Kelvin I Chima

Damien Do Couto

Estelle Leroux

Silvia Gardin

Nick Hoggmascall

Marina Rabineau

Didier Granjeon

Christian Gorini

Seismic stratigraphy and depositional architecture of Neogene intraslope basins, offshore western Niger Delta

Keywords: West Africa, Niger Delta, Seismic sequence stratigraphy, Filled ponded basins, Mass-transport deposits, Turbidite systems, Chronostratigraphy. 1

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Introduction

The depositional architecture of the deep-water systems located on divergent margins is predominantly controlled by gravity tectonics (e.g. [START_REF] Doust | Divergent passive margin basins[END_REF][START_REF] Damuth | Neogene gravity tectonics and depositional processes on the deep Niger Delta continental margin[END_REF][START_REF] Cohen | Sedimentation and shale tectonics of the northwestern Niger Delta front[END_REF]Morely and Guerin, 1996;[START_REF] Graue | Mud volcanoes in deep-water Nigeria[END_REF][START_REF] Cobbold | Structural consequences of fluid overpressure and seepage forces in the outer thrust belt of the Niger Delta[END_REF].

Gravity gliding and gravity spreading are the two main aspects of gravity tectonics that deform the sedimentary wedges that prograde into deep-water environments [START_REF] Morley | Deep-water fold and thrust belt classification, tectonics, structure and hydrocarbon perspectivity[END_REF]. While gravity gliding is characterized by the rigid translation of a rock mass down a slope, gravity spreading results from flattening and lateral spreading of a rock mass under its own weight [START_REF] Ramberg | Gravity, deformation and the Earth's crust in theory, experiments and geological application 2nd Edition[END_REF]Dejong and Sholten, 1973;[START_REF] Alves | Geomorphologic features related to gravitational collapse: M A N U S C R I Psubmarine landsliding to lateral spreading on a Late MioceneeQuaternary slope (SE Crete, eastern Mediterranean)[END_REF][START_REF] Alves | Submarine slide blocks and associated soft-sediment deformation in deepwater basins: A review[END_REF]. In the Niger Delta, gravity spreading driven by delta wedge progradation above overpressured marine shales has been described as the primary control of accommodation and depositional patterns (e.g. [START_REF] Doust | Divergent passive margin basins[END_REF][START_REF] Damuth | Neogene gravity tectonics and depositional processes on the deep Niger Delta continental margin[END_REF][START_REF] Cohen | Sedimentation and shale tectonics of the northwestern Niger Delta front[END_REF]Morely and Guerin, 1996;[START_REF] Graue | Mud volcanoes in deep-water Nigeria[END_REF][START_REF] Cobbold | Structural consequences of fluid overpressure and seepage forces in the outer thrust belt of the Niger Delta[END_REF].

The slope of the Niger Delta, characterized by a lower subsidence rate than its regional sedimentation rate, has been described as comprising a 'stepped profile', compared to the 'above-grade slope profile' typical of the central Gulf of Mexico-where the rate of salt driven subsidence is always greater than that of sediment supply [START_REF] Prather | Controls on reservoir distribution, architecture ad stratigraphic trapping in slope settings[END_REF]. Consequently, 'shallow ponded basins/perched accommodation' described by Prather et al. (2012a), but herein referred to as 'filled ponded basins', dominate the intra-slope regions of the Niger Delta (Booth et al., 2000;[START_REF] Prather | Calibration and visualization of depositional process models for above-grade slopes: a case study from the Gulf of Mexico[END_REF]. These 'filled ponded basins' are dominated by healed-slope accommodation that is always overfilled or bypassed compared to the underfilled ponded basins in the central Gulf of Mexico [START_REF] Prather | Controls on reservoir distribution, architecture ad stratigraphic trapping in slope settings[END_REF]. Although the lack of ponded accommodation in the upper-and mid-slope regions of the Niger Delta renders it susceptible to erosion and bypass, thin reservoir facies associated with channel and overbank deposits in these areas could still be

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4 good drilling targets [START_REF] Prather | Controls on reservoir distribution, architecture ad stratigraphic trapping in slope settings[END_REF]. Furthermore, hydrocarbon traps are better developed in filled ponded systems, where reservoir facies occur directly below mud-rich pelagic/hemipelagic deposits [START_REF] Prather | Controls on reservoir distribution, architecture ad stratigraphic trapping in slope settings[END_REF].

Following the discovery of hydrocarbons in the deep-water Niger Delta in recent decades, exploration is now focused on understanding the control on depositional architecture and reservoir distribution in the deep-water setting of this region (e.g. [START_REF] Chapin | Integrated seismic and subsurface characterization of Bonga Field, offshore Nigeria[END_REF][START_REF] Adeogba | Transient fan architecture and depositional controls from near-surface 3D seismic data, Niger Delta continental slope[END_REF]. Consequently, the impact on submarine channel evolution and depositional patterns by mobile shale structures has received considerable attention (e.g. [START_REF] Adeogba | Transient fan architecture and depositional controls from near-surface 3D seismic data, Niger Delta continental slope[END_REF][START_REF] Olusola | Effect of Growing Structures on Stratigraphic Evolution, Channel Architecture, and Submarine Fan Distribution, Niger Delta, West Africa[END_REF][START_REF] Clark | Interactions between coeval sedimentations and deformation from the Niger Delta deep-water fold belt[END_REF]Prather et al., 2012a[START_REF] A N U S C R I P T A C C E P T E D Accepted Manuscript 34 Jobe | Rapid adjustment of submarine channelarchitecture to changes in sediment supply[END_REF][START_REF] Jolly | Growth history of fault-related folds and interaction with seabed channels in the toe-thrust region of the deep-water Niger Delta[END_REF][START_REF] Hansen | Submarine channel evolution, terrace development, and preservation of intra-channel thin-bedded turbidites: Mahin and Avon channels, offshore Nigeria[END_REF]. The depositional architecture of the Pliocene and Quaternary filled ponded basins on the western slope of the Niger Delta has already been described and includes MTDs and turbidite systems draped by hemipelagites (e.g. [START_REF] Adeogba | Transient fan architecture and depositional controls from near-surface 3D seismic data, Niger Delta continental slope[END_REF][START_REF] Olusola | Effect of Growing Structures on Stratigraphic Evolution, Channel Architecture, and Submarine Fan Distribution, Niger Delta, West Africa[END_REF]. Similar depositional architectures have been described in the deep-water fold-and-thrust belt of the Niger Delta (e.g. [START_REF] Posamentier | Seismic geomorphology and stratigraphy of depositional elements in deep-water settings[END_REF][START_REF] Clark | Interactions between coeval sedimentations and deformation from the Niger Delta deep-water fold belt[END_REF]. Recently, academic seismic-stratigraphic studies were conducted on the deeper Neogene stratigraphic records of the upper slope of the western Niger Delta, although lacking calibration [START_REF] Benjamin | Canyon-confined pockmarks on the western Niger Delta slope[END_REF][START_REF] Benjamin | Seafloor and buried mounds on the western Niger Delta Slope[END_REF]. Industry-based study of Neogene stratigraphy reveals that highly amalgamated channel fill deposits are less compartmentalized than less amalgamated overbank deposits [START_REF] Chapin | Integrated seismic and subsurface characterization of Bonga Field, offshore Nigeria[END_REF].

Despite the plethora of observations on the upper slope of the western Niger Delta, lack of published borehole and bio/chronostratigraphic data has limited seismic facies calibration and age estimates for the entire Neogene succession in this region. Similarly, no study has investigated the interactions between shale tectonics and submarine channels/reservoir M A N U S C R I P T

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distribution in the study area (Fig. 1). Hence, the objectives of this study are to: [START_REF] Richardson | Structure and evolution of mass transport deposits in the South Caspian Basin, Azerbaijan[END_REF] identify and date major sequence-stratigraphic surfaces (Neogene to present day) preserved on the upper slope of the western Niger Delta; (2) calibrate seismic facies by defining depositional environments to better the understanding of depositional patterns and their controlling factors;

(3) investigate the interaction between shale tectonics and depositional patterns with a view to predicting reservoir distribution in the associated 'filled ponded basins'. This study not only provides age constraints on the Neogene stratigraphy of the western Niger Delta, but also describes the effect of the interplay between autogenic and allogenic forcing on the depositional architecture and potential reservoir distribution in 'filled ponded basins'.

Geological setting

The Niger Delta is located along the Equatorial Atlantic margin of West Africa (Fig. 1). The delta is underlain by sub-aerial and submarine sediments covering an area of ∼140,000 km 2 ; and its thickness reaches 12 km [START_REF] Allen | Late Quaternary Niger Delta and adjacent areas: sedimentary environments and lithofacies[END_REF][START_REF] Evamy | Hydrocarbon habitat of Tertiary Niger Delta[END_REF][START_REF] Doust | Divergent passive margin basins[END_REF]. The progradation of this siliciclastic wedge since the Late Eocene has been estimated as ∼300 km [START_REF] Doust | Divergent passive margin basins[END_REF]Fig. 1). Known oil and gas resources rank the delta the twelfth largest petroleum province in the world [START_REF] A N U S C R I P T A C C E P T E D Accepted Manuscript 36 Tuttle | The Niger DeltaTuttle[END_REF]. The Niger Delta is subdivided into three major structural domains: (i) an extensional zone below the continental shelf, (ii) a transitional zone below the upper slope, and (iii) a compressional zone located at the toe of the slope [START_REF] Doust | Divergent passive margin basins[END_REF][START_REF] Damuth | Neogene gravity tectonics and depositional processes on the deep Niger Delta continental margin[END_REF][START_REF] Connors | Compressive anticlines of the mid-outer slope, central Niger Delta (abs[END_REF][START_REF] Morgan | Structural controls on the positioning of submarine channels on the lower slopes of the Niger Delta[END_REF]Billoti and Shaw, 2005; Fig. 1).

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6

The stratigraphy of the Niger Delta is dominated by three major lithostratigraphic units: the Akata, Agbada and Benin Formations [START_REF] Short | Outline of geology of Niger Delta[END_REF][START_REF] Evamy | Hydrocarbon habitat of Tertiary Niger Delta[END_REF][START_REF] Avbovbo | Tertiary lithostratigraphy of the Niger Delta[END_REF][START_REF] Doust | Divergent passive margin basins[END_REF]; Fig. 2). The Akata Formation comprises marine shales that range in thickness from 7,000 m beneath the continental shelf to 2,000 m in the distal portion [START_REF] Doust | Divergent passive margin basins[END_REF]. Its thickness reaches 5,000 m in the deep fold-and-thrust belt due to the repeated occurrence of thrust ramps [START_REF] Wu | Slope tectonics. Comparisons and contrasts of structural styles of salt and shale tectonics of the northern Gulf of Mexico with shale tectonics of offshore Nigeria in Gulf of Guinea[END_REF][START_REF] Corredor | Structural styles in the deep-water fold and thrust belts of the Niger Delta[END_REF].

The Agbada Formation constitutes the deltaic unit of the Niger Delta. The thickness of this paralic unit reaches 3,500 m [START_REF] Evamy | Hydrocarbon habitat of Tertiary Niger Delta[END_REF][START_REF] Avbovbo | Tertiary lithostratigraphy of the Niger Delta[END_REF][START_REF] Doust | Divergent passive margin basins[END_REF]. The depositional setting of this lithostratigraphic unit ranges from delta-front, delta-topset to fluvio-deltaic environments [START_REF] Avbovbo | Tertiary lithostratigraphy of the Niger Delta[END_REF]. The Benin Formation consists of alluvial to upper coastal plain deposits up to 2,000 m thick [START_REF] Avbovbo | Tertiary lithostratigraphy of the Niger Delta[END_REF]. The Agbada and Benin Formations are associated with clay-filled channels that were incised during the Late Miocene and Plio-Quaternary (Fig. 2).

The present study is focused on the mobile shale belt located on the upper continental slope downward the transition between the extensional and compressional belts, offshore western Niger Delta (Fig. 1; red square). It covers 638 km 2 at a distance of ~120 km from the present day coastline (Fig. 1), at depths ranging from 900 to 1,150 m within the Akata Formation, which is located at water depths between ~810 to ~3240 m. The area is characterized by 'filled ponded basins' located at the flanks of mobile shales and ~3 km wide NE-SW trending submarine channel complex extending more than 25 km into the 3D block studied (Fig. 3). The reservoir compartmentalization of the block has already been investigated by [START_REF] Chapin | Integrated seismic and subsurface characterization of Bonga Field, offshore Nigeria[END_REF] and seabed mounds and pockmarks by [START_REF] Benjamin | Canyon-confined pockmarks on the western Niger Delta slope[END_REF]; [START_REF] Benjamin | Seafloor and buried mounds on the western Niger Delta Slope[END_REF].

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Dataset and methods

Dataset

The dataset used in this study includes high-resolution 3D seismic and borehole data, comprising well logs, core and biostratigraphic data.

Seismic data

The 3D seismic data were processed as a zero-phase source wavelet in American reverse standard polarity. Thus, an increase in acoustic impedance corresponds to a trough (blue loop) in the wavelet, while a decrease in acoustic impedance is represented by a peak (red loop) in the wavelet. The horizontal resolution of the data is characterized by a stacking bin spacing of 25 m

x 25 m (in-line and cross-line spacing), and a seismic recording sampling interval of 4 milliseconds two-way travel time (TWT). The dominant frequency of the data is 30 Hz over the first 3 seconds (TWT) but decreases between 20-25 Hz below a depth of 3 km. Using average interval seismic velocity of 1,639.4 m/s and applying the velocity-wavelength equation (λ/4; [START_REF] Brown | Interpretation of Three-dimensional Seismic Data[END_REF], the vertical resolution of the data was ~13.7 m over the Pliocene-Quaternary intervals but decreased between 25 and 30 m at depths below 3 km.

Borehole data

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The lithology, facies and depositional environments of five boreholes, labelled FM-1, FM-2, FM-3ST1, FM-4ST1 and FM-5 located in the southern part of the study area (Fig. 3), were analyzed. The wells penetrated respective total depths of 4,154 m, 3,627 m, 2,924 m, 3,458 m and 3,497 m below the seafloor. Petrophysical (gamma-ray, resistivity, neutron, density and sonic) logs were acquired mostly in the deeper stratigraphic intervals, e.g. between 1,140-4,154 m in the deepest FM-1 well and 1,718-2,119 m in the shallowest FM-3ST1 well. Four of the five boreholes contain biostratigraphic data, whereas core data were only available in two boreholes.

Methods

Seismic and sequence stratigraphic analysis

A standard seismic-stratigraphic analysis approach [START_REF] Mitchum | Seismic Stratigraphy and Global Changes of Sea Level, Part 7: Seismic Stratigraphic Interpretation Procedure: Seismic Stratigraphy: Applications to Hydrocarbon Exploration[END_REF] was used for the 3D seismic data based on the recognition of reflection termination patterns such as onlap, erosional truncations, seismic facies/configuration and vertical stacking patterns. Previously described depositional architectures include MTDs, turbidite systems and hemipelagic drapes. In the present study, MTDs and turbidite deposits were identified based on seismic criteria and 3D geomorphology (e.g. [START_REF] Posamentier | Seismic geomorphology and stratigraphy of depositional elements in deep-water settings[END_REF][START_REF] Richardson | Structure and evolution of mass transport deposits in the South Caspian Basin, Azerbaijan[END_REF]. MTDs were recognized (i) by their basal shear zone or basal continuous surface, which delimits the overlying deformed materials and the underlying, relatively continuous materials [START_REF] Frey Martinez | 3D seismic interpretation of slump complexes: examples from the continental margin of Israel[END_REF][START_REF] Alves | Geomorphologic features related to gravitational collapse: M A N U S C R I Psubmarine landsliding to lateral spreading on a Late MioceneeQuaternary slope (SE Crete, eastern Mediterranean)[END_REF][START_REF] Gamberi | Mass-transport complex evolution in a tectonically active margin (Gioia Basin, Southeastern Tyrrhenian Sea)[END_REF][START_REF] Alves | Submarine slide blocks and associated soft-sediment deformation in deepwater basins: A review[END_REF] and (ii) by their top, characterized by rugose/irregular topographies [START_REF] Richardson | Structure and evolution of mass transport deposits in the South Caspian Basin, Azerbaijan[END_REF]. In terms of internal architecture, MTDs may be chaotic or transparent depending on (i) the nature of materials inherited from the source area M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT (Masson et al., 2006), and (ii) the distance from source area (e.g. [START_REF] Alves | Faulting of salt-withdrawal basins during early halokinesis: effects on the Palaeogene Rio Doce Canyons System (EspiritoSanto Basin, Brazil)[END_REF][START_REF] Omosanya | A 3-dimensional seismic method to assess the provenance of Mass-Transport Deposits (MTDs) on salt-rich continental slopes (Espírito Santo Basin, SE Brazil)[END_REF][START_REF] Rovere | Geomorphometry of a submarine mass-transport complex and relationships with active faults in a rapidly uplifting margin (Gioia Basin, NE Sicily margin)[END_REF][START_REF] A N U S C R I P T A C C E P T E D Accepted Manuscript 35 Omosanya | Seismic geomorphology of Cenozoic slope deposits and deltaic clinoforms in the Great South Basin (GSB) offshore New Zealand[END_REF]. Turbidite systems were recognized based on the presence of sandy submarine channel deposits, which are generally characterized by high to moderate seismic reflectivity and high root mean square (RMS) amplitudes on 3D paleo-geomorphological maps (see also [START_REF] Posamentier | Seismic geomorphology and stratigraphy of depositional elements in deep-water settings[END_REF][START_REF] Catuneanu | Principles of sequence stratigraphy[END_REF]. Drapes were interpreted as the result of a combination of pelagic/hemipelagic suspension fall-out as well as from the fine-grained suspended portion of turbidity flows [START_REF] Kneller | Depositional effects of flow nonuniformity and stratification within turbidity currents approaching a bounding slope: deflection, reflection and facies variation[END_REF].

Well logs, lithofacies and depositional environment

The standard approach to well-log analysis (e.g. [START_REF] Mitchum | Seismic Stratigraphy and Global Changes of Sea Level, Part 7: Seismic Stratigraphic Interpretation Procedure: Seismic Stratigraphy: Applications to Hydrocarbon Exploration[END_REF] is the display and visual inspection of accurately scaled gamma-ray, neutron-density cross plots, resistivity and sonic logs. Integrated analysis of these logs and calibration with core data helped delineate lithofacies and associated depositional environments.

Bio-and chronostratigraphic analyses

We analyzed existing biostratigraphic data, chiefly calcareous nannofossils and planktonic foraminifera, from ditch cutting samples available in four boreholes. This allowed us to constrain the ages of Neogene stratigraphic intervals. In addition to 2D seismic stratigraphy, we conducted 3D seismic geomorphological analyses to identify erosional features and channel architectures, and to distinguish main depositional elements (mass-flow deposits, channel deposits, overbank deposits, mud volcanoes and hemipelagic units). This was achieved through a detailed analysis of seismic attributes, horizon slices and amplitude maps (after [START_REF] Posamentier | Seismic geomorphology and stratigraphy of depositional elements in deep-water settings[END_REF][START_REF] Catuneanu | Principles of sequence stratigraphy[END_REF].

3D Seismic geomorphology

Seismic attributes are all observations extracted from seismic data that directly or indirectly facilitate hydrocarbon identification, lithological discrimination or geological interpretation [START_REF] Chopra | Seismic attributes for prospect identification and reservoir characterization[END_REF]2007;[START_REF] Taner | Seismic attributes revisited[END_REF]. We generated RMS and Variance Edge/Coherence attributes from the original 3D seismic data using Petrel™ software. The RMS attribute computes the square root of the sum of squared amplitudes over the geological interval of interest and facilitates lithofacies discrimination (e.g. [START_REF] Catuneanu | Principles of sequence stratigraphy[END_REF]. The variance attribute (e.g. Fig. 3D) emphasizes the correlation of seismic traces by assigning light colors to areas where seismic traces correlate (low variance) and dark colors to areas where there is no correlation (high variance; Fig. 3D; see [START_REF] Catuneanu | Principles of sequence stratigraphy[END_REF]. This attribute was used to delineate structural and stratigraphic discontinuities such as faults, mobile shales, turbidite systems, MTDs etc.

Results and interpretation

This section presents the results of seismic facies analysis and lithofacies description, depositional environments and depositional architectures within the stratigraphic intervals we [START_REF] Chopra | Seismic attributes for prospect identification and reservoir characterization[END_REF][START_REF] Alves | Submarine slide blocks and associated soft-sediment deformation in deepwater basins: A review[END_REF] in the studied 3D volume is characterized by seismic wipe out in mobile shales zones (e.g. Fig. 5).

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT studied. Seismic artefacts (sensu

Seismic stratigraphic units

Seven major seismic stratigraphic units were identified in the Neogene stratigraphic records of the upper slope of the western Niger Delta (Fig. 5).

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Unit 1 This is the oldest stratigraphic unit identified in the study area. This unit has not been drilled. It is characterized by high amplitude seismic packages whose base corresponds to the top of the reflection-free (low reflectivity) seismic unit (Fig. 5). Seismic reflectivity in Unit 1 generally decreases toward the north corresponding to a vertical, irregular, seismic wipeout zone that extends upwards from 6 to 1.8 seconds (twt) deforming the overlying seabed (Fig. 5). This feature was interpreted as mobile shale. Unit 1 thins from north and south toward the mobile shale zone in the north (Fig. 5). The overall thinning of seismic Unit 1 from the south to the north toward mobile shale indicates syn-kinematic deformation linked to the movement of mobile shale during the deposition of Unit 1 (Fig. 5).

Unit 2

The base of Unit 2 shows baselap (apparent onlap) surface overlain by parallel to subparallel, moderate to high amplitude seismic packages that generally decrease in reflectivity northward due to the presence of mobile shale (Fig. 5). Seismic packages within this unit are generally subparallel and thin northward except within a landward dipping normal fault near the mobile shale zone (Fig. 5).

Unit 3.

The base of unit 3 is also a baselap (apparent onlap) surface overlain by parallel continuous, high amplitude seismic packages whose reflectivity decreases northward toward the mobile shale zone (Fig. 5). Although Unit 3 reaches maximum thickness within a fault-

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controlled accommodation in the southern part, it thins northward toward the mobile shale zone (Fig. 5).

Unit 4

The base of unit 4 is locally erosional and marks a transition from underlying parallel, continuous high amplitude seismic packages to overlying moderate to high amplitude strata (Fig. 5). Unit 4 is characterized by localized high amplitude seismic packages that generally converge or thin away from their central parts toward their lateral limits (Fig. 5). These seismic packages are interpreted as amalgamated channel levee deposits (dotted purple circles in Fig. 5). Although Unit 4 also reaches maximum thickness in fault-controlled accommodation in the southern part, it thins northward toward the mobile shale zone (Fig. 5).

Unit 5

Unit 5 is marked at the base by an erosional event overlain by thin, discontinuous/chaotic, low amplitude seismic packages that grade to relatively continuous, high amplitude seismic packages near the mobile shale zone (Fig. 5). Seismic geometries in Unit 5 vary from subparallel (north and south) in the mobile shale zone to convergent/thinning toward the mobile shale zone (Fig. 5). Unit 5 is characterized by amalgamated, high amplitude seismic packages corresponding to the channel levee deposits described above (dotted purple circles in Fig. 5). An over 80 m-thick transparent seismic unit interpreted as an hemipelagic drape overlies this unit (Fig. 5). This hemipelagic layer was truncated by younger channel levee systems (CLSs) that locally incised the underlying deposits by up to 40 m (Fig. 5). The thickness of Unit 5 remains uniform in the south but thins northward toward the mobile shale zone.

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Unit 6

The base of Unit 6 is erosional and marks a major change with the onset of erosive MTDs and CLSs (Fig. 5). This erosional event can be traced throughout the studied 3D block except in the northeast, where it could not be interpreted due to the combined effect of signal wipe out (artefacts) and erosion (Fig. 5). Three erosive mass-transport deposits labelled lower, middle and upper MTDs were identified in unit 6 (Figs. 5,6).

The lower MTD covers an area of ~155 km 2 and extends 3-5 km laterally with a maximum thickness of ~148 m (using an average interval velocity of 1,639.4 m/s). Its upper limit is relatively gentle with localized erosional scours (Fig. 5). This MTD truncates the underlying CLS, reaching its maximum thickness at the northern and southern flanks, but drapes the central area, where the CLS thins out (e.g. Figs. 5, 6 C, D, G).

The middle MTD covers an area of ~175 km 2 with a lateral distance of 6-8 km and reaches maximum thickness of ~246 m in the hanging-wall block bounded by normal faults labelled P, Q (see also [START_REF] Doust | Divergent passive margin basins[END_REF][START_REF] Cohen | Sedimentation and shale tectonics of the northwestern Niger Delta front[END_REF][START_REF] Graue | Mud volcanoes in deep-water Nigeria[END_REF]Figs. 5, 6B, F). The middle MTD is followed by relatively continuous/chaotic, high amplitude seismic packages interpreted as turbidite feeder channels. Unlike the lower MTD, the middle MTD is highly deformed by synthetic and antithetic normal faults with inferred offsets of a few meters, and 20-25 m relief incised into the underlying CLS (Figs. 5,6F).

The upper MTD is the third and most laterally extensive MTD in Unit 6 (Fig. 5). It covers an area of 187 km 2 with a lateral distance of 12 km and reaches a maximum thickness of ~205 m downstream (Fig. 6A,E). This MTD also incises the underlying CLS, but was truncated by younger turbidite feeder channels and affected by extensional deformation (Figs. 5,6A,E).

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The upper MTD is overlain by CLSs, ultimately draped by a hemipelagic layer that was truncated by younger CLSs (Fig. 5).

Unit 7

Unit 7 is the youngest seismic unit identified in the study area. Its base marks another major change in the sedimentary record, characterized by overall high-frequency and highamplitude seismic packages (Fig. 5). These seismic packages are interpreted as amalgamated channel levee/lobe deposits and alternate with transparent seismic packages corresponding to hemipelagites (Fig. 5). The amalgamated channel levee/lobe deposits are locally truncated by turbidite feeder channels (Fig. 5). Unit 7 is topped by the seafloor, which is directly underlain by a thick hemipelagic deposit (Fig. 5).

Lithofacies and depositional environment

The integrated analysis of well logs in the study area shows that the intervals characterized by average to moderately high gamma-ray log values, low resistivity, high neutrondensity separation and high sonic values correspond to hemipelagic layers, overbanks and/or mud-rich turbidites (e.g. Fig. 7). However, intervals defined by low gamma-ray, high resistivity, low neutron-density separation and low sonic values are interpreted as amalgamated channel deposits (Fig. 7). Facies vary from amalgamated channel sands at the base of submarine channel deposits to overbank and/or hemipelagic drape at the top (Fig. 7).

Bio/chronostratigraphic analyses and correlation with seismic data

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Despite the location of the study area on the upper slope, which is prone to mass-wasting and sediment reworking, the integration of calcareous nannofossil and planktonic foraminifera biohorizons increased the reliability of the age model presented in this study.

We fixed the first occurrences (FOs), last occurrences (LOs), and the corresponding depths of key Neogene planktonic foraminifera (pf) and calcareous nannofossils (cn), in the biostratigraphic distribution sheet of the FM-1, FM-2, FM-3ST1 and FM-4 boreholes. We assigned ages to these fossil markers using equivalent ages derived from recently published studies [START_REF] Anthonissen | Cenozoic and Cretaceous Biochronology of Planktonic Foraminifera and Calcareous Nannofossils[END_REF][START_REF] Backman | Biozonation and biochronology of Miocene through Pleistocene calcareous nannofossils from low and middle latitudes[END_REF][START_REF] Agnini | Biozonation and biochronology of Paleogene calcareous nannofossils from low and middle latitudes[END_REF][START_REF] Raffi | A Cenozoic calcareous nannofossil biozonation from low and middle latitudes: A synthesis[END_REF]. Accordingly, the FO of Sphenolithus ciperoensis (cn) was assigned to ~24.4 Ma; the LO of Cyclicargolithus abisectus (cn) to ~23.5 Ma; the FO of Globoquadrina dehiscens (pf) to ~22.4 Ma; the FO and LO of Sphenolithus heteromorphus (cn) to ~17.5 Ma and ~13.53 Ma respectively; the LO of Neogloboquadrina acostaensis (pf) to ~9.8 Ma; the LO of Cartinaster coalitus (cn) to ~9.7 Ma (Fig. 7).

With the aid of time-depth relationship (checkshot) data, we depth-matched this bio/chronostratigraphic information with their corresponding seismic horizons. Finally, we attributed relative ages to the identified seismic reflectors based on: (i) the presence of fossil markers in at least two wells, with the exception of the LOs of S. ciperoensis (24.4 Ma) and C. abisectus (23.5 Ma), which were only recorded in the deepest FM-1 borehole. (ii) Relative depths of the identified seismic reflectors with respect to the nearest biohorizons (Figs. 5, 7, 8).

The basal limit of Unit 1 has not been drilled and could not be assigned a precise age (Figs. 5,8). It tops the transparent seismic unit previously interpreted as deep-marine shales deposited during Eocene sea-level highstand e.g. Doust and Omatsols (1990). Also, it marks a

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transition from the transparent seismic unit to the high amplitude seismic packages interpreted as turbidite deposits (amalgamated channels/lobes; e.g. Figs. 9, 13). The base of Unit 1 could be associated with an increase in sediment supply to the Niger Delta following the regional uplift that affected the area in the Oligocene (~34-28 Ma; [START_REF] Chardon | Stabilization of large drainage basins over geological time scales: Cenozoic West Africa, hot spot swell growth, and the Niger River: hot spot swell growth and Niger River[END_REF][START_REF] Grimaud | Cenozoic sediment budget of West Africa and the Niger delta[END_REF].

The baselap (apparent onlap) associated with the base of Unit 2 (purple line in Fig. 5) occurs 200 m below the FO of Sphenolithus ciperoensis (~24.4 Ma), which occurs at the depth of 4,017.3 m in the FM-1 borehole (Fig. 7). The base of Unit 2 also could not be assigned a precise age since it has not been drilled (Fig. 7). The geometries of successive apparent onlaps (Fig. 5) suggests it is associated with a non-deposition period in the deep basin probably during maximum flooding of the shelf. Also, since it lies 200 m below the 24.4 Ma biohorizon, it could be a good candidate for the sea-level transgression described in the Chattian [START_REF] Haq | Chronology of fluctuating sea levels since the triassic[END_REF] Fig. 8).

The base of Unit 3 (pink in Fig. 5) occurs 195 m above the LO of Cyclicargolithus abisectus (~23.5 Ma) and the FO of Globoquadrina dehiscens (~22.4 Ma; Figs. 5,7). The base of this unit, defined by apparent onlaps, could also be a good candidate for a sea-level transgression in the Burdigalian [START_REF] Haq | Chronology of fluctuating sea levels since the triassic[END_REF]Fig. 8) ,8), it could be related to the long-term Burdigalian sea-level transgression. It was tentatively assigned to the ~18.5 Ma (see [START_REF] Haq | Chronology of fluctuating sea levels since the triassic[END_REF]Fig. 8).

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The base of seismic Unit 4 occurs 160 m above the LO of Sphenolithus heteromorphus (~13.53 Ma) and corresponds to a regional erosional truncation. This limit could be linked to the short-term sea-level fall in the Serravallian (~13.8 Ma) according to the [START_REF] Haq | Chronology of fluctuating sea levels since the triassic[END_REF]Fig. 8), but probably occurred a little later in the Niger Delta. Since it is younger than 13.53 Ma, it was tentatively assigned to the Serravallian (~12.5 Ma).

The base of Unit 5 corresponds to a regional truncation and occurs 120 m above the LO of Catinaster coalithus (~9.7 Ma). Since this erosional event is younger than 9.7 Ma, it was assigned a tentative age of ~9.5 Ma on the assumption that it could be associated with the major progradation event recorded in the eastern Niger Delta during the Tortonian ~9.5 Ma [START_REF] Jolly | Growth history of fault-related folds and interaction with seabed channels in the toe-thrust region of the deep-water Niger Delta[END_REF].

On top of seismic Unit 5, we identified a transparent unit characterized in the FM-3ST1 borehole by abundant and diverse nannofossils and foraminifera. The LO of Discoaster quinqueramus (~5.5 Ma) allowed us to confidently date this hemipelagic layer as uppermost Messinian. At the top of this hemipelagic layer lies a barren interval, which is followed by an interval characterized by the simultaneous occurrence of Reticulofenestra pseudoumbilicus (cn), Globorotalia plesiotumida (pf), Globorotalia miocenica (pf), Discoaster pentaradiatus (cn), Globorotalia exilis (pf), Discoaster brouweri (cn) and Gephyrocapsa spp (cn). The cooccurrence of D. brouweri and D. pentaradiatus, whose extinction levels should be distinct in the uppermost Pliocene [START_REF] Backman | Biozonation and biochronology of Miocene through Pleistocene calcareous nannofossils from low and middle latitudes[END_REF], together with the Pleistocene markers such as H. sellii and Gephyrocapsa spp, suggests that the top of this interval has been affected by reworking. The presence of a regional erosional event corresponds to this interval on seismic sections (e.g. red line in Figs. 5, 9B, 10C, 11C). It is characterized by the onset of erosive CLSs and MTDs.

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Although we were unable to precisely define the Messinian-Pliocene transition due to poor biostratigraphic data, the onset of sediment-gravity flows following the hemipelagic drape dated ~5.5 Ma could be associated with sea-level fall following the transgressive event of the uppermost Messinian [START_REF] Haq | Chronology of fluctuating sea levels since the triassic[END_REF]. Therefore, the base of Unit 6 could be related to a sealevel fall in the Middle Pliocene (~3.7 Ma; see [START_REF] Haq | Chronology of fluctuating sea levels since the triassic[END_REF] curve recalibrated by [START_REF] Gorini | Late Neogene sequence stratigraphic evolution of the Foz do Amazonas Basin, Brazil[END_REF]2014;Fig. 8). This interpretation seems to agree with the progradation event recorded in the eastern Niger Delta during the Middle Pliocene (3.7 Ma; [START_REF] Jolly | Growth history of fault-related folds and interaction with seabed channels in the toe-thrust region of the deep-water Niger Delta[END_REF].

Similarly, assuming that the base of the laterally extensive (upper MTD) and associated CLS, mark the onset of the major cooling event at the base of the Gelasian ~2.6 Ma (see [START_REF] Miller | The Phanerozoic Record of Global Sea Level Change[END_REF]; Fig. 8), the base of Unit 7, characterized by erosive CLSs and the onset of strongly reflective seismic packages could be associated with a sea-level fall during the Middle Pleistocene. It was assigned ~?1.9-0.9 Ma on the assumption that it could mark the onset of the high-frequency/high-amplitude sequence cycles [START_REF] Miller | The Phanerozoic Record of Global Sea Level Change[END_REF]; Fig. 8). The assigned 1.9 Ma seems to agree with a published line located a few kilometers from our study area (see [START_REF] Bellingham | The Deep-water Niger Delta: An Underexplored World Class Petroleum Province[END_REF].

3D seismic geomorphology and depositional architecture

Detailed analysis of interval attribute maps, horizon and time slices focused on Units 5, 6 and 7 (Fig. 5). It allowed us to characterize the depositional architecture within the younger stratigraphic units in detail. RMS amplitude extraction at 3.3 seconds (twt) at the top of parallel, continuous high amplitude seismic packages (C bh facies), revealed the presence of NE-SW trending sinuous channels (Fig. 9A). These channels are characterized by their low amplitudes in

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ACCEPTED MANUSCRIPT 20 their axes, showing high amplitudes in their overbanks (Fig. 9A). They are interpreted to have incised older amalgamated channel/lobe deposits but were draped with mud during their abandonment (see also [START_REF] Chapin | Integrated seismic and subsurface characterization of Bonga Field, offshore Nigeria[END_REF][START_REF] A N U S C R I P T A C C E P T E D Accepted Manuscript 34 Jobe | Rapid adjustment of submarine channelarchitecture to changes in sediment supply[END_REF]. A ~5 m core data retrieved in the FM-1 borehole within the (C bh ) facies revealed high net-to-gross channel sand, characterized by blocky gamma-ray log patterns (Fig. 9B). Downstream, a ~2 m core data retrieved in the FM-3ST1 borehole is characterized by interbedded sand and shales that grade downward to dominantly shales characterized by irregular gamma-ray log patterns (Fig. 9B,D). Lateral variation in the gamma-ray log from blocky in the FM-1 borehole to irregular patterns in the FM-3ST1 borehole points to an overall decrease in grain size in this case from the channel axis to overbank deposits characterized by thin interbedded sands and shales.

The RMS amplitude map of the erosive event at the base of Unit 6 revealed the presence of a ~2 km wide, E-W oriented, low-amplitude, moderate sinuosity submarine channel complex (labelled 1; Fig. 10A,B) and three channel levee systems (CLSs labelled 2, 3 and 4), confined to the southern part of the mobile shale zone (Fig. 10 A,B). The main CLS developed tripartite, moderate sinuosity networks with overall E-W paleo-flow direction (Fig. 10A,B). CLS 1 terminated on a moderate amplitude, lobe-like feature (Fig. 10 B). CLSs 3 and 4 are characterized by higher amplitudes and crossed paleo-relief associated with a buried fold/shalecored anticline (Fig. 10A,B).

RMS amplitude extraction at the base of Unit 7 reveals the presence of a submarine channel complex (SE in Fig. 11B; labelled channel 1), CLSs 2, 3 and 4 (SE in Fig. 11B) and bifurcating, moderate sinuosity CLSs located in the NW corner of the 3D volume (Fig. 11B).

The NW CLSs are interpreted as bypass channels that incised older sand-rich amalgamated channels/lobe deposits. The updip continuity of the main submarine channel complex is obscured by mobile shale/erosion (Fig. 11B). Furthermore, the horizon slice at the base of the high reflectivity seismic packages in Unit 7 reveals the presence of erosive submarine channel complex 1 and high sinuosity CLSs 2 and 3 (Fig. 12A,B). A seismic line through this channel complex revealed its erosive character and wide U-shaped geometry. This stage marks a significant increase in the width of the channel, the development of higher sinuosity, a northward shift toward the mobile shale zone and overall NE-SW orientation compared to the previous stages (Figs. 10,11 and12).

An RMS amplitude extraction at 1.9 seconds within the high amplitude seismic packages revealed a NE-SW trending, moderate sinuosity submarine channel complex (labeled 1; Fig. 13B) and four E-W oriented sinuous CLSs (labelled 2, 3, 4 and 5; Fig. 13B). These channels are all confined to the southern part of the mobile shale zone (Fig. 13). The submarine channel complex 1 captured the CLSs 2 and 3 downslope, where the latter developed abandoned meander loops (Fig. 13). CLS 4 is bordered by a high amplitude sediment wedge interpreted as levee deposits (Fig. 13). The linear, high amplitude channel-like feature located downslope of CLS 4 is interpreted as an avulsion channel (see also [START_REF] Posamentier | Seismic geomorphology and stratigraphy of depositional elements in deep-water settings[END_REF]Fig. 13).

The horizon slice at 1.8 seconds (below the present day seafloor) shows the presence of irregular, low coherent seismic areas in the north, flanked by planar radial discontinuities interpreted as radial faults (Fig. 14A). These low coherent features were interpreted as mobile shales (Figs. 2,5,(12)(13)(14). Linear scars were also observed in a hanging-wall fault block adjacent to the mobile shale zone (Fig. 14). A seismic line through these lineated features revealed coherent to slightly deformed high amplitude seismic packages with a somewhat erosive base. These lineaments were interpreted as scars associated with mobile shale-derived MTDs (Fig.

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14D

). This MTD shows a lateral distance, area and maximum thickness of ~2.3 km, ~13 km 2 and ~60 m, respectively (Table 1). The presence of an E-W oriented, moderate sinuosity submarine channel complex characterized by rough edges and entrenched CLSs can also be observed on this map (Fig. 14).

Discussion

Here, we focus on the sedimentary processes in intraslope basins, interactions between shale tectonics and submarine channel evolution, depositional patterns and their implications for reservoir distribution. We prefer the term 'filled ponded basins' to 'shallow ponded basins' used by Prather et al. (2012a) in the description of the depositional architecture in the intraslope basins we studied. We think this term is more appropriate as the 3D block we studied is located in a supply-dominated deep-basin between 900-1,150 m, where deep-water processes dominate, and where the high rate of sediment supply filled the accommodation created by shale tectonics.

Interaction between shale tectonics and submarine channel complex

Detailed seismic and 3D geomorphological analyses (Fig. 15) revealed that the main submarine channel complex described in Units 6 and 7 (Figs. 10-15) evolved in three stages from the Pliocene to the present day. The evolution of the submarine channel from relatively moderate to high sinuosity over the Pliocene and the Pleistocene could be related to shale tectonics (Figs.

5, 10-15; see also [START_REF] Olusola | Effect of Growing Structures on Stratigraphic Evolution, Channel Architecture, and Submarine Fan Distribution, Niger Delta, West Africa[END_REF][START_REF] Clark | Interactions between coeval sedimentations and deformation from the Niger Delta deep-water fold belt[END_REF][START_REF] Jolly | Growth history of fault-related folds and interaction with seabed channels in the toe-thrust region of the deep-water Niger Delta[END_REF][START_REF] Hansen | Submarine channel evolution, terrace development, and preservation of intra-channel thin-bedded turbidites: Mahin and Avon channels, offshore Nigeria[END_REF].

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During stage 1 (Middle Pliocene-Early Pleistocene), the channel was characterized by narrow morphology, low sinuosity and confined to the south of mobile shale in an E-W orientation 15). The narrow low sinuosity of the channel during this stage is typical of young submarine channels with a relatively high slope gradient (see also [START_REF] Posamentier | Seismic geomorphology and stratigraphy of depositional elements in deep-water settings[END_REF]Ferry et al., 2005;[START_REF] Heiniö | Knickpoint migration in submarine channels in response to fold growth, western Niger Delta[END_REF][START_REF] A N U S C R I P T A C C E P T E D Accepted Manuscript 34 Jobe | Rapid adjustment of submarine channelarchitecture to changes in sediment supply[END_REF][START_REF] Jolly | Growth history of fault-related folds and interaction with seabed channels in the toe-thrust region of the deep-water Niger Delta[END_REF][START_REF] Hansen | Submarine channel evolution, terrace development, and preservation of intra-channel thin-bedded turbidites: Mahin and Avon channels, offshore Nigeria[END_REF]. The E-W orientation of the channel south of the mobile shale zone suggests that the later was active from the Middle Pliocene to the Early Pleistocene, leading to the channel's confinement 15).

Stage 2 (Early to Middle Pleistocene) marks a northward shift of the submarine channel complex toward the mobile shale zone and a transition from an E-W to a NE-SW orientation (Figs. 12,15). Its higher sinuosity, broad U-shaped morphology are typical features of mature submarine channels with relatively gentle slope gradients (Fig. 12; see also [START_REF] Posamentier | Seismic geomorphology and stratigraphy of depositional elements in deep-water settings[END_REF]Ferry et al., 2005;[START_REF] A N U S C R I P T A C C E P T E D Accepted Manuscript 34 Jobe | Rapid adjustment of submarine channelarchitecture to changes in sediment supply[END_REF][START_REF] Heiniö | Knickpoint migration in submarine channels in response to fold growth, western Niger Delta[END_REF][START_REF] Jolly | Growth history of fault-related folds and interaction with seabed channels in the toe-thrust region of the deep-water Niger Delta[END_REF][START_REF] Qin | Quantitative seismic geomorphology of a submarine channel system in SE Brazil (Espirito Santo Basin): Scale comparison with other submarine channel systems[END_REF][START_REF] Hansen | Submarine channel evolution, terrace development, and preservation of intra-channel thin-bedded turbidites: Mahin and Avon channels, offshore Nigeria[END_REF]. The channel's northward adjustment toward the mobile shale zone (Figs. 12,15) suggest relative quiescence of mobile shales.

Stage 3 (Middle Pleistocene to present day) marks a return of the channel to its original E-W paleo-flow direction, south of the mobile shale zone (Figs. 1D,(14)(15). The downward shift of the submarine channel complex suggests that renewed activity of the mobile shale led to the deflection of the submarine channel complex southward away from the mobile shale zone. The presence of NE-SW oriented, high sinuosity submarine channel complexes 1 and 2 at 1.9 seconds (twt; middle part of Unit 7; Fig. 13) suggests that shale tectonics and the final southward deflection of the channel complex occurred a little later than the Middle Pleistocene. The presence of mud volcanoes on the present day seafloor supports the ongoing activity of the

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mobile shale (Fig. 5). The presence of entrenched sinuous channel levee system within a valley (Fig. 14) indicates increasing confinement due to successive erosional and aggradational (depositional) stages (Fig. 13; channel complexes 1 and 2). A large valley containing the channel develops complex geometries as the valley wall becomes unstable (large scars Fig. 14A), with the development of inner terraces and a central sinuous channel (labeled in part B).

The modification of the channel valley could result from the increasing confinement and focused energy of turbidity currents (see also Gee and Gawthorpe 2007, and others) during their activity.

The present day hemipelagic drape (Fig. 14 C) shows that the last channel complex described in this study was not active in the Holocene.

Reservoir distribution in filled ponded basins

Fig. 13 is a detailed example of the recent entrenched channel levee system (CLS) in Unit 7. The generally high amplitudes on the upstream bank of channel levee system (e.g. CLS 4; Fig. 13) suggest that sand-rich sediment was deposited both in the channel axis and on the overbank.

In addition, the chaotic high amplitude reflection packages (HARPs) at the erosional base of the main submarine channel complexes (e.g. Figs. 13, 14; labeled HARPs) are interpreted as coarse grained turbiditic channel fill deposits (after Prather et al., 1998;[START_REF] Heiniö | Knickpoint migration in submarine channels in response to fold growth, western Niger Delta[END_REF][START_REF] Gamboa | Classifications of submarine confluences and implications for intra-canyon sediment distribution[END_REF]. However, the occurrence of high RMS amplitudes downstream of the channels (e.g. CLS 4 in Fig. 13A,B; labelled avulsion channel) show that turbidity flows periodically spilled sandy deposits on the overbank environment. The presence of high amplitudes associated with the avulsion channel (Fig. 13 A,B) compared to lower amplitudes in the downstream part of the main CLS 4 (Fig. 13), suggests that sand-rich sediment was diverted

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from the main channel during the avulsion event. Similar observations have been reported in the De Soto Canyon, Gulf of Mexico, where upstream avulsion due to a breached levee resulted in the deposition of the avulsion lobe and concomitant decrease in both water and sediment discharge downstream of the main channel [START_REF] Posamentier | Seismic geomorphology and stratigraphy of depositional elements in deep-water settings[END_REF].

As a first conclusion, these detailed images of reservoir distribution in Unit 7 are a good analog for the Middle to Late Miocene turbidite deposits described by previous authors as amalgamated channels and overbank deposits [START_REF] Chapin | Integrated seismic and subsurface characterization of Bonga Field, offshore Nigeria[END_REF]. However, our study shows that reservoirs topped by hemipelagic layers and/or mud-rich turbidites are Late Oligocene to present day in age (see Figs. 5,7,9). Our study also shows that the geometries and the distribution of reservoir could be controlled by both shale tectonics and sediment supply, with prevalence of hemipelagic deposition during the periods of sea level rise (see Figs. 7 and8).

Reservoir facies were confirmed by borehole data (see Fig. 9). Lobe-like deposits (e.g. Fig. 10 B) are generally rare and not discussed further in this paper due to lack of sufficient evidence to fully characterize their architecture.

Shale tectonics vs MTD types and distribution:

Two types of MTDs were recognized based on seismic facies, internal architecture and aerial extent (Figs. 5,6, 14 D and Table 1): (1) Mobile shale-derived MTDs and (2) regional MTDs.

Detailed analysis of kinematic indicators enabled us to infer the source area of the locally derived type 1 MTDs, but the limited size of the 3D volume we studied did not allow us to observe the headwall scars of the type 2 MTDs. However, analysis of their thickness maps helped us to infer their provenance.

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Type 1 (Mobile shale-derived MTDs)

The type 1 MTDs described in seismic Unit 7 are thin and laterally confined adjacent to mobile shales (Fig. 14. D; Table 1). Their strong reflectivity distinguishes them from the type 2 MTDs, and their overall coherent internal architecture indicates that they are close to their source area (mobile shales).

Type 2 (Regional MTDs)

The type 2 MTDs described in Unit 6 (labelled lower, middle and upper MTDs; Figs. 5,6; Table 1) are characterized by thick, chaotic, discontinuous low amplitude/transparent seismic packages that are bounded at the base by laterally extensive erosional scours. Their thick, lateral distribution, low amplitudes and chaotic internal architecture indicate that they were reworked far from their source areas (shelf-margin/upper slope).

The distribution of the regional MTDs could be controlled by general slope (Fig. 6), syndepositional faulting (Fig. 6 B) and differential compaction/cementation in the underlying substratum (Fig. 6 D).

The lower MTD increases in thickness from the NE downslope to the SW suggesting possible source area upslope (Fig. 6C; see also [START_REF] Alves | Faulting of salt-withdrawal basins during early halokinesis: effects on the Palaeogene Rio Doce Canyons System (EspiritoSanto Basin, Brazil)[END_REF]. However, the local increase in MTD thickness on both side of CLSs (Figs. 5,6C,G), suggest that the degree of compaction/cementation in the underlying CLSs could be a controlling factor at local scale. The differential erosion in sandy CLSs and muddy deposits could also explain the observed variation in thickness of the lower MTD (see erosional truncations at its base, Fig. 6 G).

We observed a NE-SW decrease in thickness of the middle MTD downstream (6B, F) due to its confinement within fault bounded accommodation (Hanging-Wall; HW; Fig. 6F). This

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observation suggests that the deposition of MTD in the intraslope basins of the Niger Delta was controlled by syn-depositional normal faults (P, Q? Fig. 6 F) that were active during the emplacement of MTD (Fig. 6B,F).

The thickness of the upper MTD increases downstream and was locally controlled by the degree of compaction in the underlying CLSs (Fig. 6A). The overall downstream increase in thickness of this MTD suggests a response to regional accommodation development (see also [START_REF] Alves | Faulting of salt-withdrawal basins during early halokinesis: effects on the Palaeogene Rio Doce Canyons System (EspiritoSanto Basin, Brazil)[END_REF]. The foregoing observations strongly suggest that the type 2 MTDs originate from shelf-margin and/or upper slope. ,C,D;11 A,C). However, relative quiescence and/or low subsidence/uplift rates from the Early to Middle Pleistocene resulted in more mature CLSs characterized by complex terraces, overbank deposits and channel avulsion (e.g. Fig. 13). The erosional nature of the main submarine channel complex during mobile shale deformation suggest that a dynamic equilibrium was maintained between changing local slope gradients and sedimentation rates (see also [START_REF] Adeogba | Transient fan architecture and depositional controls from near-surface 3D seismic data, Niger Delta continental slope[END_REF] , 2012;[START_REF] Jolly | Growth history of fault-related folds and interaction with seabed channels in the toe-thrust region of the deep-water Niger Delta[END_REF][START_REF] Hansen | Submarine channel evolution, terrace development, and preservation of intra-channel thin-bedded turbidites: Mahin and Avon channels, offshore Nigeria[END_REF]. Similar observations have been made in the adjacent lower Congo Basin (Ferry et al., 2005).

In addition, the control of the distribution of MTDs by syn-depositional faults suggests that mass-wasting was coeval with the growing faults. Equivalent observations have been reported in the lower Congo Basin (Ferry et al., 2005).

At a basin scale, the depositional sequence is characterized by a succession of MTDs and turbidite deposits topped by hemipelagic layers (see Fig. 6G). Moreover, the Plio-Pleistocene sedimentary records show that eustasy and sediment supply could control the depositional architecture at regional scale (e.g. Figs. 5,6E,G,H;. The absence of thick hemipelagic drape above the older submarine channel complex (e.g. CLS 1 Fig 10 C,D) suggests that this hemipelagic drape could have been eroded during stage 1 of the submarine channel complexes or that the former was below resolution. However, the presence of a thick hemipelagic layer above the present day submarine channel complex (just below the present day seafloor; Figs. 5,10 C,D;[12][13][14] indicates that it was draped and abandoned during the last postglacial sea level rise.

These observations suggest that channel complexes were most active during the low eustatic sea level but that hemipelagic sedimentation prevailed during high eustatic sea level.

Conclusions

The Neogene stratigraphy of the western Niger Delta's slope is characterized by seven Note the vertical succession of facies from channel axis, overbank and/or hemipelagic deposits.

See text for a detailed explanation. • Channel fills and overbank deposits act as reservoirs while hemipelagites act as seals

  a,b* Corresponding author : Kelvin Ikenna Chima; Sorbonne University; Institut des Sciences de la Terre (ISTeP), Case 129, Tour 56/66, fifth floor, 4 Place Jussieu, 75252, Paris Cedex. E-mail: kelvini.chima@gmail.com

4. 1 .

 1 Seismic facies analysis and seismic stratigraphic units 4.1.1 Seismic facies analysis Six seismic facies were identified in the studied 3D volume (Fig. 4) using the seismic facies classification schemes proposed by Prather et al. (1998) and Hanson et al. (2017). These include: (1) chaotic, discontinuous low amplitude seismic facies (B l ), generally characterized by an erosive base, irregular top and deformed internal architecture interpreted as mass-transport deposits (MTDs); (2) chaotic, discontinuous, high amplitude seismic facies (B h ) interpreted as turbidite feeder channels; (3) inclined, coherent/moderately deformed, high amplitude seismic facies interpreted as slides/slump blocks; (4) parallel, continuous/convergent, high amplitude seismic facies (C bh ) interpreted as amalgamated channels/lobes; (5) convergent thinning high amplitude seismic facies (C th ) flanked by seismic packages that onlap negative topographic relief features interpreted as levees and channel fill deposits, respectively; (6) transparent, continuous seismic facies (D) interpreted as hemipelagic drapes.

  . The older fossil marker-the LO of C. abisectus (~23.5 Ma), which was only recorded in the FM-1 borehole, occurs at the same depth as the younger fossil marker-the FO of G. dehiscens (~22.4 Ma), recorded in both FM-1 and FM-2 boreholes. These fossil markers are separated by ~1.1 Ma and should not coexist. Hence, this datum may represent a condensed interval or alternatively, C. abisectus could represent a reworked component. Since, the base of seismic Unit 3 lies 195 m above the FO of G. dehiscens ~22.4 Ma-Aquitanian (Figs. 7

5. 4

 4 Control of depositional systems in filled ponded basins: Our results show that the depositional architecture of the filled ponded basins in the western Niger Delta is controlled by shale tectonics, sediment supply and eustasy. At local scale, basin tectonics is associated with mobile shale deformation and syndepositional faults, controlling submarine channel morphology and depositional patterns in these filled ponded basins. Inferred high uplift/subsidence rates driven by the mobile shale in the Middle Pliocene and Early Pleistocene resulted in high slope gradients characterized by erosionally confined channels with low sinuosity (e.g. Figs. 10B

  major seismic units dated as Chattian, Burdigalian, Serravallian, Tortonian, Middle Pliocene, Middle Pleistocene and present day. Major changes in the sedimentary record occurred in the Pliocene-Pleistocene, characterized by erosional MTDs, turbidites and channel levee systems . In the Plio-Pleistocene upper sequences, we observed changes in the morphology of submarine channel complexes over time (low sinuosity/highly erosive versus high sinuosity/depositional) that were controlled by the interaction between shale tectonics (creating accommodation, and controlling the seafloor profile) and sediment supply.Younger sequences of the Niger Delta upper slope could be a good analog for sedimentary processes in intraslope basins; at the margin scale, depositional architecture in the filled ponded basins on the upper slope of the western Niger Delta is controlled by the interaction between local basin tectonics, eustasy and sediment supply. Reservoir facies are characterized by amalgamated channel fills and overbank deposits. Periodic levee breaching by high-energy turbidity currents results in channel avulsion and diversion of sand-rich sediments downstream of avulsion channels. Our detailed analysis of the depositional architecture of the Tortonian and Plio-Pleistocene sedimentary units provides useful analog for deeper exploration of the older stratigraphic units. interpretation. Many thanks to Schlumberger for providing the Petrel (TM) Software used in 3D data interpretation. Many thanks to Dia Ninkabou, Quentin Soulet, Douglas Lisboa (Sorbonne University) and David Anomneze (Camac Petroleum, Nigeria) for software support. This paper benefited from insightful discussion with Miguel Mora-Glukstad (Shell Nigeria). Mark Newall, Celestine Ugwu, Otuka Umahi, Adebukonla Kalejaiye, Segun Akinrolabu, Obobi Onwuka, Ifekelunma Umeogu, Jerome Odiwanor and Jude Omuije (Shell Nigeria) are thanked for organizing a discussion session during manuscript drafting. Bilal Haq provided insightful review comments on the first draft. Marzia Rovere and Kamaldeen Olakunle Omosanya are thanked for their high quality and constructive review comments/suggestions, which greatly improved the quality of this paper. Daphne Goodfellow proof read the revised manuscript. We thank the invited editor Tiago M. Alves not only for granting us sufficient time to make revisions, but also for pointing out subtle errors in the final submission.

Fig. 1 .Fig. 2 .

 12 Fig. 1. A: Superposed relief and bathymetric map of the Niger Delta showing the extensional,

Fig. 3 .

 3 Fig. 3. Physiography of the study area. A, B: Uninterpreted and uninterpreted time slice (at 4.9

Fig. 4 :

 4 Fig. 4: Overview of the seismic facies observed in this study, corresponding line drawings and

Fig. 7 :

 7 Fig. 7: Representative borehole (FM-1 well) drilled to a total depth of 4.15 km down to the

Fig. 8 :

 8 Fig. 8: Chronostratigraphic chart showing standard stages, long-term and short-term sea level

  Fig. 11. A: Uninterpreted RMS amplitude map of a channel levee system near the top of Unit 7

Fig. 12 .

 12 Fig. 12. A: Horizon (variance attribute) slice at the base of the high reflectivity seismic packages

Fig. 13 .

 13 Fig. 13. A: Uninterpreted RMS amplitude map at 1.9 seconds (twt) draped on horizon (variance

Fig. 14 .

 14 Fig. 14. A, B: Uninterpreted and interpreted horizon (variance attribute) slice at 1.8 seconds (twt)

Fig. 15 .•

 15 Fig. 15. Shale tectonics and submarine channel complex evolution from Middle Pliocene to

Table 1 :

 1 Summary of statistical measurements on MTDs

	Name	Length (m)	Thickness (m)	Area (km 2 )
	Mud-derived MTD	2.3	75	13
	Upper MTD	12	205	187
	Middle MTD	6-8	246	175
	Lower MTD	3-5	148	155

Acknowledgments

The authors would like to thank the Federal Republic of Nigeria for funding the PhD of Kelvin Ikenna Chima under the auspices of the Petroleum Technology Development Fund (PTDF) Scholarship Scheme. We are indebted to Shell Nigeria for providing data and permission to publish this study. The first author thanks the management and staff of Alex Ekwueme Federal University Nudfu-Alike Ikwo (AE-FUNAI), Ebonyi State, Nigeria for their continued support and encouragement. Many thanks to Olusegun Obilaja, Adekemi Afolayan, Babatope Atitebi, Richard Streatfield and Timi Tralagba for their invaluable support with data. The Institute des Sciences de la Terre (ISTeP), Paris provided the computer workstation used in data analysis and


Keywords: Method, Environmental Monitoring, Armed Conflict, Syria, GIS, Environmental Index, Risk map

Today, armed conflict affects some twenty countries, covering an area making up 11% of the surface area of the Earth. Any degradation of nature in these areas represents a harmful depletion of the world's natural heritage. Despite this, environmental issues are neglected during these periods of conflict, considered secondary to the urgency of restoring peace and safeguarding human life. Yet their consequences are potentially severe. In these areas, it is future generations who will suffer the effects of the current devastation for a very long time.

In this context, the method developed in this study, named (Geographic Information System) for Environmental Monitoring in Wartime, can be used to calculate a risk indicator for environmental degradation, spatial monitoring and risk management. This will make it possible to identify the main threats to protected areas, catalogue the damage caused to the environment by armed conflicts, and create a dynamic risk map. In this paper, GIS-EMW has been applied to calculate a risk indicator for environmental degradation in Syria.

Introduction:

Today, armed conflict affects some twenty countries. The relationship between humankind and nature is changing radically in these areas of armed conflict; extreme circumstances of lack of food, medicine, drinking water and sanitation forces mankind to develop a different relationship with its environment. Any degradation of nature in these areas represents a harmful depletion of the world's natural heritage. Despite this, environmental issues are neglected during these periods of conflict, considered secondary to the urgency of restoring peace and safeguarding human life. Yet their consequences are severe and their damage irreversible 1 . In these areas, it is future generations who will suffer the effects of the current devastation for a very long time.

Currently, scientists' intervention is limited to assessing damage in post-conflict periods.

Viewpoints around these issues need to evolve in order to develop strategies and tools for the monitoring and active protection of the environment, of nature reserves, and of biodiversity in armed conflict zones in times of war. Geographic information systems, remote sensing and spatial modelling now offer new potential for understanding and analysing observations concerning armed conflict's impact on nature, in order to intervene and prepare better for the post-conflict period. These tools will also be used to document breaches of environmental regulations in real time.

In this context, the method developed can be used to calculate a risk indicator for environmental degradation, spatial monitoring and risk management. This will make it possible to identify the main threats to protected areas, catalogue the damage caused to the environment by armed conflicts, and create a dynamic risk map. This risk indicator takes account of the conservation priority levels of the area concerned, the site's level of degradation risk, and the ecological services it offers to humans.

Calculating this risk makes it possible to identify those areas with the highest environmental degradation risk, and to determine the best measures that should to be taken. This methodology could be used to inform a decision support tool coupled with a GIS (Geographic Information System) for Environmental Monitoring in Wartime (i.e. a GIS-EMW) with a view to controlling environmental damage in armed conflict zones and to propose the intervention strategies needed to limit this damage and plan a rapid resumption of monitoring, focusing on the conservation of biodiversity and natural resources during the post-conflict reconstruction phase. These tools will also be used to identify the services nature provides to mankind, and the changes mankind brings to its immediate environment in these zones, in order to propose solutions and eco-aware approaches that meet the needs of mankind in these extreme conditions, while maintaining a balanced relationship with nature.

Method:

Objective: Draw up a map that shows the levels of potential threats from risks of degradation in the protected areas according to several indices. All of this will form part of a GIS in a 10 km analysis grid, in terms of spatial accuracy.

At each site, the natural habitats will be categorised at site-level according to the global standard Land Cover Classification System (LCCS), developed by the United Nation's Food and Agriculture Organization (FAO), and the United Nations Environment Program (UNEP).

Three types of index will be collected.

The first type of index depends on the degree of conservation priority of the natural habitat (directly correlated to the environment's biodiversity).

The second type of index depends on the level of risk of degradation to the site due to armed conflict.

The third index is designed to evaluate ecosystem services with a view to determining which benefits, goods and services can be directly or indirectly derived from ecosystems by humans.

The nature of the indices are specified below:

Group I indices: Conservation Priority

Index 1: Local-Level Rarity of natural habitat (R). For each analysis window, this index takes the maximum value of 1 when the habitat is rare, here defined as marginal, declining and threatened habitats [START_REF] Rameau | Définition et évaluation de la qualité d'un écosystème forestier[END_REF]. This index will be evaluated by an expert according to available bibliographical sources. This index takes the minimum value of 0.0001 when there are no rare habitats within the area concerned. Index 2: Surface Area of Habitat (S). For each analysis window, this index is to be assigned the maximum value of 1 when the surface area of a rare natural habitat is less than double the minimum required for habitat maintenance. This index will be evaluated by an expert according to available bibliographical sources. This index takes the minimum value of 0.0001 when the surface area of a rare natural habitat is more than double the minimum required for habitat maintenance. Index 3: Presence of Threatened Plant & Animal Species (Sp). This index takes the maximum value of 1 when endangered species occur within the analysis cell. This index takes the minimum value of 0.0001 when no threatened species occur within the analysis cell.

Group II indices: Degradation risks relating to armed conflicts are subject to three essential factors (i.e. space, time and damaging effect) Index 4: Proximity to Combat Zones (Comb). This index corresponds to how far away the combat zones are located in relation to the habitat, where the value 0.0001 is to be assigned when there are no combat zones nearby (i.e. within a radius of 10 km). This index takes the value 1 if the pixel occurs within a combat zone, otherwise it is to be assigned the value 0.0001. Index 5: Proximity to Illegal Oil Drilling Sites or Unauthorised Extraction of Minerals (EXT1). A value of 1 is to be assigned to each analysis cell when the site is being used for illegal mineral extraction (i.e. within a radius of 10 km). This value is based on the proximity of the pollution caused by either of the illegal extraction types, otherwise it takes the value 0.0001. Index 6: Proximity to Refugee Camps. The value 1 is to be assigned at each analysis window when refugee camps are located nearby, so as to take account of the exploitation of biological resources (EXT2) e.g. collection of firewood (Chechabo 2011) within each site. It takes the value 0.0001 when there are no refugee camps in the vicinity. Index (t) accounts for Time, which takes a value of 0.01 for each day of fighting occurring within the area, and takes the value 1 when there have been 100 or more days of fighting.

Group III: Ecological Service indices (ES):

Group III indexes are to be assigned the value 0.0001 if the area does not offer any of the services concerned by this index, the value 1 if the area does offer the ecological services listed [START_REF] Maresca | Évaluation des services rendus par les écosystèmes en France. Les enseignements d'une application du Millennium Ecosystem Assessment au territoire français[END_REF], or the value 0.5 if the area offers at least one of the ecological services listed. For any given geographic grid cell, an environmental monitoring index in armed conflict zones is defined (image 1)

EM Conflict = (𝐶𝑃𝐼+𝐼𝐷+𝐸𝑆) * 𝑡 3
Where CPI is the Conservation Priority Index for that grid cell. This is calculated using the three indices R, S and Sp.

𝐶𝑃𝐼 = 𝑅 + 𝑆 + 𝑆𝑝 3

ID is the Degradation Risk Index for each grid cell. This is calculated using the three indices.

𝐼𝐷 = 𝐶𝑜𝑚𝑏 + 𝐸𝑥𝑡1 + 𝐸𝑥𝑡2 3

ES is the Ecological Services Index 𝐸𝑆 = 𝐿𝑆 + 𝑅𝑆 + 𝐴𝑆 + 𝑆𝑆 Index (t): This value approximates 0 when the harmful effects are occasional, and takes the value 1 when the damaging effects have affected the site for at least 100 days.

Image1 : Components of the comprehensive EMC Index (Environmental Monitoring Index in Armed Conflict Zones)

Case Study, Syria:

The armed conflict in Syria, which has been going on now for about 8 years, has had significant negative impacts on the environment. Failure to comply with international conventions for the protection of world natural heritage in wartime has led to serious environmental degradation and destruction. In this context, the EMC Index was calculated for Syria divided into 2454 (10x10 Km) grid cells. The UTM coordinates (WGS84) of Syria are:

Zone 37S E: 499708.67 N: 3851094.3

The Different Indices:

Conservation Priority Index (CPI):

The Conservation Priority Index was calculated using indices R, S and Sp, where natural reserve areas in Syria were considered locally rare natural habitats containing threatened plant and animal species; the map of nature reserves in Syria was used [START_REF] Mobaied | La nature dans les zones de conflits armés. Le cas de la Syrie[END_REF].

The minimum surface area indicating the susceptibility of a habitat to degradation caused by events such as armed conflict has been estimated at 0.5 hectares for forests as well as land use in arid zones (FAO 2012, FAO, 2017). For the nature reserve relating to the Marine Ecosystem, and in the absence of a definition in terms of the ideal size for marine protected areas (Brennan Jacot, 2009), 1000 ha was considered to be more than double the minimum required for habitat maintenance. One hectare was considered the minimum surface area required for maintaining breeding areas.

The ICP was calculated for each grid cell (Map1).

Environmental Monitoring Index in Armed Conflict Zones (EMC)

For each of the geographic grid cells, the EMC Index was calculated (Map 5)

Map5: Environmental Monitoring Index (EMC) Map in Syria

Analysis and perspective of method used:

Application of the GIS-EMW method was facilitated by the abundance of existing data on Syria, particularly relating to oil operations, which have been mapped by a number of researchers and journalists, but the same cannot be said for all armed conflict zones around the world.

In this context, this study demonstrated the contributions vis-à-vis the GIS-EMW method made by remote sensing, i.e. the knowledge and techniques whereby certain physical and biological features of location points observed can be determined on the basis of remote measurements, without actually needing to be in physical contact with those location points.

(CNRTL Website).

By example artisanal oil refineries could be visible on satellite images and their environmental impact could be mentioning in armed conflict zones in the absence of other sources of data [START_REF] Zwijnenburg | Human health and environmental risks of civilian operated makeshift oil refineries in Syria, PAX Google Scholar[END_REF].

Many other index can support the model suggested in this study, such as vegetation indexes to identify and measure the vegetation degradation and to detecting and mapping burned surface [START_REF] Pereira | A comparative evaluation of NOAA/AVHRR vegetation indexes for burned surface detection and mapping[END_REF][START_REF] Faour | Detecting Changes in Vegetation Trends in the Middle East and North Africa (MENA) Region Using SPOT Vegetation[END_REF] To develop the use of the contributions of remote sensing in this method, images of the Copernicus Programme of ESA, which delivers optical images SENTINEL2 all the 6 days could be an important tool. SENTINEL2 provides Multi-spectral data with 13 bands in the visible, near infrared, and short wave infrared part of the spectrum with a 10 m would be an enormous advantage, which allows to follow-up on vast territories. (What is Copernicus, 2018). GIS analysis can be performed with free software also like QGIS also, which has all the good virtues to carry out this work.

The environmental monitoring Index map classes each of the grid cells in Syria according to zone priority in terms of potentially severe damage to the environment, which deprives humans of the resources and services, hitherto -generously -offered. The protection and restoration of such zones is more urgent during and after armed conflicts.

The GIS-EMW model proposed here is flexible and extensible. It is structured in such a way that it can be easily and quickly modified as new data becomes available for each of the geographic grid cells.

This method offers a tool that can be used in all armed conflict zones with a view to creating a useful database for the rehabilitation of those areas and for the management of environmental problems caused by war. This method could be applied to monitor the risk of other events affecting the environment. It can also be used for scientific studies within the numerous fields addressed during and after conflicts.

Index 7 :Index 8 :Index 9 :Stage 2 :

 7892 Life Support Services (LS) offered by the natural area are: Regulatory Services (RS) offered by the natural area are: Abstraction & Supply Services (AS) and Goods offered by the natural area Merging of results to develop a comprehensive index.

Map1:

  Conservation Priority Index (CPI) Map in Syria Degradation Risk Index (DI) The Degradation Risk Index was calculated using the three indices -Proximity to Combat Zone (Comb), Proximity to Illegal Oil Extraction Sites (EXT1), Proximity to Refugees Camps (EXT2). Comb was calculated using Conflict Data for the Syrian Arab Republic, collated by the Armed Conflict Location & Event Data Project (ACLED)), which records data on fighting and bombing in Syria. (ACLED, 2019) The Proximity to Illegal Oil Extraction Sites Index, EXT1, was calculated using maps showing illegal oil drilling sites in Syria (IEA, 2014 and Financial times, 2016) The Proximity to Refugee Camps Index, EXT2, was calculated using maps showing the location of refugee camps (US Department of State Humanitarian Information Unit, 2014, and Relief Works Agency, 2018). The Degradation Risk Index was calculated for each grid cell (Map2). Map 2: Degradation Risk Index (DI) Map in Syria Ecological Services Index (ES) For each of the grid cells, the natural habitats in Syria were classed according to the FAO's 2015 Land Cover Classification System (LCCS) (Di Gregorio, 2016) (Map3). A value for the Ecological Services Index (SE) was assigned to each habitat type (Map

When many species are in danger of becoming extinct, when damage causes irreversible degradation of the fragile ecosystems, when natural resources are irretrievably destroyed or contaminated.

Acknowledgment:

We thank Luc Semal, researcher at the Center for Ecology and Conservation Sciences (Cesco, UMR 7204) for his suggestions that significantly contributed to improving the quality of this study. We thank the anonymous reviewers for their helpful and constructive comments that significantly contributed to improving the quality of the publication.