generating mechanisms (Fig 2.1), among which the most common are:

Figure 2 . 1 :

 21 Figure 2.1: Causes of overpressure. From Grauls (1997).

Figure 2 . 2 :

 22 Figure 2.2: Passive margin with basinward-dipping detachment and thus a significant component of gravity gliding (not to scale). Basinward tilting is enhanced by differential thermal subsidence and cratonic uplift and is reduced by proximal loading subsidence. Modified from Rowan et al. (2004).

Figure 2 . 3 :

 23 Figure 2.3: Passive margin with failure dominated by gravity spreading (not to scale). (a) Progradational deposition along the outer shelf and upper slope increases the differential sediment load and thus drives spreading. (b) and (c) Upper-slope bypass and distal deposition on the lower slope and abyssal plain reduces the surficial slope and the gravity potential, slowing or stopping spreading. From Rowan et al. (2004).

Figure 2 . 4 :

 24 Figure 2.4: Schematic model representing a possible structural zonation generated by extensive surface deformation. Modified from Perovano (2008).

Figure 3

 3 Figure 3.1: Location map of the Foz do Amazonas Basin, and its boundaries with the Para-MaranhãoBasin. Modified fromMello et al. (2001)



  Post-rift: the upper megasequence corresponds to the beginning of marine sedimentation on a passive margin and is characterized by three distinct sedimentary phases: o First phase: Upper Albian and Lower Paleocene (Danian) marine transgressive and regressive intervals, composed of interbedded shales and fluvio-deltaic sandstones of the Limoeiro Formation (Fig. 3.2).

Figure 3 . 2 :

 32 Figure 3.2: Stratigraphic chart of the Foz do Amazonas Basin. Simplified from Figueiredo et al. (2007).

Figure 3 . 3 :

 33 Figure 3.3: Regional seismic profiles with interpretation showing the structural zoning and the ages of some intervals of the basin filling sequence. For locations, see map A. Structural map (B) with domains due to gravity tectonics in the Amazon Fan. Modified from Cobbold et al. (2004).

Figure 3

 3 Figure 3.4: Thickness map in time (ms) of the sedimentary cover on the level of basal detachment (Upper Cretaceous) above which gravity tectonic structures have developed in the Amazon Fan. D1 and D2 represent the main depocenters, which correspond to the NW and SE structural compartments of the Amazon Fan. Modified from Oliveira et al. (2005).

  3.5-3.6), based on seismic interpretations from Silva et al (1999):  Upper Albian (c.100Ma)  Upper Cretaceous-Paleocene (c. 65Ma)  Upper Miocene (c.10.5Ma) These authors identified, through 2D seismic interpretation, a diversity of structures associated with the mechanism of thin-skinned extension, developed above one or multiple deep detachment surfaces: (1) a proximal extensive domain, where normal synthetic listrics faults predominate (Figs. 3.5-3.6) and (2) a distal compressive domain, characterized by the presence of reverse faults and thrust-fold structures (Figs. 3.5-3.6).The remarkable structures associated with the deep-water gravity-driven fold-andthrust belts vary in geometry and structural complexity along the margin. Structures are significantly more complex along the Northwestern Compartment, where the major depocenters are located, exhibiting evidence of long-lasting deformation from multiple partiallyoverlapping fronts. More deformation means more stress acting in the structure bodies and faults. In the southeastern compartment, the system is restricted to a pair of active reverse faults causing no major sea-bottom relief (inactivated belt).[START_REF] Perovano | Modelagem fisica experimental de mecanismos de dedormação gravitacional simulando multiplos intervalos superpressurizados : Apliacação à Bacia da Foz do Amazonas[END_REF] performed a series of scaled physical experiments (sandbox models) in order to replicate possible scenarios involving the two main stages of deformation in the Amazon Fan (Upper Miocene c.10.5Ma), including basal detachments. The authors concluded that the different levels of detachment seem to be controlled mainly by the distribution of hydrocarbon generation (gas), while other inductive mechanisms, such as mechanical compaction, were of secondary importance.

Figure 3 . 5 :

 35 Figure 3.5: Structural map of gravity tectonic structures in the Foz do Amazonas Basin. Numbers refer to seismic lines not shown here. NWSC (NW Structural Compartment), SESC (SE Structural Compartment). From Perovano et al (2009).

Figure 3 . 6 :

 36 Figure 3.6: Interpreted seismic profiles (in two-way time) crossing the extensional and compressional domains of the Amazon fan and illustrating variations in the structural framework between the SE (profile 2A) and NW (profile 2B) structural compartments (modified from Perovano et al., 2009). Dashed lines show the main interpreted detachment surfaces, from bottom to top (lower blue surface -Albian c.100Ma ; intermediate green surface -Cretaceous-Tertiary limit c.65Ma; upper yellow surface -Middle Eocene c.40Ma), all from Silva et al. (1999).

  These authors identified a Moho reflection on the multi-channel seismic reflection lines as a discontinuous, low amplitude, low frequency single reflector (Fig.3.7). The difference in two-way travel time between top oceanic basement and the Moho reflector indicates that the crust has variable thickness, is anomalously thin along the majority of the lines length and thickens towards the Ceara Rise (Fig.3.7).

Figure 3 . 7 :

 37 Figure 3.7: Crustal model of the Foz do Amazonas basin from Rodger (2008). The red lines relate to the seismically defined transition from upper to lower oceanic crust (layers 2 to 3) and the seismic Moho from ACE refraction data along Line 323B.

  index is type II (marine environment)(Fig 3.9).Oils from the second group of source rocks have been recovered in the Para-Maranhão area, in the Foz do Amazonas province (Fig.3.8), and described as Lower Tertiary dark-grey shale potential source rocks deposited in marine environment (Travosas Formation). However, the Tertiary marine source rocks have not been drilled up to now. The TOC average is around 3% and the kerogen is Type II (Fig 3.9).

Figure 3 . 8 :

 38 Figure 3.8: Location map showing potential source rocks based on oil types sampled in Niger Delta and Foz do Amazonas. From Mello et al (2000).

Figure 3 . 9 :

 39 Figure 3.9: Crossplot of hopanes, steranes ratios versus C29 sterane and Oleanane index versus C29

  3.10), underlain by rifted continental crust of Precambrian and early Mesozoic age. In total, five sites were drilled in a present-day depth transect from 3200-1900 meters below sea level (mbsl), which resulted in the recovery of c. 650 m of black shales. Critical intervals recovered include multiple penetrations of oceanic anoxic events (OAEs) 2 and 3. Site 1257 recovered a 44-m-thick sequence of laminated black shales of Santonianlatest Cenomanian age that include OAEs 3 and 2. Total organic carbon (TOC) content reaches 16 wt%, and hydrogen index and oxygen index values indicate that the black shales contain Type II kerogen, indicating a marine source for the organic matter (Figs. 3.10-3.11). Site 1258 recovered a c.56-m-thick black shale sequence that contains the lower part of OAE 3, a complete OAE 2, and an expanded succession of laminated shales of Cenomanian-middle Albian age. OAE 2 is represented by an interval of distinctly laminated black shales with TOC values up to 28 wt%. Rock-Eval analyses indicate Type II kerogen, which is consistent with a marine origin of the organic matter (Figs. 3.10-3.11). Site 1259 recovered c.56-m-thick laminated black shale succession of Cenomanian-Santonian age, including OAEs 3 and 2, with a maximum TOC content of 29 wt% (Figs. 3.10 -3.11). Site 1260 recovered a c.93-m-thick black shale succession of Coniacian-Albian age, with TOC content up to 14 wt%. This interval contains OAE2, and the lower part of OAE 3 (Figs. 3.10-3.11). Site 1261 recovered a c.89-m-thick Santonian to Upper Cenomanian succession of laminated black shales and laminated foraminiferal limestones, including OAEs 3 and 2 (Figs. 3.10-3.11).

Figure 3 .

 3 Figure 3.10: Sites drilled on the Demerara Rise during IODP Leg 207, together with seafloor bathymetry compiled from the seafloor pick of the indicated seismic survey track lines. Squares in map at right indicate the location of the Demerara Rise (solid red square) relative to the Amazon Fan (dashed bordeaux square). Map after www.odsn.de. From Erbacher et al, (2004).

Figure 3 .

 3 Figure 3.11: Comparison of original total organic carbon (TOC) content and Corg/Ntotal values of black shales (defined as >1% TOC) for black shale units of the five sites drilled during Leg 207 on the Demerara Rise. Increasing C/N values with higher TOC concentrations probably reflect preservational conditions that favored burial of carbon and recycling of the nitrogenous components of marine organic matter. From Erbacher et al, (2004).

Figure 3 .

 3 Figure 3.12: Lithostratigraphy log of the Limoeiro Formation detailed on wells 1-APS29-AP and 1-APS-44-AP (for location, see Fig. 3.15), plotted with gamma-ray (GR) log data and Total Organic Carbon (TOC) data in well 29. From Boulila et al., (2020).

Figure 3 .

 3 Figure 3.14: (A) Correlation of 4.7 Myr orbital cycles with seismically detected second-order SL sequences in Well 44. (B) Correlation of 9.5 Myr orbital cycles in Well 29 with the main SL sequences that compose the Limoeiro Formation (Figueiredo et al., 2007). The maximum flooding surfaces (MFS) identified in Wells 44 and 29 might indicate the ages of detachment surfaces of the Foz do Amazonas Basin. In our model, we chose two MFS' candidates to define the age of Upper Cretaceous detachments: first one as MFS 6 (c. 69.7 -70Ma) identified on well 44, and second as MFS 4 (c. 81Ma), identified in both wells. From Boulila et al. (2020).

Figure 3 . 15 :

 315 Figure 3.15: Location of industry wells available for this study on the Amazon fan. Wells in orange dots were used as calibration data. Wells in black/white dots were used as hydrocarbon classification (see tables 3.1 and 3.2).

Figure 3 .

 3 Figure 3.16: Fault map of the Miocene showing the growth faults near the shelf edge, thrust faults in a northeastern direction together with interpreted natural oil seeps. From Miranda et al. (1998).

Figure 3 .

 3 Figure 3.17: Location map showing the Upper Amazon fan study area (black polygon) developed by Ketzer et al (2018) and the multibeam echo sounder survey area in inset. Black hexagons show the location of plumes and stars represent sites where gas hydrates were recovered in cores. The isobaths line of 600m (in red) represents the approximate depth of the edge of the methane hydrate stability zone (GHSZ) in the Amazon Fan.

Figure 3 .

 3 Figure 3.18: Schematic diagram showing the potential of biogenic gas thanks to the Amazon discharge of sediment and organic matter from the continent to the deep sea. Part of organic matter deposited in the deep-sea fan will be altered to methane by microbial activity, which will migrate upward to be trapped as free gas, gas hydrates and authigenic carbonates. However, another part of the gas will migrate along structural pathways including extensive and compressive fault systems formed by gravity tectonics of the Amazon fan, and seep from the seafloor. From Ketzer et al (2018).

Figure 4

 4 Figure 4.1: Seismic reflection and well data available from the Amazon Fan.

  a) Bibliographic review to define the current state of the art of gravity tectonics and overpressure mechanisms.

  c) Time-to-depth conversion of the stratigraphic and structural interpretation along two seismic cross-sections: A-B (SE compartment) and C-D (NW compartment) (see details in Chapter V, section V.3.3 and in Chapter VI, section VI.) d) Cross-balancing of the two sections A-B (SE) and C-D (NW) in depth (see details in Chapter V, section V.3.4 and in Chapter VI, section VI.) e) 2D restoration from seismic interpretation in depth:The KronosFlow software, developed by IFPen, allows a 2D geological model to be built, taking into account both the extensional and compressional deformations and fault displacements through time. For each step of restoration, some hypothetical constraints are imposed: (1) paleobathymetry profile; (2) detachment as a mobile level; (3) baseline fixed between the base of the model (120Ma) and the detachment (no horizontal movement, only vertical). For more details, see chapter V, section V.3.4.

  g)Boundary conditions to build the 2D thermal model:This stage consists in calculating the temperature of the basin over time, based on some constraints: paleosurface and bottom temperatures, thermal conductivity and radiogenic heat production. For the bottom temperatures, the constraints are: 1) heat flow;(2) thermal gradient;(3) temperature of the upper mantle; (4) heat flow from the upper mantle[START_REF] Hantschel | Fundamentals of Basin and Petroleum Systems Modeling[END_REF].In order to calculate the heat flow through a basin, at all ages, it is necessary to have a model of crustal thickness to constrain calculations of heat generation through time at the base of the basin. Due to KronosFlow software limitations, it was not possible to work with a crustal model directly integrated to the sedimentary model. Therefore, I built a simplified crustal model for sections A-B and C-D using TemisFlow (Petroleum system modeling software). To build the hypothetical crustal model (Fig. 4.2), I used: (1) the Moho profile (t1) calculated by Rodger (2008) and Watts et al (2009); (2) the seismic basement interpreted (t2) on the sections A-B and C-D, converted in depth; (3) a value of initial average thickness of the crust o fc.32km (t0) (4) the stretching factor β calculated from the classical theory of McKenzie(1978), where β = t0/(t1-t2).

Figure 4

 4 Figure 4.2: 2D Present-day crustal models (sections A-B and C-D) used to build the thermal scenario (using TemisFlow software). The Moho profile is that identified by Rodger (2008) and Watts et al (2009).

(

  2012) and Tremblin et al (2016). The first group of authors compiled stable oxygen isotope values from benthic foraminiferal species that are sensitive to climatic changes for the period 115-65 Ma and their relation with paleotemperatures of ocean surface waters (Fig 4.3; Friedrich et al 2012). Values were compiled from several parts of the world (North Atlantic Ocean, southern high latitudes, Pacific Ocean, subtropical South Atlantic Ocean, Indian Ocean) (Fig 4.3). However, Tremblin et al (2016) introduced a discussion on prominent paleoceanographic and climatic feature in the context of overarching pCO2 decline and the establishment of an Antarctic circumpolar current (Fig. 4.4). These authors used recent developments in coccolith biogeochemistry to reconstruct equatorial Atlantic sea surface temperature (SST) and atmospheric pCO2 values from pelagic sequences preceding and spanning the EOT (Eocene-Oligocene Transition). All analyses were based the IODP sites Leg 925 and 929 on the Ceara Rise (Fig. 4.4).

Figure 4

 4 Figure 4.3: Stable oxygen isotope compilation of benthic foraminifera for 115-65 Ma. Black North Atlantic Ocean, gray -southern high latitudes, red -Pacific Ocean, blue -subtropical South Atlantic Ocean, green -Indian Ocean. OAE -oceanic anoxic event; EAG -Equatorial Atlantic Gateway. From Friedrich et al (2012).

Figure 4

 4 Figure 4.4: Reconstructed equatorial Sea surface Temperature (SST) during the Early Eocene to the Early Oligocene. (A) Multiproxy temperature estimates from the TDP (in red) indicate a warm and relatively stable Eocene climate. The blue curve denotes published TEX 86 in the equatorial Atlantic showing relatively similar temperatures in the Eocene and the Early Oligocene. (B) Coccolithδ 18Oderived temperatures from sites ODP 925 and 929. Early to Middle Eocene cooling highlights the termination of the greenhouse period. After climate ups and downs in the Middle Eocene (30 -32), including the Middle Eocene Climatic Optimum (MECO), a Late Eocene 6 °C warming predates the pronounced cooling seen coeval with the onset of glaciation during the Early Oligocene. From Tremblin et al, (2016).

Figure 4

 4 Figure 4.5: Example of paleosurface temperatures calculated for 8-0 Ma along section A-B, using the relation developed by[START_REF] Cardoso | Caracterizacao do campo térmico atual e evoluçao téermica da Plataforma Continental do Estado do Rio de Janeiro[END_REF] and[START_REF] Gutierrez | Campo Termico da Crosta na Regiao Sedimentar Bahia Sul e na Area Pre-Cambriana adjacente: Reavaliaçao com base em dados corrigidos da Margem Continental[END_REF].

Figure 5 . 1 :

 51 Figure 5.1: Bathymetry of the Equatorial Margin of Brazil showing the location of the Amazon Fan andavailable seismic profiles and wells. The Amazon Fan is composed of NW and SE structural compartments, as defined by[START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF] andOliveira et al (2005). Gravitational collapse has resulted in paired domains of extensional and compressive faults across the shelf and upper slope. The available regional dataset comprises multi-channel seismic reflection profiles and drilling results from industry and ODP/DSDP wells. For this study, we use a composite seismic profile across the SW compartment referred to as A-B, shown in red, calibrated with reference to two wells: 1-BP-3-APS and 1-APS-31A-AP (also shown in red).

  [START_REF] Figueiredo | Bacia da Foz do Amazonas[END_REF], the syn-rift phase of the Foz do Amazonas Basin took place between the Valanginian and the Albian and was marked by the deposition of fluvio-deltaic, lacustrine and marine sediments filling pull-apart half-grabens (Cassiporé and Codó Formations; Fig.5.2). The post-rift phase saw the widespread deposition of open marine mudstones and siltstones of the Limoeiro Formation between latest Albian and Maastrichtian (from c. 101 to c. 66 Ma;[START_REF] Figueiredo | Bacia da Foz do Amazonas[END_REF] Boulila et al., 2020). From Paleocene to late Miocene (c. 66 to 8 Ma), shelf sedimentation resulted in a mixed carbonate-siliciclastic platform up to 4000 m thick, laterally equivalent to thinner deep-water calcilutites and mudstones deposits (Marajo, Amapá and Travosas

Figure 5 . 2 :

 52 Figure 5.2: Foz do Amazonas Basin stratigraphy chart, focusing on the Amazon Fan. Detachment levels (Albian c. 100Ma and Upper Cretaceous c. 69.7Ma) and likely levels of Oceanic Anoxic Events (OAE1d c. 101.1Ma, OAE2 c. 93.5Ma and OAE3 c. 83.6Ma), representing the potential source rocks have been added. Modified from Figueiredo et al. (2007).

Figure 5 . 3 :

 53 Figure 5.3: Methodology applied to seismic and well data to investigate the evolution of pore fluid pressure in the Amazon Fan.

  Figure 5.4: Stratigraphic interpretation of the AB seismic profile (inset for location). The succession includes three main intervals: (1) from Lower Aptian to Upper Cretaceous (120 Ma, 102Ma, 101.1Ma, 100Ma, 93.5Ma, 83.6 Ma), delimited on top by a second detachment surface (dark blue horizon, c. 69.7Ma); (2) from Upper Cretaceous to Middle Miocene (66 Ma, 38 Ma, 33.5Ma, 28Ma, 24Ma, 8Ma); (3) from Middle Miocene to present-day (8Ma, 5.5Ma, 3.7Ma, 2Ma and seabed), which corresponds to the Amazon Fan. Gravity tectonics identified along the section are characterized by listric normal faults beneath the shelf and upper slope (F1 to F5) and a compressional domain to seaward (duplexes and thrust faults, Cp1 to Cp3), both rooted on a detachment surface within the Upper Cretaceous (c. 69.7Ma). The slope break is located between F4 and F5.

  Fig. 5.4 includes extensional faults across the outer shelf and upper slope and compressional structures to seaward (cf. Fig. 5.1) The main extensional structures are five normal faults (F1 to F5), F1 and F2 rooted on the lower detachment surface and F3-F5 on the upper detachment (dark blue horizon on Fig. 5.4). F4 and F5 reach up to seafloor where they are expressed as scarps. The compressional structures are thrust-folds expressed as duplexes at the detachment front (Cp1 on Fig. 5.4), synthetic and antithetic fault sets (Cp2 on Fig. 5.4) and some buckling to seaward (Cp3 on Fig. 5.4

Figure 5 . 5 :

 55 Figure 5.5: Lithofacies model along seismic profile A-B (location in Fig. 5.1), based on information from regional wells and the literature. Also shown are the locations of well 1-BP-3-APS and of pseudo-wells used for the time-depth conversion.

Figure 5 . 6 :

 56 Figure 5.6: Porosity-depth plot calculated for 1-BP-3-APS, constructed from two different data sets: (1) from seafloor to 342m, from site 931' reports (ODP, Leg 155, see Fig. 5.1), and (2) from 998m to 5618.5m, from acoustic log data. Porosities between 342m and 998m were extrapolated from datasets 1 and 2.

Figure 5

 5 Figure 5.7: Porosity-permeability user-defined relation derived from measurements in well 1-BP-3-APS.

  compiled stable oxygen isotope values of benthic foraminifera sensitive to climatic changes for the period 115-65 Ma, and their relation with paleotemperatures. Tremblin et al (2016) used recent developments in coccolith biogeochemistry to reconstruct equatorial Atlantic sea surface temperature (SST) and atmospheric pCO2 values from pelagic sequences preceding and spanning the EOT.

Figure 5

 5 Figure 5.8: Present-day crustal model used to build the thermal scenario. The Moho profile is that identified by[START_REF] Watts | Seismic structure, gravity anomalies and flexure of the Amazon continental margin, NE Brazil[END_REF], shown together with their interpretation of continental to oceanic crustal types (arrows in blue). For our study, we used the top basement and the limit of oceanic crust as interpreted from 2D seismic line (arrows in red).

  Deformation resumes at 2 Ma (F3 and compressive faults C, shown in red on Fig.5.9), and intensifies from 1. F4 and C, shown in red). During this phase, F1 and F2 are no longer active. A sequential forward propagation of the extensional domain is observed, at 1.6-1.2 Ma (F3, F4, shown in red), 0.8 Ma (F4, shown in red) and at 0.4 Ma (F4 and F5, shown in red). The compressive domain is also active from 2 Ma, with subsurface fold propagation faults deforming layers up to seafloor (Fig.5.9). From 0.4 Ma to present, a deep duplex forms at the buried front of the compressive domain, breaking overlying sediments; during the same period, older compressive structures are squeezed and become narrower (Fig.5.9). However, these faults are not observed to propagate toward the surface. At the present-day, only normal faults reach the seafloor where they are expressed as scarps (F4 and F5, shown in red on Fig.5.9).

Figure 5

 5 Figure 5.9: 2D restoration along section A-B from 8 Ma to the present day (output from KronosFlow). Active faults are shown in red, inactive in black.

Figure 5 .

 5 Figure 5.10: Results of modeling pore pressure and temperature evolution along section A-B from 8 Ma to the present-day (outputs from TemisFlow). At left: Active faults are shown in red, inactive in black. Overpressure tends to compartmentalize between faults, notably since 2 Ma when sedimentation rates in the Amazon Fan intensified. At right: Temperature results are calibrated in terms of the clay diagenesis window (70-175°C), which remained above the detachment surface from 8 Ma to the present-day. The three OAE source rock levels are efficient over time, reaching the gas expulsion window until 24 Ma (see Fig. 5.14).

Figure 5 .

 5 Figure 5.11: Comparison of present-day pore pressures and temperatures in well 1-BP-3-APS with 1D outputs from the 2D model simulations: (a) pressure profile calculated for well 1-BP-3-APS (in grey) using the method of Eaton (1975) compared with 2D model output at the same site (in green); (b) temperatures measured in well 1-BP-3APS compared with 2D model output.

(

  1984) gave a range of 70-150°. The temperature range of 70-175°C modelled along section A-B over the last 8 Ma is shown in Fig.10. Most of the section is dominated by mudstones, including the detachment level and the overlying sedimentary succession. The modelled temperature history clearly supports smectite-illite transition within the Amazon Fan throughout its growth and at the present-day (Fig. 5.10). However, the window of pore water generation resulting from this clay mineral transformation is found to have mainly remained above the detachment surface from 8 Ma to the present-day (Fig. 5.10).

Figure 5 .

 5 Figure 5.13: Mass of gas expelled over the last 100 Ma from OAE1d (in blue), OAE2 (in green) and OAE3 (in yellow) source rocks levels (see Fig. 5.2). The curves were extracted at locations along section A-B for each OAE source rock, and grouped to obtain a gas expulsion range. Almost all gas generated was expelled before the onset of deposition of the Amazon Fan around 8 Ma ago.

Figure 5 .

 5 Figure 5.14: Cross-plot of density vs compressional velocity for well 1-BP3-APS, applied to shale/mudstones compaction/unloading behavior. Loading is identified when velocity and density increase proportionally (tendency line in brown), and unloading when velocity decreases and density remains constant (tendency line in violet).

Figure 5 . 15 :

 515 Figure 5.15: Synthesis of deformation over the last 8 Ma along section A-B obtained from mechanical modeling using OptumG2 (A) and the associated evolution of the ratio of fluid pore pressure (λ) along the indicated faults and the detachment (B). To calculate λ, we fixed the friction of faults and detachment to a standard value for clay (𝜆 = 0.4, Di Toro et al., 2011).

Figure 5 .

 5 Figure 5.16: 2D Normalized virtual velocities in horizontal and vertical directions obtained from OptumG2, from 8 Ma to 0.8 Ma: (1) 8 Ma, F1 and F2 are active and propagate up to C1, C2 and C3. For the simulation here shown 𝑔 = 0.94. (2) 1.6 Ma, F3 and F4 are active and propagate up to C1 (𝑔 = 0.98). (3) 1.2 Ma, F3 and F4 are active and propagate only C1 (𝑔 = 0.86). (4) 0.8 Ma, F4 active and propagate up to C1, C2 and C3 (𝑔 = 0.94).

Figure 5 .

 5 Figure 5.17: 2D Normalized virtual velocities in horizontal and vertical directions obtained from OptumG2, from 0.4 Ma to 0 Ma. Two alternatives are considered at 0.4 Ma: (1) F4 active and propagation to C1, C2 and C3 (𝑔 = 0.95); (2) F5 active with propagation to C1 (𝑔 = 0.96); (3) 0 Ma, F4 and C1 active (𝑔 = 0.89).

Figure 5 .

 5 Figure 5.18: Synthesis of modeling results along section A-B, grouped into four main stages: (a) 8 Ma, (b) from 5.5 -3.7Ma, (c) from 2-1.2Ma and (d) from 0.8 to the present-day. Colors indicate the distribution of source rocks and the migration and entrapment of hydrocarbons generated from them beneath the fan, and the distribution of overpressure and the bounds of the smectite-to-illite transition window within the fan. Detachment and fault activities related to gravitational deformation are indicated at each stage (active red, inactive in black) over time, associated evolution of the ratio of fluid pore pressure (λ, graph in blue). The seaward migration of the shelf break (SB) is indicated, and the inset maps at right indicate the changing directions of sediment supply to the Amazon Fan.

Figure 5 .

 5 Figure 5.A1: Schematic plots showing: (1) left track, acoustic slowness from 1-BP-3-APS well, the normal compaction trend (𝑁𝐶𝑇) of the transit time in the normal pressure condition (∆𝑡 𝑛 ) and pore

  For shales, claystones and mudstones, we relied on the[START_REF] Horsrud | Estimating mechanical properties of shale from empirical correlations[END_REF] correlation defined as: 𝑈𝐶𝑆 = 1.35 * 𝑝𝑜𝑤(𝑉 𝑝 , 2.6) * 1.80, 𝑈𝐶𝑆 in MPa (B.5) For sandstones, we used the McNally equation (McNally, 1987): 𝑈𝐶𝑆 = 185165 * 𝑒𝑥𝑝(-0.037 * ∆𝑡), 𝑈𝐶𝑆 in psi. (B.6)

Figure 6 .

 6 Figure 6.2: Methodology applied to seismic and well data to investigate the evolution of pore fluid pressure in the Amazon Fan, simplified from Souza et al., 2020.

Figure 6 .

 6 Figure 6.3: Stratigraphic interpretation of the CD seismic profile. The sedimentary succession includes three main intervals: (1) from Lower Aptian to Upper Cretaceous (120 Ma, 102 Ma, 100.35 Ma, 100 Ma, 93.5 Ma, 83.6 Ma, c.81Ma?), delimited at the top by the uppermost of three detachment surfaces (dark blue horizon, c. 69.7 -70? Ma); (2) from Upper Cretaceous to Middle Miocene (66 Ma, 38 Ma, 33.5 Ma, 28 Ma, 24 Ma, 8 Ma); (3) from Middle Miocene to presentday (5.5 Ma, 3.7 Ma, 2 Ma and seabed), which corresponds to the Amazon Fan. The gravity tectonics system identified along the section is characterized by listric normal faults beneath the shelf and slope (F1 to F6) and a compressional domain to seaward (duplexes and thrust faults, C1 to C3), both rooted on a detachment surface within the Upper Cretaceous (c. 69.7 -70? Ma). The lower of two Upper Cretaceous detachment surfaces (c.81Ma?) is associated with outermost thrust fault (C4). The slope break is located between F4 and F5.

Figure 6 . 4 :

 64 Figure 6.4: Lithofacies model along seismic profile C-D (location in Fig.1), based on information from regional wells and the literature. Shown at top are the projected locations of wells 1-APS-36-AP, 1-APS31A-AP, 1-APS52-AP, 1-APS-45B-AP, 1-APS-22-AP, 1-BP-2-APS as well as of pseudo-wells used for the time-depth conversion.

Figure 6 .

 6 Figure 6.5: Present-day crustal model across the NW compartment used to build the thermal scenario for section C-D. The Moho profile as identified by Rodger (2008) and Watts et al. (2009), shown togetherwith their interpretation of continental to oceanic crustal types (arrows in blue). For our modeling, we used the top basement and the limit of oceanic crust as interpreted along the seismic section C-D (arrows in red).

Figure 6 . 6 :

 66 Figure 6.6: 2D structural restoration along section C-D from 8 Ma to the present day (output from KronosFlow). Active faults are shown in red, inactive in black.

Figure 6 . 7 :

 67 Figure 6.7: Results of pore pressure and temperature evolution modeling along section C-D from 8 Ma to the present-day (outputs from TemisFlow). Active faults are shown in red, inactive in black. Top left: Overpressure. Top right: Temperature results calibrated in terms of the clay diagenesis window (70-175 °C). Bottom left and right: hydrocarbon mass of gas accumulated by primary and secondary cracking.

Figure 6 .

 6 Figure 6.8: Comparison of present-day temperatures in wells located on outer shelf (1-APS-31A, 1-APS-52, 1-APS-45B) and Upper Fan (1-BP2-APS) with 1D outputs from the 2D model simulation: temperatures measured in wells compared with 2D model output (dotted lines in dark blue and brown).

Figure 6 .

 6 Figure 6.9: Mass of gas expelled over the last 100 Ma from OAE1d (in blue, c.100.5 -101.1? Ma), OAE2 (in green, 93.5 Ma) and OAE3 (in yellow, 83.6 Ma) (see Fig.2). The curves were extracted at locations along section C-D for each OAE source rock, and grouped to obtain a gas expulsion range. The expelled gas window for OAE1d lies between c. 100-70 Ma. The OAE2 and OAE3, the gas generated has been expelled since 70-65Ma and these source rocks levels seem to be still expelling after the deposition of the Amazon Fan.

Figure 6 .

 6 Figure 6.10: (A) Synthesis of deformation over the last 8 Ma along section C-D obtained from mechanical modeling using OptumG2 (A) and (B) the associated evolution of the ratio of fluid pore pressure (λ) along the indicated faults and the detachment. To calculate λ, we fixed the friction of faults and detachment to a standard value for clay (𝜆 = 0.4, Di Toro et al., 2011).

Figure 6 .

 6 Figure 6.12: 2D Normalized virtual velocities in horizontal and vertical directions obtained from OptumG2, from 2 Ma to 0.4 Ma: (1) 2 Ma, F2 is active and propagate up to C1/C2. For the simulation here shown 𝑔 = 0.94. (2) 2 Ma, F2 is active and propagate up to C3/C4 (𝑔 = 0.96). (3) 0.4 Ma, F3 is active and propagate up to C1/C2 and C3/C4 (𝑔 = 0.97). (4) 0.4 Ma, F4 and F3 are active and propagate up C3/C4 (𝑔 = 0.96).

Figure 6 .

 6 Figure 6.12: 2D Normalized virtual velocities in horizontal and vertical directions obtained from OptumG2, at present day: F3 and F4 are active and propagate up to C1/C2 and C3/C4. For the simulation here shown 𝑔 = 0.98.

  Mechanisms of inflationary overpressure, notably smectite-illite transformation and hydrocarbon generation, were also active during the growth of the Amazon fan, but were of secondary importance. In the SE compartment, model results indicate the smectite-illite temperature window was within the succession of the Amazon fan, but mainly above the detachment surface (c.69.7 Ma, Fig.7.1, left). However, in the NW compartment, the smectiteillite temperature window was lower relative to the detachment surface and in part positioned on it over time (Fig.7.1, right). On both compartments, this overpressure mechanism has undoubtedly an important role on pore pressure evolution and gravity tectonics, but as a secondary contribution.Hydrocarbon of thermogenic origin generated beneath the Amazon Fan has had differing consequences in the SE and NW compartments. In the SE compartment, the results of 2D basin modeling show that thermogenic gas accumulated in layers beneath the detachment surface(c.69.7 Ma, Fig 7.1) and did not play an important role in gravity tectonics.

Figure 7 . 1 :

 71 Figure 7.1: Synthesis and comparison of modeling results along sections A-B and C-D, grouped into four main stages: (a) 8 Ma, (b) from 5.5 to 3.7 Ma (A-B) and 5.5Ma (C-D), (c) from 2 to 1.2 Ma (A-B) and 3.7-2Ma (C-D) and (d) from 0.8 to the present-day (A-B) and from 1.6 to present-day (C-D). Colors indicate the distribution of source rocks and the migration and entrapment of hydrocarbons generated from them beneath the fan, and the distribution of overpressure and the extent of the smectite-to-illite transition window within the fan. Detachment and fault activities related to gravitational deformation are indicated at each stage (active red, inactive in black) over time, associated evolution of the ratio of fluid pore pressure (λ, graph in blue). The seaward migration of the shelf break (SB) is indicated, and the inset maps at right indicate the changing directions of sediment supply to the Amazon Fan.

  7.2). The OCT is also observed on sections A-B and C-D, although the quality of the seismic data is quite poor. The crustal model of[START_REF] Rodger | A Marine Geophysical Study of the Amazon Continental Margin, North Eastern Brazil[END_REF] and[START_REF] Watts | Seismic structure, gravity anomalies and flexure of the Amazon continental margin, NE Brazil[END_REF] brought important information for temperature modeling, suggesting the Amazon Fan was deposited on post-rift cooling crust and the mantle was replaced beneath the extended continental crust. They also concluded that the Amazon Fan was emplaced on thin crust (oceanic-continental transition), supported by a strong mantle, based on the sedimentary load and a flexure model.The 2D seismic interpretations of sections A-B and C-D across the SE and NW compartments (Fig.7.2) indicate a difference between the two areas: the post-rift sedimentary record of the SE compartment was deposited mainly above continental crust and the OCT, whereas in the NW compartment the post-rift succession was mainly deposited above oceanic crust (Fig.7.2). Comparing the three sections (A-B, X-Y and C-D, Fig.7.2), which cover distances of up to 200km in the strike direction and 100 km in the dip direction, they cross at least three distinct crustal types, including abrupt changes in crustal thickness.

Figure 7 .

 7 Figure 7.3 shows a plot of basal heat flow over time output from the thermal modelling

  7.2).Over the last c.40Ma, basal heat flow decreased in both compartments (c.50mW/m² in the SE, c.40mW/m² in the NW, Fig.7.3C), with a more pronounced decrease in the last 8Ma during deposition of the Amazon Fan. Rapid sedimentation associated with the percolation of water in the pores causes a blanketing effect, which accounts for the decline of heat flow until the present-day (Fig.7.3C). In terms of overpressure mechanisms, in the SE compartment the mass of gas from OAE source rocks was already completely expelled (see Fig.5.13), while in

Figure 7 . 2 :

 72 Figure 7.2: Synthesis of the crust model built for sections A-B (SE compartment) and C-D (NW compartment), and comparison with section X-Y (central-NW compartment) by Rodger (2008). At left, the three sections are plotted against the simplified structural framework of the Offshore Amazon Basin from Cruz (2018), based on an integrated analysis of seismic reflection and gravity and magnetometric data. The stars in blue and orange represent the basal heat flow extracted from NW and SE compartments, shown in the Figure 7.3.

Figure 7 . 3 :

 73 Figure 7.3: Basal Heat Flow in the NW (lines in blue) and SE (lines in orange) compartments, calculated from 2D thermal modeling. Boxes A, B and C correspond to the late syn-rift, early post-rift and late post-rift (Amazon fan) phases, respectively. Note higher initial heat flow in the NW compartment, versus higher late post-rift heat flow in the SW compartment with a sharp decrease due to thermal blanketing in response to the growth of the Amazon fan.

Table 3 .

 3 2: Summary of data available from exploration wells (see Fig.3.15) on total organic carbon (TOC), vitrinite reflectance (Ro), the source rocks levels drilled and hydrocarbon gas potential. Remarkable biogenic gas potential in Amazon Fan series (Tucunaré, Pirarucu and Orange Fm.).

		Wells	Average TOC (%)	Ro (/)	Source rocks levels drilled -Limoeiro Fm Biogenic Gas Potential -Amazon Fan series
		1-APS-09	0,35	0,86	x
	SE	1-APS-14 1-APS-41	0,37 0,3	0,61	x x
		1-APS-51A	0,58	0,35	x
		1-APS-05	0,3	0,86	x
		1-APS-18	3,01		x
		1-APS-25	0,32		x
		1-APS-27A	0,49	0,48	x
	NW	1-APS-29	3	0,69	x
		1-APS-31A	1,17	0,78	x
		1-APS-36	0,36	0,41	x
		1-APS-44A	1,93	0,7	x
		1-APS-45B	0,75	0,79	x

Table 4 .

 4 1: Summary of wells available for the study within the SE and NW compartments, and the data they provide on direct pressure measurements, temperature, total organic carbon (TOC), vitrinite reflectance and overpressures levels.

	Wells	Pressure Temperature	TOC	Vitrinite Reflactance Overpressure Level

Table 4 .

 4 2: Summary of input parameters for modeling along sections A-B (SE compartment) and C-D (NW compartment).

	Section A-B	Parameters	Section C-D
		Source Rocks
	√	OAE 1d, 2 and 3 (based on Erbacher et al., 2000 and Boulia et al, 2020)	√
	8%	Original TOC (based on Erbacher et al., 2000)	8%
	50m each	Assumed thickness (based on wells and

Table 5 .

 5 1: Well data set available for this paper.

	Wells	Water depth (mbsl)	Total depth (mbsl)	Composite log	Sonic	Check shots	Gamma-ray
									mud weight, temperature,
	1BP-3-AP	754	5625	√	√		√	√	LOTs
	Site 931	3475	421						porosities
	1-APS-01-AP	21	4812.8	√	√
	1-APS-04-AP	87	4917	√	√
	1-APS-05-AP	88	4354	√	√
	1-APS-06A-AP	84.7	4714.6	√	√
	1-APS-07-AP	74	4630	√	√
	1-APS-11-AP	80	3931	√	√
	1-APS-12-AP	91	4407	√	√	√
	1-APS-14-AP	88.6	5000	√	√
	1-APS-17-AP	81.4	5092	√	√	√
	1-APS-18-AP	84	3987	√	√	√
	1-APS-20B-AP	144.5	3542	√	√
	1-APS-22-AP	107	3704	√	√
	1-APS-24-AP	22	4847	√	√
	1-APS-25-AP	73	4400	√	√	√
	1-APS-27A-AP	95	2415.5	√	√	√
	1-APS-29-AP	98.7	4592	√	√
	1-APS-30-AP	125.6	3736.5	√	√	√
	1-APS-32-AP	96	3969	√	√	√
	1-APS-35-AP	80.7	3998	√	√	√
	1-APS-36-AP	89	4673	√	√
	1-APS-41-AP	36	4211	√	√
	1-APS-44A-AP	140	4123	√	√	√
	1-APS-45B-AP	104.7	4432	√	√	√
	1-APS-47B-AP	19.8	4485	√	√
	1-APS-51A-AP	438	3986	√	√
	1-APS-52-AP	103.7	4698.5	√	√	√
	1-APS-53-AP	180	3572	√	√	√
	1-APS-54A-AP	115	3984	√	√	√
	1-BP-2-AP	967	5152	√	√

Acknowledgements

Acknowledgments

The authors gratefully acknowledge the support granted during the completion of this study by Petrogal-Galp Brazil SA and CNPq (Brazilian National Research Council, grant agreement N°. 201531/2016-0), in the form of PhD scholarship made to Juliana M. Gonçalves de Souza. We also acknowledge the Brazilian Agência Nacional do Petróleo, Gás Natural e Biocombustíveis (ANP) for seismic and stratigraphic data made available to Universidade Federal Fluminense (UFF) and Universidade do Estado de Rio de Janeiro (UERJ). Daniel Praeg was supported by funding from the European Union's Horizon 2020 research and innovation program under Marie Skłodowska-Curie grant agreement No. 656821, and by the Brazilian Coordination of Superior Level Staff Improvement IODP /CAPES-Brazil (Edital 38/2014). Our special thanks are also due to Beicip-Franlab, Optum Computational

Engineering and SMT Kingdom for the use of educational licenses. Finally, we would like to thank for all the suggestions and corrections made during the review process. Special thanks to MSc. Eduardo de Mio for his critical and insightful comments that much improved the quality of this paper. We also thank Alexandre Lethiers for diligently drafting all the figures included in this paper.

muito obrigada to Alexandre Lethiers for diligently drafting all the figures included in this manuscript. I gratefully acknowledge to the partnership CNPq (Brazilian National Research Council) and Petrogral-Galp, for the financial support of this research, in form of PhD grant.

Boulila, S., Brange, C., Cruz, A. M., Laskar, J., Gorini, C., Reis, T. and Silva, C. G., 2020. Astronomical pacing of Late Cretaceous third-and second-order sea-level sequences in the Foz do Software: JewelSuite Geomechanics (Baker Hughes) https://www.bhge.com/digital/field-planning-optimization/jewelsuite-geomechanics OpenFlow Suite 2019 http://www.beicip.com/openflow-suite-2019 OptumG2 2018 https://optumce.com/products/optumg2/

The authors suspect that the strong peak in Well 44 may match the equivalent OAE2 (c. 94.17 Ma) interval in Well 29, and they assumed the GR peak in Well 29 match the OAE1d (c.100.35Ma). From Boulila et al., (2020).

Chapter IV Data and Methods

__________________________________________________________________________

Chapter V Overpressure evolution in the SE compartment of the Amazon deep-sea fan

__________________________________________________________________________

Abstract

The Amazon Fan provides a natural laboratory to study the generation of overpressure, due to rapid late Cenozoic burial that has resulted in gravitational collapse above shale detachments. Here we examine collapse systems for the first time using the techniques of petroleum systems analysis. We propose an integrated methodology based on numerical modeling constrained by the structural restoration of a seismic profile across the southwestern fan. The results provide information on the evolution of pore pressure and temperature and their implications for the operation of the detachment and overlying extensional and compressional faults during the deposition of up to 6 km of sediment over the last 8 Ma. The modeled thermal history implies that fluid release by smectite-to-illite transformation has taken place within the thickening sedimentary succession, but has not significantly contributed to pore pressures along the detachment. Modeling of hydrocarbon generation and migration from source rocks beneath the fan indicates gas accumulated in successions at depths of 10 2 -10 3 m beneath the detachment without influencing pore pressures along it. In contrast, model results indicate that overpressures have varied in response to disequilibrium compaction. Fault activity within the collapse system took place during phases of higher sedimentation rates, and ceased from 5.5-3.7 Ma when sediment supply to the SE fan decreased. From 2 Ma, renewed sediment flux and shelf-slope progradation led to a basinward migration both of overpressure along the detachment and of fault activity above it. We conclude that gravity tectonics in the Amazon Fan over the last 8 Ma have been mainly controlled by overpressures due to disequilibrium compaction, with secondary contributions from clay mineral transformation.

Present-day pressure conditions show that the southeastern Amazon Fan is not at equilibrium and gravity driven deformation could occur at any time.

Keywords: overpressure evolution, overpressure mechanism, gravitational collapse, basin and mechanical modeling, Amazon Fan

V.3.1 Well data

The study uses data from 32 industry wells on the continental shelf and upper slope and from Ocean Drilling Program (ODP) site 931 on the middle slope (Fig. 5.1, Table 5.1).

From all wells, composite logs, sonic profiles and check shot data were used as input to the interpretation and time-depth conversion of seismic data (Table 5.1; see below). Additional data from two wells, 1-BP-3-APS and 1-APS-31A-AP (Fig. 5.1, Table 5.1), were used as input to geomechanical and basin modeling: density and gamma ray logs were used from both wells, while from 1-BP-3-APS we also used the resistivity and calibration data including direct formation pressure and temperature measurements, leak-off tests (LOTs) and well markers.

In addition, porosity data from ODP site 931 (Table 5.1;Flood at al., 1995) were used as input to 2D basin modeling.

Table 5.2: Interval velocities attributed to pseudo-wells and to well 1-BP-3-APS as control on the timedepth conversion of section A-B (see Fig. 5.5). Velocities for ages younger than 69.7Ma were based on sonic log and check shots from industry wells; for 69.7-120Ma interval age, velocities were based on [START_REF] Watts | Seismic structure, gravity anomalies and flexure of the Amazon continental margin, NE Brazil[END_REF].

V.3.4 2D Structural Restoration

The depth-converted profile was restored using KronosFlow (version 2019), a software package developed by IFPen (Institut Français du Petrole et Energies Nouvelles). The structural evolution is reconstructed based on the relation between faults and sediment thicknesses. Different geological scenarios are built step-by-step and used to simulate the evolution of physical properties (in our case, pressure, temperature and hydrocarbon generation) with a robust calculator called Arctem [START_REF] Callies | A new kinematic tool for petroleum system modeling in complex structural settings: Application to the foorhills region of Kurdistan[END_REF]. Each reconstruction step involves sequential 2D fault displacements associated with material deformation and mechanical decompaction. The best fitting scenario is finally chosen. A coherent mesh through time is built at each step, on which petroleum system modeling is done.

A 2D starting model in KronosFlow requires an initial geometry in depth and an absolute age for each horizon. We interpreted seismic section A-B starting from the stratigraphic

V.5.2 2D basin modeling of pressure and temperature evolution

The results of modeling pore pressure and temperature evolution along section A-B over the last 8 Ma are shown together in Fig. 5.10.

The pore pressure evolution is shown on the left of Fig. 5.10 (active faults are shown in red). At 8 Ma, we do not observe significant overpressure, with most values lower than 10 MPa. Normal faults F1 and F2 and the compressive domain are active (Fig. 5.10).

Overpressure increases from 5.5 Ma to 3.7 Ma, but no deformation is observed. Sedimentation rates increase in the fan from 2 Ma, and overpressure increases to 10-20 MPa (Fig. 5.10).

From 1.6 Ma to 0.8 Ma, overpressure propagates within the sediment column and along the detachment, increasing significantly (10-40 MPa) between faults F3 and F4 (Fig. 5.10) and near the detachment. Faults F3 and F4 and the compressive domain are active (shown in red in Fig. 5.10). From 0.4 Ma to the present-day, overpressure is well-developed from the extensional domain to the front of the section, and is seen to be compartmentalized by active faults and along the detachment.

The results obtained from 2D thermal modeling are shown on the right of Fig. 5.10, calibrated in terms of two temperature-dependent processes that represent potential mechanisms for overpressure generation (see next section). The smectite-to-illite temperature window is seen to remain above the detachment throughout the simulation (Fig. 5.10). Also shown is the total quantity of oil and gas accumulated over time (hydrocarbon saturation) following migration from three OAE source rocks (see Fig. 5.2), which is seen to be trapped along thickness against faults located bellow the detachment (Fig. 5.10). These results depend on the modeled thermal history of the area, which is not well-constrained. However, a comparison between bottom hole temperatures measured in well 1-BP-3-APS at around 5.5 km depth and a 1D output form the 2D temperature simulation for the present-day yields a reasonable fit (Fig. 5.11b), suggesting a good calibration. migrate upward to the detachment level or into overlying successions it if there were structures present that breach the detachment. Thus, from 8 Ma to the present-day, our results provide no evidence of gas migration along the detachment (Fig. 5.12).

Our modeling results provide evidence of a downslope migration of the deformation caused by detachment overpressure during the progradation of the Amazon Fan during the last 8 Ma (Fig. 5.16, 5.17). We argue that disequilibrium compaction during the accumulation of up to 6 km of sediment has been the predominant overpressure mechanism along the detachment throughout the deposition of the Amazon Fan.

Sediment supply to the SE compartment of the Amazon fan decreased due to the migration of the sediments to the NW from 5.5 Ma, and deformation rates reduced to a minimum (Fig. 5.18b). From 2 Ma, when the sedimentation rate became massive, the basinward migration of the rapidly accumulating sediment load again induced disequilibrium compaction mechanisms. The migration of overpressure along the detachment led to high deformation rates.

At the present day, the SE compartment of the Amazon Fan continues to be characterized by some overpressure (Fig. 5.18d). According to our analysis of well 1-BP-3-APS, a pore pressure increase takes place below 3500 m and to at least 5500 m depth based on the method of Eaton (Fig. 5.7), while the method of Bowers points to fluid expansion mechanisms. Since no gas has been found in this interval, we favor clay mineral transformation, consistent with the depth of the smectite-to-illite temperature window (1500-6000 meters). Biogenic gas might be acting as a present-day overpressure mechanism within the Amazon Fan succession. This gas generation could be related to a tertiary marine deltaic source rocks (type II organic matter), related to supply from Amazon Fan, described by [START_REF] Mello | Foz do Amazonas Area: the last frontier for elephant hydrocarbon accumulation in the South Atlantic realm[END_REF]. Geochemical modeling applied to ODP site 938 at 2801.01 m water depth on the mid-fan indicates that sedimentation rates and organic carbon content within the upper 400 m Appendix A -Pore pressure estimation from 1D Geomechanical model Here we detail the methods of [START_REF] Eaton | The Equation for Geopressure Prediction from Well Logs[END_REF] and [START_REF] Bowers | Pore Pressure Estimation from Velocity Data; Accounting for Overpressure Mechanisms Besides Undercompaction[END_REF][START_REF] Bowers | Determining an appropriate pore-pressure estimation strategy[END_REF] and their application to estimating present-day pore pressures in well 1-BP-3-APS (Fig. 5.6). The two methods are based on different assumptions. Eaton's method assumes disequilibrium compaction (undercompaction) to be the main overpressure mechanism. However, with the transition zone into the overpressure zone in the BP-3-AP, the normal compaction trend (NCT), which means the trend of travel time property plotted as function of depth through the normal pressure data, shows the existence of deflection data trend (from 80µs/m to 150µs/m, see fig.

5.A1), representing possible overpressure due to fluid expansion mechanisms. Bower's method (1995,2001) allows capturing the depth at which such fluid expansion mechanisms take place. We thus. used this approach to evaluate the present-day pore pressure considering the mineral transformation, i.e., clay transformation and presence of gas. Density-acoustic crossplot from 1-BP-3-APS was used to identify unloading and shows clear unloading response.

A.1 -Eaton's Method

The [START_REF] Eaton | The Equation for Geopressure Prediction from Well Logs[END_REF] approach is used to estimate abnormal pore pressure caused by compaction (disequilibrium compaction or under-compaction). Such compaction induces insitu physical properties changes (compaction, porosity, and effective stress changes) and variations of resistivity or transit time from the expected trend line are interpreted as overpressure [START_REF] Eaton | The effect of overburden stress on geopressure prediction from well logs[END_REF].

In this study, raw resistivity data was not available due to commercial sensitivity, we thus used sonic log data (Fig. 5.A1). The method includes a fudge factor, the Eaton coefficient, which enables the user to alter the trend line. An Eaton coefficient of 3.0 was established in the Gulf of Mexico [START_REF] Eaton | The Equation for Geopressure Prediction from Well Logs[END_REF], and most trend-line pore pressure methods are variations of the method of [START_REF] Eaton | The Equation for Geopressure Prediction from Well Logs[END_REF] with locally calibrated coefficients. The coefficient 3.0 was not

Appendix B -2D mechanical model set-up

In this section, we present the mechanical rock properties estimated using the available log data from wells 1-BP-3-APS and 1-APS-31A-APS. These properties include internal friction angle and cohesion. Rock mechanical laboratory testing on core samples are the most accurate methods for estimation of rock strength, but they never can lead to a continuous profile of rock strength [START_REF] Rabe | Development of a New Correlation to Estimate the Unconfined Compressive Strength of a Chicontepec Formation[END_REF][START_REF] Rabe | Coupled fluid flow and geomechanics: a case study in Faja del Orinoco[END_REF][START_REF] Chang | Empirical relations between rock strength and physical properties in sedimentary rocks[END_REF] along wellbore and the tests were not executed on the study area. These parameters can be estimated using borehole as a continues source of rock strength to estimate internal friction and cohesion by using correlations of proposed in the literature based on acoustic data, that reflects the effect of lithology, porosity and fluid content.

B.1 -Internal friction angle

To calculate the Internal Friction Coefficient from sonic log, we used the 𝐿𝑎𝑙 𝑉 𝑝 correlation applicable to all types of lithology [START_REF] Lal | Shale stability: drilling fluid interaction and shale strength[END_REF]:

where 𝑉 𝑝 is compressional velocity in 𝑘𝑚/𝑠. Results are given in Table Supplementary material.

B.2 -Pore fluid pressure

The pore fluid pressure is taken into account through effective frictions defined as: structures can be divided into NW and SE structural compartments, as defined by [START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF] and Oliveira et al (2005). Grey lines show the available regional dataset of multi-channel seismic reflection profiles, dots show industry wells and ODP/DSDP drill sites. In this chapter, we use seismic profile C-D across the NW compartment; in Chapter VII, the results are compared with those obtained along section A-B across the SE compartment.

VI.2 MODEL SET-UP AND CALIBRATION FROM WELL DATA

The study is based on the interpretation of 2D seismic reflection data along a profile 275 km long, here referred to as section C-D, calibrated with reference to 7 industry wells as a calibration data (Fig. 6.1). The methods used (Fig. 6.2) are identical to those detailed in Chapters IV and V [START_REF] Souza | Controls on overpressure evolution during the gravitational collapse of the Amazon deep-sea fan[END_REF]).

Here, we address the setting-up and calibration of the models used to analyse section C-D based on information obtained from wells (see Fig. 6.1). We first present the stratigraphic and structural interpretation of the seismic profile (Fig. 6.3), followed by the construction of a model of lithofacies distribution along the section (Fig. 6.4) and its conversion from travel-time to depth (Table 6.1, Fig 6.4). For the last, the approach applied was the layer-cake method, which is well detailed on the chapert V, section V.3.3.

Because of limited information from wells in the NW compartment on sediment porosities and permeabilities, these input data were obtained from well 1-BP-3-APS located in the SE compartment (Fig. 6.1). Mechanical parameters were obtained and extrapolated from 1-APS-31A-AP and 1-BP-3-APS (Fig. 6.1).

VI.2.1 2D Seismic interpretation (TWTT)

The interpretation of stratigraphic units and structures along seismic profile C-D (Fig.

6

.1) is presented in Fig. 6.3. The interpreted succession is simplified to accommodate limitations of seismic data quality and structural complexity and well control (Fig. 6.3).

We recognize 16 horizons which, together with the seafloor, define 17 stratigraphic units younger than 120 Ma. The units are offset by extensional and compressional faults that root on two different detachment surfaces within Upper Cretaceous strata. The upper of these two detachments (Fig. 6.3, dark blue horizon, c. 69.7 -70 Ma ?) is considered a regional detachment [START_REF] Silva | Cenozoic tectonics of Amazon Mouth Basin[END_REF][START_REF] Perovano | O processo de colapso gravitacional da secao marinha da Bacia da Foz do Amazonas -Margem Equatorial Brasileira[END_REF], 2011[START_REF] Souza | Controls on overpressure evolution during the gravitational collapse of the Amazon deep-sea fan[END_REF]) and probably

VI.2.3 2D basin modeling set-up

VI.2.3.1 Boundary conditions

To build our 2D model, we use the same method as in Chapter VII [START_REF] Souza | Controls on overpressure evolution during the gravitational collapse of the Amazon deep-sea fan[END_REF].

First, we built a crustal model using the Moho profile (from [START_REF] Rodger | A Marine Geophysical Study of the Amazon Continental Margin, North Eastern Brazil[END_REF][START_REF] Watts | Seismic structure, gravity anomalies and flexure of the Amazon continental margin, NE Brazil[END_REF] and the top of the basement interpreted from seismic line C-D (Fig. 6.5), together used to calculate a stretching factor [START_REF] Mckenzie | Some remarks on the development of sedimentary basins[END_REF] based on an assumed initial crust geometry (upper crust 20km thick and lower crust 12km thick, Fig. 6.5). We then built the thermal history, using the heat flow profiles obtained from the crustal model together with paleosurface temperatures, based on paleobathymetries at depth derived from [START_REF] Cardoso | Caracterizacao do campo térmico atual e evoluçao téermica da Plataforma Continental do Estado do Rio de Janeiro[END_REF] and [START_REF] Gutierrez | Campo Termico da Crosta na Regiao Sedimentar Bahia Sul e na Area Pre-Cambriana adjacente: Reavaliaçao com base em dados corrigidos da Margem Continental[END_REF], and on paleosurface temperatures at sea level (0 m) from [START_REF] Friedrich | Evolution of middle to Late Cretaceous oceans -A 55 m.y. record of Earth's temperature and carbon cycle[END_REF] and [START_REF] Tremblin | Eocene-Oligocene sea surface temperature and pCO2 estimates[END_REF].

The last step consists of modelling clay mineral transition and hydrocarbon gas generation along the section. We assume three potential source rock levels corresponding to Ocean Anoxic Events (Fig. 6.3): (1) OAE1d, dated c. 100.5 -101.1? Ma, (2) OAE2, dated c.

Ma and

(3) OAE3, dated c. 83.6 Ma. Kerogen is defined as type II (marine carbonate environment [START_REF] Mello | Geochemical and molecular studies of the depositional environments of source rocks and their derived oils from the Brazilian marginal basins[END_REF][START_REF] Requejo | Hidrocarbon source facies in the Niger Delta petroleum province as indicated by the molecular characteristics of oil[END_REF][START_REF] Mello | Foz do Amazonas Area: the last frontier for elephant hydrocarbon accumulation in the South Atlantic realm[END_REF]. The kinetics are based on PT-beds in Aptian source rocks in Brazil [START_REF] Penteado | Study of biodegradation process along the Carnaubais trend, Potiguar basin (Brazil) -Part 1[END_REF]. The original total organic carbon (TOC) is defined as 8% for all levels based on [START_REF] Erbacher | [END_REF], with an average thickness of 50 m.

VI.3.1 2D restoration of growth and deformation of the NW Amazon Fan

The 2D restoration of section C-D obtained using KronosFlow is presented in Fig. 6.6.

Extensional activity (fault F1, shown in red) and a basinward propagation of compressional deformation both start at 8 Ma (Fig. 6.6). During this phase, carbonate sedimentation predominates on the shelf [START_REF] Cruz | Neogene evolution and demise of the Amapá carbonate platform, Amazon continental margin, Brazil[END_REF]. From 5.5 Ma, siliciclastic supply increases in the NW compartment, marking the onset of Amazon fan growth, and the progressive burial of carbonates on the inner NW shelf [START_REF] Cruz | Neogene evolution and demise of the Amapá carbonate platform, Amazon continental margin, Brazil[END_REF]. The first evidence of forward propagation of the extensional domain is the abandonment of F1 and the activation of fault F2 at 5.5 Ma (Fig. 6.6, in red). At the same time, forward propagation is observed in the outer compressional deformation with the activation of faults C3 and C4 (Fig. 6.6, in red). This thrust-fold remains active from 3.7 Ma to 2 Ma, with short pulses of deformation on C1 and C2 associated with extensional activity on F2 (Fig. 6.6, in red), and a continuous activity of C3 and C4 (Fig. 6.6, in red). From 1.6 Ma to present, the forward propagation of extensional deformation intensifies, with activity on extensional faults F3 and F4 as well as on all the compressional faults C1, C2, C3 and C4 (Fig. 6.6, shown in red). F1 and F2 are no longer active. As a result of compressional deformation, the subsurface fold propagation faults deform the layers up to seafloor (Fig. 6.6) in a relief that persist to the present-day (Fig 5 and 6.6). At present, only normal faults reach the seafloor although without deforming it (F3 and F4, shown in red on Fig.

6.6).

VI.3.3 Evolution of deformation along the detachment and faults

We used the OptumG2 software to constrain the evolution of pore pressure along the detachment and faults in section C-D, using the same procedure as in Chapter VII [START_REF] Souza | Controls on overpressure evolution during the gravitational collapse of the Amazon deep-sea fan[END_REF]. We ran a series of trial and error tests, and selected the effective friction along the detachment and faults that best reproduced the observed deformation. We kept scenarios with a factor of safety, defined as the ratio between the load necessary to generate a collapse and the gravitational force (𝑔 = 9.8𝑚/𝑠²), lower than 1, and thus confirming the gravitational collapse.

The effective frictions were then converted in pore pressure ratio (𝜆) assuming a fixed friction for the faults and detachment of 0.4, typical of clays [START_REF] Saffer | Comparison of smectite-and illite-rich gouge frictional properties: application to the updip limit of the seismogenic zone along subduction megathrusts[END_REF].

The pore fluid pressure ratios from 8 Ma to present are synthesized in Fig. 6.10, while the vertical and horizontal movements for each model step are given in Figs 6.11, 6.12 and 6.13.

Chapter VII

Discussion -Comparison of results from the SE and NW compartments __________________________________________________________________________ In addition, an important volume of the gas generated was expelled prior to the onset of deposition of the Amazon fan (Chapter V, Fig. 5.13; [START_REF] Souza | Controls on overpressure evolution during the gravitational collapse of the Amazon deep-sea fan[END_REF]. In contrast, in the NW compartment, gas generated from the same OAE source rocks was expelled much later (see Chapter VI, Fig. 6.10) and continues to be expelled today. The 2D basin modeling results point to a greater contribution of gas by secondary cracking than by primary cracking (Chapter VI, Fig. 6.8). The gas generated has migrated and accumulated in layers beneath and along the detachment since the onset of the Amazon fan, also migrating through the F1 fault to seafloor (Fig. 7.1, at right). The contribution of thermogenic gas as an overpressure mechanism is evident and it plays a strong role in gravity tectonics.

These basin modeling reconstructions are consistent with present-day evidence of hydrocarbons in wells drilled in both compartments. In our dataset, gas was found at different levels in at least six industry wells drilled in the NW compartment (1-APS-18, 1-APS27A, 1-APS29, 1-APS-31A, 1-APS45B and 1-BP-2, see Fig. 3.15), and geochemistry reports for some of these wells (1-APS-18, 1-APS27A, 1-APS29, 1-APS-31A, 1-APS45B, see Fig. 3.15) confirm the gas as being of thermogenic origin. These wells are located within 90 km of section C-D.

In contrast, in the SE compartment, only well 1-APS-51A (see Fig. 3.15) found thermogenic gas.

Gas has been suggested as an overpressure mechanism within the Amazon fan by several authors [START_REF] Silva | Cenozoic tectonics of Amazon Mouth Basin[END_REF][START_REF] Perovano | Modelagem fisica experimental de mecanismos de dedormação gravitacional simulando multiplos intervalos superpressurizados : Apliacação à Bacia da Foz do Amazonas[END_REF][START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF]. In the NW compartment, [START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF] suggested hydrocarbon gas from thermogenic origin, and they proposed timing of gas expulsion from Cenomanian-Turonian source rocks (equivalent to OAE2) around 6Ma. My results pointed out the beginning of gas expulsion of OAE2 from c.

70-65Ma and still expelling after the onset of the Amazon Fan (see Fig. 6.9). In both cases, the results confirm the presence of thermogenic gas in the NW fan. However, the timing suggested by [START_REF] Cobbold | Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation[END_REF] seems to be doubtful due they did not present how these results were achieved.

VII.1.3

Effect of the thermal structure of the crust on overpressure mechanisms

The differences between the SE and NW compartments discussed above invite a broader reflection on overpressure mechanisms within the Amazon: what could cause the observed spatial variations? Here I present a hypothesis that spatial variations in thermal history of the Foz do Amazonas basin may play a key role in the style of gravity tectonics.

The thermal history of a basin affects not only hydrocarbon generation, but also the smectite-illite transformation, both of which are temperature-dependent processes. The thermal history of the Foz do Amazonas basin remains poorly constrained, due to limited information on the composition and geometry of the crust, which impacts on input parameters for crustal stretching (beta factor) and radiogenic heat generation, as well as other calibration data (temperature, vitrinite reflectance).

The few published works on the Foz do Amazonas basin shed light on : (1) how the crust evolved during rifting and break-up of the Brazilian Equatorial margin, based on 3D gravity anomaly inversion (Kusznir et al., 2018); and (2) the current state of the crust, based on 2D seismic refraction data and gravity modeling (Fig. 7.2, [START_REF] Rodger | A Marine Geophysical Study of the Amazon Continental Margin, North Eastern Brazil[END_REF][START_REF] Watts | Seismic structure, gravity anomalies and flexure of the Amazon continental margin, NE Brazil[END_REF]. [START_REF] Cruz | Integrated geological and geophysical studies applied to understanding the evolution of the Offshore Amazon Basin[END_REF] argued that post-rift sedimentation along the margin was controlled by distinct tectonic domains, which he identified using seismic reflection profiles and gravity and magnetic data. He proposed three sub-basins within the Offshore Amazon Basin: Cassiporé, Araguari and Machadinho (Fig. 7.2, at left).

SB

Chapter VIII

Conclusions and Perspectives

__________________________________________________________________________

VIII CONCLUSIONS AND PERSPECTIVES

The research undertaken for this thesis proposed to investigate the evolution of pore pressure during growth and gravitational collapse of the Amazon Fan. To do so, I have developed an innovate integration of methods, based on modeling techniques commonly used in the oil & gas industry. As a dataset, I used the interpretation of three commercial 2D seismic reflection profiles across the shelf and slope, in order to build two sections across the southeast (A-B) and northwest (C-D) structural compartments of the Amazon Fan gravity tectonic system.

Both sections were stratigraphically and geomechanically constrained by well data and these data were used to undertake the 2D kinematic restoration, followed by forward modeling of the pore fluid pressure and temperature over the last 8 Ma. The main conclusions of this investigation are:

 The Foz Amazonas Basin is a natural laboratory to study the evolution of pore fluid pressure over time, and to investigate the main overpressure mechanisms acting in the basin, and affecting gravity tectonics within the Amazon Fan;

 The progradation of the Amazon Fan was accompanied by the basinward migration of the deformation front, which, according to the modeling results, corresponds with a seaward migration of overpressure along the detachment surface;

 The main control on gravity tectonics was sediment supply, as reflected in episodic activity within the SE compartment where faults were active only during periods of higher sedimentation; in contrast, in the NW compartment continuous fault activity corresponds with constant sediment input over the last 8 Ma;

 Disequilibrium compaction (undercompaction) is the primary mechanism of overpressure in the Amazon Fan, driven by high rates of sedimentary input from the Amazon River that resulted in rapid growth over a relatively short time (5-10 km thickness of sediment over the last 8 Ma);

Chapter IX References __________________________________________________________________________