Romain Jatiault

Lies Loncke b Damien Dhont

Dominique Dubucq

Patrice Imbert

Geophysical characterisation of active thermogenic oil seeps in the salt province of the Lower Congo Basin. Part II: A regional validation

Keywords: Oil slicks, Thermogenic fluid seeps, Pockmarks, Asphalt, Salt diapir, Lower Congo Basin, Gas hydrates, Double BSR

The Lower Congo Basin is known to discharge a substantial volume of oil towards the sea surface, from more than one hundred seafloor seep sites distributed throughout the deep province of the Lower Congo Basin. A large geochemical coring survey confirmed the presence of oil on the seabed. The combination with the seismic data considerably improved the identification of the origin of the oil slicks on the seabed. Multiple specific geophysical characteristics of thermogenic hydrocarbon seep sites were highlighted in a previous detailed analysis of seismic datasets. This study aims to test the characteristics previously identified at regional-scale. The active discharge zone is limited to the distal province of the basin. It is characterized by strong compression / shortening due to the sliding of the post-salt super-sequence, resulting in numerous salt diapirs that control the location of seafloor oil seep sites. The paper describes the bathymetric, reflectivity, amplitude and sub-bottom profiler characteristics of a group of thermogenic seep sites. They correspond essentially to submarine mounds or pockmarks of complex and irregular shape surrounded by hummocky mounds. Active oil pockmarks are systematically associated with positive amplitude anomalies on the seabed and are linked to vertical high-amplitude columns rooted in the seismic reflector and associated with the base of the gas hydrate stability zone. The sub-bottom profiler data shows that the hummocky mounds are connected

Introduction

Characterizing the natural transfer of hydrocarbons from petroleum reservoirs to the seafloor has strong implications on the understanding of the evolution of sedimentary basins (Gay et al., 2006a;[START_REF] Cartwright | Seal bypass systems[END_REF]Andresen et al., 2011, Anka et al., 2013;[START_REF] Marcano | Major controlling factors on hydrocarbon generation and leakage in South Atlantic conjugate margins: A comparative study of Colorado, Orange, Campos and Lower Congo basins[END_REF][START_REF] Serié | Subsurface fluid flow in the deep-water Kwanza Basin, offshore Angola[END_REF], deep marine ecosystems inventories [START_REF] Charlou | Physical and chemical characterization of gas hydrates and associated methane plumes in the Congo-Angola Basin[END_REF][START_REF] Ondreas | ROV study of a giant pockmark on the Gabon continental margin[END_REF][START_REF] Jones | Asphalt mounds and associated biota on the Angolan margin[END_REF] or submarine geohazards [START_REF] Bünz | Fluid flow impact on slope failure from 3D seismic data: a case study in the Storegga Slide[END_REF][START_REF] Hill | Deepwater Angola part I: Geohazard mitigation[END_REF], see the review by Anka et al. (2012). Pockmarks are among the most common expression of focused fluid flow at the seafloor [START_REF] King | Pockmark on the Scotian Shelf[END_REF][START_REF] Hovland | Seabed pockmarks and seepages: impact on geology, biology, and the marine environment[END_REF][START_REF] Judd | Seabed fluid flow: the impact on geology, biology and the marine environment[END_REF]. Their size and geometry depend on sediment host properties and hydrocarbon flux [START_REF] Hovland | Methane and minor oil macro-seep systems-their complexity and environmental significance[END_REF]. The ascent of fluids through the sedimentary series causes vertical successions of acoustic anomalies on seismic sections [START_REF] Cunningham | AAPG Memoir 73, Chapter 8: Seismic Evidence for Widespread Gas Hydrate Formation[END_REF][START_REF] Heggland | Seismic evidence of vertical fluid migration through faults: applications of chimney and fault detection[END_REF]Gay et al., 2006b). They are called either pipes, defined as vertical successions of increasing reflection amplitudes (typically narrow, i.e. up to a few 100m diameter) and known to commonly form large clusters, or chimneys, defined as large-scale (typically > 1 km diameter) and solitary features without any particular internal reflection [START_REF] Løseth | 1000 m long gas blowout pipes[END_REF][START_REF] Loncke | Mud volcanoes, gas chimneys, pockmarks and mounds in the Nile deep-sea fan (Eastern Mediterranean): geophysical evidences[END_REF][START_REF] Cartwright | Seal bypass systems[END_REF][START_REF] Løseth | Hydrocarbon leakage interpreted on seismic data[END_REF][START_REF] Hustoft | Effects of rapid sedimentation on developing the Nyegga pockmark field: Constraints from hydrological modeling and 3-D seismic data, offshore mid-Norway[END_REF][START_REF] Moss | 3D seismic expression of km scale fluid escape pipes from offshore Namibia[END_REF][START_REF] Løseth | 1000 m long gas blow-out pipes[END_REF][START_REF] Ho | Vertical evolution of fluid venting structures in relation to gas flux, in the Neogene, Quaternary of the Lower Congo Bain[END_REF][START_REF] Karstens | Seismic chimneys in the Southern Viking Graben-Implications for palaeo fluid migration and overpressure evolution[END_REF]. In the case of abundant gas migration towards the seabed, hydrates precipitate at the interface between free gas and hydrate domains. This forms a seismic reflector with a polarity opposite to that of the seabed, known as the Bottom Simulating Reflection (BSR) [START_REF] Shipley | Widespread Occurrence Of Possible Gas-Hydrate Horizons From Continental Slopes As Identified On Seismic Reflection Profiles[END_REF]Hovland et al., 1997;[START_REF] Mackay | Origin of bottom-simulating reflectors: geophysical evidence from the Cascadia accretionary prism[END_REF][START_REF] Berndt | Seismic character of bottom simulating reflectors: examples from the mid-Norwegian margin[END_REF][START_REF] Haacke | Gas hydrate, fluid flow and free gas: Formation of the bottom-simulating reflector[END_REF]. The identification of sea surface oil seepage slicks is used throughout the petroleum exploration workflow to validate source rock maturation, from targeting [START_REF] Kornacki | Impact of oil and gas vents and slicks on petroleum exploration in the deepwater Gulf of Mexico[END_REF][START_REF] Thrasher | Understanding geology as the key to using seepage in exploration: The spectrum of seepage styles[END_REF][START_REF] Williams | The Role of Satellite Seep Detection in exploring the South Atlantic's Ultradeep Water, in Surface exploration case histories: Applications of geochemistry, magnetic, and remote sensing[END_REF][START_REF] O'brien | Yampi Shelf, Browse Basin, North-West Shelf, Australia: a test-bed for constraining hydrocarbon migration and seepage rates using combinations of 2D and 3D seismic data and multiple, independent remote sensing technologies[END_REF][START_REF] Stalvies | Establishing hydrocarbon charge to the Ashmore Platform, Bonaparte Basin, Australia: A natural seeps study[END_REF] to prospect validation [START_REF] Jauer | Old marine seismic and new satellite radar data: Petroleum exploration of north west Labrador Sea, Canada[END_REF]. Positioning the outlines of multi-temporal slicks on the same map commonly shows radiating features from the central source area (De Beukelaer et al., 2003;[START_REF] Jatiault | Monitoring of natural oil seepage in the Lower Congo Basin using SAR observations[END_REF]. The spatial imagery was able to detect 50% of potential oil-supplying seep sites identified on exploration seismic in the Gulf of Mexico (Garcia- Basin (see details in section 2.2). This complementary study aims at testing the recognition criteria defined for a single oil seep on a larger seep population recognised in the Lower Congo Basin. The combination of sea-surface slicks database, geochemical cores and geophysical anomalies (highresolution surveys used for geo-hazards evaluation and exploration 3D seismic) made it possible to inventory oil-seep sites over a 35*50 km wide area. The comparison of oil-supplying seeps with the whole set of fluid-flow features at the seafloor and in the shallow subsurface will be used to better discriminate and define seismic features that can potentially be diagnostic of oil seep sites. This study discusses about the significance of fluid-related seismic features with an updated methodology integrating for the first time sea surface slicks and geophysical data in the Lower Congo Basin. Active oil seep sites will be compared with potential seep sites identified from geophysical attributes in the Lower Congo Basin.

Pineda et al., 2010).

Recently

The Lower Congo Basin

Geological setting and petroleum systems

The study area located between 10 to 12 °E and 6 to 8°S corresponds to a large oil-seeping province in deep offshore Angola characterised by water depths ranging from roughly 1000 to 3000 m. The chrono-stratigraphic framework of the LCB is divided into 3 major phases after the South Atlantic opening during the Early Cretaceous . The pre-salt (Berriasian to Barremian) succession consists of extensive lacustrine deposits including the oil-prone (mix type I/II kerogens) Bucomazi Fm (Fig. 1c;[START_REF] Burwood | Angola: source rock control for Lower Congo Coastal and Kwanza Basin petroleum systems[END_REF]MacHargue, 1990;[START_REF] Schoellkopf | Petroleum systems of offshore, Cabinda, Angola[END_REF][START_REF] Brownfield | Geology and total petroleum systems of the west-central coastal province (7203)[END_REF]. The transitional phase (Late Barremian to Aptian) marks the end of the crustal extension followed by a period of thermal subsidence [START_REF] Brice | Tectonics and sedimentation of the South Atlantic Rift Sequence: Cabinda, Angola[END_REF][START_REF] Mchargue | Stratigraphic Development of Proto-South Atlantic Rifting in Cabinda, Angola -A Petroliferous Lake Basin[END_REF][START_REF] Fort | Salt tectonics on the Angolan margin, synsedimentary deformation processes[END_REF]. The confined environment of the margin together with a drop in the main sea level enabled the deposition of a thick evaporite layer (1000 m thick in average; [START_REF] Marton | Evolution of the Angolan Passive Margin, West Africa, With Emphasis on Post-Salt Structural Styles[END_REF]Fig. 1b). The postsalt super-sequence is subdivided into three main episodes. The aggrading stage started with the development of a carbonate-ramp (Pinda Fm; 113 -95 Ma) under stable climatic and restricted marine conditions [START_REF] Haq | The new chronostratigraphic basis of Cenozoic and Mesozoic sea level cycles. Timing and depositional history of eustatic sequences: constraints on seismic stratigraphy: Cushman Foundation for Forminiferal Research[END_REF][START_REF] Séranne | Early Oligocene stratigraphic turnover on the West Africa continental margin: a signature of the Tertiary greenhouse-to-icehouse transition?[END_REF][START_REF] Séranne | South Atlantic continental margins of Africa: a comparison of the tectonic vs climate interplay on the evolution of equatorial west Africa and SW Africa margins[END_REF]. This period continued with a progressive deepening of the basin due to thermal subsidence, leading to the deposition of the oilprone Iabe Fm (99 -65 Ma, ~10 % TOC;[START_REF] Burwood | Angola: source rock control for Lower Congo Coastal and Kwanza Basin petroleum systems[END_REF][START_REF] Cole | Petroleum geochemical assessment of the Lower Congo Basin[END_REF] and subsequent (~4% TOC) Landana Fm (65 -45 Ma;Cole et al., 2000, Schoellkopf and[START_REF] Schoellkopf | Petroleum systems of offshore, Cabinda, Angola[END_REF]Fig. 1c). The pre-salt series are the most prolific source intervals in the basin as Tertiary source rocks are immature over most of it. The Eocene/Oligocene transition corresponds to a period of amplified sea level variations ("Ice house" period; Séranne, 1999). It is marked by a regional unconformity, corresponding to a 10 Ma hiatus controversially interpreted as originating from the African superswell [START_REF] Nyblade | The african superswell[END_REF] or climate fluctuations. The 500 m thick eroded series were transported towards the basin [START_REF] Teisserenc | Sedimentary basin of Gabon--geology and oil systems[END_REF][START_REF] Séranne | Seismic super-units and post-rift evolution of the continental passive margin of southern Gabon[END_REF][START_REF] Anka | The long-term evolution of the Congo deep-sea fan: A basin-wide view of the interaction between a giant submarine fan and a mature passive margin (ZaiAngo project)[END_REF]. The prograding stage (35 -5 Ma) corresponds to the establishment of the paleo-Congo during the early Oligocene. It was coupled with an "ice house" period, resulting in the deposition of siliciclastic sediments (Malembo Fm).

The Miocene-Pliocene transition is marked by the shift of the Congo River to its present-day location.

The development of an important upwelling system during the Early Quaternary (Uenzelmann-Neben, 1998) resulted in fining-upward sedimentation, leading to fine-grained deposits in the deep sea fan.

The post-salt super-sequence is affected by gravitational deformation, sliding over the Aptian salt décollement layer. Salt tectonics subdivided the basin into an upslope extensional domain, a translated area and a downslope compressive province characterised by salt diapirs, which confined the sedimentation to minibasins [START_REF] Oluboyo | Salt tectonic controls on deepwater turbidite depositional systems: Miocene, southwestern Lower Congo Basin, offshore Angola[END_REF]. [START_REF] Gay | Sinuous pockmark belt as indicator of a shallow buried turbiditic channel on the lower slope of the Congo Basin, West African Margin[END_REF]Gay et al., 2006a;[START_REF] Gay | Geological controls on focused fluid flow associated with seafloor seeps in the Lower Congo Basin[END_REF]Andresen et al., 2011;[START_REF] Wenau | Cold seeps at the salt front in the Lower Congo Basin II: The impact of spatial and temporal evolution of salt-tectonics on hydrocarbon seepage[END_REF][START_REF] Jones | Asphalt mounds and associated biota on the Angolan margin[END_REF]

Literature review of natural hydrocarbon seeps

The LCB is affected by a large active fluid seepage system, as revealed during intensive petroleum exploration over the last decade. Seabed outcrops of Miocene series, characterised by extensive high amplitudes above the crest of the salt diapirs, correspond to preferential stratigraphic pathways for migrating fluid in the salt provinces [START_REF] Serié | Subsurface fluid flow in the deep-water Kwanza Basin, offshore Angola[END_REF] [START_REF] Unterseh | Early Recognition of Seabed and Sub-Seabed Natural Hydrocarbon Seeps in Deep Offshore Angola[END_REF][START_REF] Jones | Asphalt mounds and associated biota on the Angolan margin[END_REF]. The subsurface is affected by several geophysical features including a shallow High Impedance Layer (HIL) that reaches the seafloor at the location of depressions on the sub-bottom profiler data. The HIL is interpreted as related to carbonate concretions with a possible contribution of asphalt layers [START_REF] Hill | Deepwater Angola part I: Geohazard mitigation[END_REF], (2) a set of two parallel reflections, about 50 ms apart, whose polarity is opposite that of the seafloor. They are interpreted as a methane-related BSR overlying a second BSR that could be related to a mix of heavier gas [START_REF] Jatiault | Geophysical characterisation of active thermogenic oil seeps in the salt province of the lower Congo basin part I: Detailed study of one oilseeping site[END_REF], (3) two different positive amplitude anomalies horizontally delimited by the BSR; massive high-amplitude bodies below and vertical pipes that connect the BSR with the seafloor above.

Data and Methods

This study adopts the methodology of [START_REF] Serié | Subsurface fluid flow in the deep-water Kwanza Basin, offshore Angola[END_REF] to characterize the fluid flow phenomena in the Kwanza basin. We refined it by integrating a detailed slicks inventory together with AUV-based sub-bottom profiler data. Precise location of oil slicks origins at the sea surface better locates the location of the active seeping vents at the seafloor [START_REF] Jatiault | Monitoring of natural oil seepage in the Lower Congo Basin using SAR observations[END_REF] and to evaluate the seepage activity. In addition, the SBP provides greater discrimination between seafloor fluid-related morphotypes and shallow acoustic anomalies. All the maps shown in this publication (Fig. 3a,b; Fig. 6a,b; Fig. 7f; Fig. 9a,b and Fig. 11 a,b) cover the exact same area (red square location in Fig. 2) which has been upscaled to enhance the quality of maps although the study was conducted over a larger area that corresponds to the province imaged with 3D seismic.

SAR data and seepage slicks inventory

The entire SAR collection consists of long-range monitoring integrating 156 scenes acquired over 21 years. The variability of the SAR acquisition data distribution is high, both temporally and spatially, as seen in Table 1 and Fig. 2, respectively. The oil-seeping province is imaged with a stack of SAR scene collection varying from 50 to 150 overlapping scenes (Fig. 2). The SAR dataset consists of a combination of X-band (wavelength of 2.5 to 3.75 cm) and C-band (3.75 to 7.5 cm) data that are known to be the most suitable wavelengths to detect oil slicks at the sea surface, enabling the recognition of the thinnest oil films [START_REF] Gade | Imaging of biogenic and anthropogenic ocean surface films by the multifrequency/ multipolarization SIR-C/X-SAR[END_REF]. The oil at the sea surface reduces the wind-induced capillary waves, thereby locally decreasing the radiometric values on SAR data. The discrimination of look-alikes (i.e. platform or boat spill, low wind effects, algal blooms) is primarily based on object features (radiometric values, edge sharpness and sinuosity), the remaining potential seepage slicks database is stacked on a compilation map. The variable orientation of surface slicks controlled by the sea surface water displacement (wind and currents) creates diverging patterns whose centres identify the oil surfacing area (black stars in Fig. 2

Geochemical coring

The seafloor coring campaign was based on regional 2D seismic sections, across the deep transitional/compressive province of the LCB (blue triangles in Fig. 2). Piston cores were collected prior to the acquisition of high resolution 3D seismic presented in this paper. The entire dataset consists of 10 488 piston cores recovered during several coring campaigns between 1998 and 2010 by TDI Brooks.

The piston cores consist of a vertical succession of 20 cm-long sub-sections with a maximum depth of 5.6 m. The geochemical analyses include:

-Gas Chromatography -Mass Spectrometer (GC -MS) which provides the distribution of alkane composition (Sn alkane) assessed from the retention time (performed for 423 cores out of the whole set). The sum of individual extractible alkane portions provides an estimation of the concentration of high molecular-weight hydrocarbon (C15+ hydrocarbon concentration).

-Unresolved Complex mixture (UCM) defined as the unextractable organic carbon with chromatogram (C15+).

-Total Scanning Fluorescence (TSF) intensity measured with spectrometry technique, which helps quantify the fluorescence of aromatic components [START_REF] Abrams | Significant of hydrocarbon seepage relative to petroleum generation and entrapment[END_REF]. The maximum intensity (I max) corresponds to the maximal values of fluorescence between emission (EM) spectrum and excitation (EX) spectrum (e.g. [START_REF] Barwise | Predicting oil properties from core fluorescence[END_REF]. The ratio (R1) between maximal value of EM and EX provides an indirect estimation of °API (American Petroleum Institute; API gravity = -0.1*R1 + 59.8).

Seismic dataset

The available seismic dataset consists of a 3D seismic cube used for petroleum exploration, complemented by a high-resolution survey used for geo-hazards evaluation acquired with an Autonomous Underwater Vehicle (AUV). The seismic cube covers a 3300 km² area with a maximum record length of 4500 ms TWT (Fig. 2). The source system is composed of two airgun clusters, 50 m separated with shot interval of 25 m. The sensor system consists of 10 parallel, 100 m separated streamers, and composed of 396 individual receiver groups separated by 25 m. We used the near-offset seismic trace.

The Sub-Bottom Profiler (SBP) dataset followed a grid with a line spacing of 1000m in the slope direction and 175m in the strike direction, and was acquired in two surveys operated in 2009 by C&C and in 2011 by Fugro (Table 2). The combination of the two datasets consists of an 830 km² gridded area and 5500 km of SBP sections. The high-frequency SBP is characterised by a 10 cm vertical resolution whereas the penetration is limited to the first 100 ms TWT below seafloor. We used the amplitude attribute to display the 3D seismic impedance contrast at lithological interfaces. The RMS (Root Mean Square) amplitude was used to display SBP sections and to identify geophysical anomalies on 3D seismic. Seafloor curvature maps are computed from the first arrivals of the 3D seismic cube, using a window size of 3 profiles and an interpolation length of 5 samples.

The high-resolution survey includes bathymetry (EM2000 multibeam echo-sounder) and seafloor reflectivity acquired along the acquisition tracklines of the vehicle. In areas not surveyed by the AUV, lower-resolution seafloor data (bathymetry, seismic amplitude) were taken from the seafloor reflections picked on the 3D seismic using Sismage software [START_REF] Guillon | Sismage and the 3d visualization at total[END_REF]). In addition, the seafloor origin of a large majority of (>80 %) of recurrent oil seep sites were imaged at large scale (16 000 km²), 12.5m resolution multibeam bathymetry. The computation of the slope map with superimposed seafloor isochrones enhances the imagery of seafloor features. Seafloor curvature maps are computed from the first arrivals on the seafloor, extracted from the 3D seismic cube using Sismage software and taking estimation window size of three samples and an interpolation length of five samples.

Analyses and results

Evidence of active oil seeps

Sea surface manifestations of oil presence

The visual analysis of the 156 SAR scenes revealed more than 3000 seepage slicks characterised by elongated/curvilinear trends with variable length (Fig. 3a). Diverging black streaks and OSO

Seafloor geochemical coring

The hydrocarbon content of seafloor piston cores is strongly variable (Fig. 4a). Geochemical analysis of heavy (C15+) hydrocarbons shows that the sum of alkane (Sn) concentration (C16 -C32) ranges from 0.1 to 36 ppm while the concentration of UCM varies from 2 to 85 000 ppm. While the fluorescence (TSF) intensity changes from 1 116 to 87 240 000, 35 % of the cores have greater values than the regional background defined at 10 000 units [START_REF] Serié | Subsurface fluid flow in the deep-water Kwanza Basin, offshore Angola[END_REF]. The plot of fluorescence vs. C15+ concentration is an indirect method for quantifying the presence of heavy hydrocarbons in sub-surface sediments (Fig. 4a). the TSF intensity is at least 10 times greater than the regional background (100 000 < TSF < 1 000 000).

The presence of hydrocarbon is qualified as being sure for 20 cores having TSF values greater than 10 6 .

The observation of asphalt inclusions for 10 piston cores also confirms the migration of oil towards the seafloor (black triangles in Fig. 4a). Even if they represent only 2% (10/488) of the total core array, they correspond to the highest TSF and UCM values.

Fluid flow features on the seafloor

Seafloor features that may be related to fluid venting are categorised into 7 main families: conical pockmarks (isolated and lined-up), seafloor gullies, complex-shaped pockmarks (clusters of heterometric pockmarks and irregular isolated pockmarks), submarine mounds and scattered positive topographic hummocky features (Fig. 5).

The conical pockmark-gully continuum

Conical pockmarks can reach km-scale diameters with maximum depth of 150 m and are characterized by gentle slopes of ~15° (Fig. 5a). With a total number of 890, they are widespread throughout the study area and mostly occur in salt minibasins (Fig. 6).

Lined-up conical pockmarks (Fig. 5b), defined as a succession pool of at least 3 conical pockmarks along a main direction, occur in 3 main settings. First, they are sparsely distributed along the maximal flexural axis of salt minibasins (Fig. 6). Increase of their number and density can ultimately lead to form curvilinear gullies (Fig. 5c) corresponding to deep (>50 m) and elongated (> 1 km) incised depressions on the seafloor. Second, conical pockmarks can occur at the tip of normal faults on the seafloor in association with the sagging of minibasins. Third, they are located at the seafloor interface unconformity between uplifted slabs of Miocene-Pliocene series at diapirs and Quaternary siltstones in minibasins. The total amount of conical pockmarks, including isolated ones and aligned sets, is significant in the study area (1 750 individual features) (Table 3), which represents 85% of the total fluid escape features identified on the bathymetric data.

Submarine mounds

Submarine mounds are bathymetric highs on the seafloor with a typical relief of 5 to 20m above the surrounding seabed and diameter up to 500m (Fig. 5d). We identified only 17 such mounds; the only one of these that is imaged on HR data appears as a cluster of smaller individual mounds, whose slopes range from 10 to 30°. The high-resolution bathymetry map reveals fault sets diverging radially away from the seafloor mounds; similar radial patterns have been observed around salt diapirs by

Carruthers et al. (2013).

Submarine mounds are clearly discernible on the seafloor as patches with higher reflectivity compared to the surrounding seabed, indicating that they are formed of higher impedance material than the background silts and clays. Seafloor mounds are located above allochthonous salt diapirs like the irregular pockmark clusters (Fig. 5). Most of the submarine mounds are associated with spiky diapirs (needle-shaped salt protrusion), and have not been observed above bulbous ones. No clear distinction in terms of seepage proficiency (occurrence rate) could be established between submarine mounds and other active features.

Complex shaped pockmarks

Complex-shaped pockmarks are uneven fluid escape features on the seafloor with a main depression whose depth does not exceed a few tens of meters. Complex-shaped pockmarks occur as two distinctive patterns; (1) Clusters of heterometric pockmarks (Fig. 5e), defined as an aggregation of a large number of unit pockmarks [START_REF] Hovland | Unit-pockmarks and their potential significance for predicting fluid flow[END_REF], with clusters reaching 2-3 km and exceeding 1500m in length (Jatiault et al., 2019, Part I). They usually develop along the salt-related fault networks at the rim of a salt canopy, thereby creating a complex-shaped depression at broader scale, that contains different heterometric pockmarks;

(2) Irregular pockmarks (Fig. 5f), corresponding to sub-circular depressions with a rough aspect at the base. Local depressions are associated with a strong high-amplitude anomaly on the seafloor reflectivity map (Fig. 5). In contrast with previously described mounds, they are mostly found along landward flanks of bulbous diapirs, and never appear in association with spiky ones. We identified 109 clusters of heterometric pockmarks and 175 irregular isolated pockmarks.

Seafloor hummocky areas

The seafloor is affected by rough patches clustering over limited-size areas (10-15m in diameter and few meters high), on top of salt diapir. They were identified on high-resolution bathymetry maps and therefore remain below the 3D seismic resolution (Fig. 7a). However, seafloor hummocky induces a slight disturbance in the seafloor creating a rough aspect that may be discernible from the exploration seismic resolution using specific seismic attributes such as seabed curvature (Fig. 7a). On the HR bathymetry, hummocky areas consist of a numerous small hummocks (Fig. 7b). The discrimination of hummocky features is particularly enhanced using the curvature attribute that measures the bathymetric slope inflexions (Fig. 7c). On curvature maps, hummocky features are characterised by a central (round or elongated; Fig. 7d) high positive value over topographic features, surrounded by subcircular medium values (~0) and by negative values at the rim (Fig. 7e). We assumed that rough patches correspond to zones with a high density of seafloor hummocks and individual positive sub-circular features correspond to individual hummocks. The presence of hummock clusters is associated with complex-shaped pockmarks (clusters of heterometric and irregular pockmarks) in most cases. Underlying HIL remains buried under few tens of ms below the seafloor and connected to hummocks by shallow faults (Fig. 5g). We identified 2946 possible individual hummocks over the area imaged with 3D seismic. Ten areas with a high hummock concentration were identified on the HR survey and an additional 50 on the wider 3D survey (12.5m resolution) (Fig. 7). 3D

seismic data detects about 55% of individual hummock features visible on the HR survey. The number of discernible hummock features in clusters ranges between 10 to 210 on the 3D seismic, covering

One piston core was drilled inside a hummocky cluster visible on the 3D seismic and showed asphalt inclusions in the sediment. Hummocks have the same morphology and geophysical characteristics as asphalt mounds and their distribution is similar to that of those previously identified in the area [START_REF] Unterseh | Early Recognition of Seabed and Sub-Seabed Natural Hydrocarbon Seeps in Deep Offshore Angola[END_REF][START_REF] Jones | Asphalt mounds and associated biota on the Angolan margin[END_REF]. Seafloor hummocks are interpreted as seafloor asphalt storage that reached the seafloor through scattered emission points above the crest of salt diapirs, following a stage of oil migration across sedimentary series.

Seismic geomorphology in the sub-surface

Positive geophysical anomalies on the 3D seismic

We computed the RMS amplitude stack of the first 100 ms TWT below the seafloor reflection to identify geophysical anomalies (Fig. 8 A). Geophysical anomalies occur as three main distinct features, or categories. Above shallow salt diapirs, the RMS interval cross-cuts the top of salt, making high-amplitude anomalies that are unrelated to fluid effects. We dismissed these areas from the interpretation (purple patches in Fig. 8).

The first type of acoustic anomaly (1 on Fig. 8 A) is defined as a vertical succession of low-amplitude reflections. Low-amplitude pipes suggest acoustic dimming in the sedimentary pile where the impedance contrasts are attenuated across successive lithologic interfaces. Low-amplitude pipes are essentially spread across salt minibasins or aligned along the maximal flexural axis of salt minibasins (Fig. 8 A; Fig. 9). We identified a large number of low-amplitude pipes (582) across the area imaged using the exploration seismic (Fig. 9). The low-amplitude anomalies identified on the 3D seismic correspond to the locations of conical shaped pockmarks (isolated and aligned) and gullies (Fig. 8 C).

The lithologic interfaces are disrupted by an acoustically transparent zone in the sub-bottom profiler below the location of conical pockmarks (Fig. 5). Lithologic interfaces are either truncated by the seafloor depression or associated with a slope inflection, stacked on successive lithologic interfaces. In the former case, the active fluid seep probably corresponds to a recent initiation of the seepage activity. In the latter case the deepest interval affected can be followed down to 600 ms TWT on the exploration seismic. On the sub-bottom profiler, stratigraphic boundaries surrounding the lowamplitude anomalies remain well organised down to the acoustic penetration. Most correspond to the bulls-eye pockmarks defined by Andresen et al. ( 2011).

The second type (2; Fig. 8 A) of acoustic anomaly corresponds to moderate positive amplitude anomalies related to outcropping older sediments (mostly Neogene) above salt diapirs; these anomalies gradually dim out laterally (Fig. 8 E). Affected areas range from 1 to 10 km². Moderate amplitude anomalies are composed of consistent, extensive patches indistinctively downslope or upslope (Fig. 8 E) in relation to the depocentre shift/tectonic inversion [START_REF] Quirk | Flip-flop salt tectonics[END_REF].

Uplifting of older sediments can also be detected on the sub-bottom profiler sections where they occur as strong threadlike reflections (Fig. 8 F).

The third type (3; Fig. 8 A

Negative polarity reflections

Occasionally, consistent negative reflections crosscut stratigraphic series with all the characteristics of a methane hydrate BSR. The BSR occurs as extensive reflectors in minibasins with a distance from the seafloor ranging roughly from 250 to 500 ms TWT and is usually underlain by a thick High-Reflectivity Zone (Andresen et al., 2011).

generally occurs in discrete small patches (Fig. 10a). This difference between the overburden of the diapirs and minibasins is known to be associated with the high thermal conductivity of the salt, which concentrates heat above diapirs, resulting in a higher geothermal gradient there, and consequently a thinner hydrate stability zone [START_REF] Lucazeau | High-resolution heat flow density in the lower Congo basin[END_REF]. The BSR corresponds to the basal limit of pipelike features (type 3 anomalies) characterised by a vertical succession of PHAA and is underlain by a 100 ms-thick high-amplitude interval.

The sub-surface series are also locally affected by an occasional second negative polarity reflection.

The horizon occurs as a continuous, crosscutting surface and tends to be parallel to the first BSR. The vertical distance with the BSR is mostly constant (~50ms), or occasionally increases towards the minibasins (Fig. 10). The thermal effect associated with the presence of a salt diapir also affects the vertical location of the second BSR by decreasing the distance from the seafloor. Double BSRs are observed exclusively above salt diapirs. Both BSRs are concomitantly and locally deflected upward below seafloor depressions, which also correspond to the locations of PHAA (Fig. 10).

Subsurface high impedance layer

A consistent High-Impedance Layer (HIL) frequently appears on sub-bottom profiler sections, in the shallow sub-surface above the salt diapirs (Fig. 8 H). HIL are only identified on the high-resolution survey and consist of a high-amplitude cross-cutting reflector. High-Impedance Layers occur in patches 29 whose surface areas can reach 120 km², the average being 20 km² (Fig. 11 b). They are located above diapiric areas, mostly at the upslope flank but can also be occasionally found downslope. The TWT difference with the seafloor ranges from 0 to 80 ms TWT above diapirs and can be higher (up to 200 ms TWT) in minibasins.

Synthesis and discussion

Integrative synthesis of the oil seepage situation

A subset of 22 cores spatially corresponds to the vertical projection of recurrent oil seep sites detected from the SAR study, within a radius of < 1500 m (red dots and green triangles in Fig. 4b). Geochemical analyses were successfully performed for 19 out of 22 cores. For these sampled oil seep sites, the C15+ concentration (Sn Alkanes + UCM) shows high concentrations of heavy hydrocarbons and TSF values at least 50 times higher than the regional background. A large majority (17/19) of sampled oil seep sites fall into the "Probable" and "Sure" categories and the remaining (2/17) into the "Possible" category (red dots and green triangles in Fig. 4b). These observations validates the assumption that lateral drift in the water column averages to nil over time and confirms that, without any systematic offset, the pockmarks beneath the barycentre of OSOs corresponds to the source of expelled oils, on a regional scale as demonstrated from integration of vertical mooring data [START_REF] Jatiault | Deflection of natural oil droplets through the water column in deep-water environments: The case of the Lower Congo Basin[END_REF].

Considering that 58 pistons belong to the "Probable" and "Sure" categories, 38% (22/58) correspond to the location of recurrent seepage slicks detected with the SAR system. The oil gravity, estimated using the fluorescence technique, ranges from 10 to 40°API for active oil seep sites associated with sea surface manifestations of oil.

Even though visual confirmation of asphalt inclusion remains uncommon (10 cores; black triangles in Fig. 4a), 40% of drilled sites showing asphalt inclusions (4/10; green triangles in Fig. 4b) are located close to a recurrent seep site at the sea surface. The geochemical analysis is available for 2 cores associated with asphalt inclusions and shows that the TSF values and the concentration in C15+ reaches the highest values in the core dataset (Fig. 4b).

We identified multiple fluid-related morphotypes on the seafloor in our study area (Fig. 5). Let us examine which ones can be considered characteristic of oil-bearing seeps compared to other fluid releases, by comparing geophysical features against the results of the SAR study. The number of conical pockmarks, including aligned and isolated cones, represents 85% of the total fluid escape features identified on the bathymetric data (1 750 individual features) (Table 3). Although these features are abundant, none are presently responsible for oil seepage detected with SAR systems. Conversely, clusters of heterometric pockmarks, irregular pockmarks and submarine mounds are occasional across the margin. Respectively, 109, 175 and 17 were indexed from bathymetric analyses, making barely 15 percent of the total number of identified seafloor fluid-escape features. These 15 percent account for 98% of seafloor vents responsible for oil supply during the observation period (Table 3).

Seafloor hummocky areas are associated with oil slicks detected with SAR data for 5 out of the 10 seafloor hummocky areas detected on the HR dataset and 21 out of the 50 identified from the 3D seismic. Both the HR survey and information deduced from 3D seismic suggest that barely half of asphalt mounds fields are associated with detectable and recurrent slicks at the sea surface (21 out of entire 50 hummocky areas array).

On the subsurface, the area imaged with the 3D seismic covers 30 individual recurrent oil seep sites, 28 of which are either directly related to shallow salt diapirs, or correspond to the outcropping older sediment slabs uplifted by differential salt movements. The seafloor origin of recurrent oil seep sites visible exclusively on SAR data (100 %) corresponds to PHAA pipes in the shallow subsurface (<100ms; Fig. 9). Among the 75 individual vertical pipes identified (type 3; Fig. 8 A), about 40% of geophysical anomalies (30/75) are currently associated with prolific seep sites. Conversely, none (0%) of low amplitude pipes identified across the study area are associated with oil seeps visible on space-borne data. Among the 75 local positive geophysical anomalies (type 3) recognised using the 3D seismic, about 60 % (45/75) are associated with a BSR. Among the 30 recurrent oil seep sites imaged with 3D seismic, 21 (70 %) are emplaced directly above a shallow BSR. Double BSR are unusual in the study area but exclusively occur to oil seep sites detected with the SAR system (see red patches in Fig. 11 a).

Six of the 30 recurrent oil seep sites detected with the SAR system and imaged with 3D seismic are directly emplaced above a double BSR. All the detected oil seeps belong to the areas associated with the development of a HIL (Fig. 11 b). Moreover, the interpreted seafloor origin of oil seeps (irregular isolated pockmarks and cluster of heterometric pockmarks; Fig. 5) corresponds to areas where the HIL reach the seafloor. There is a strong geographical correlation between seafloor-reaching HIL, shallow local BSR and PHAA. At the submarine mound locations, the interface between the series affected by and unaffected by the HIL are characterised by a sharp, strong acoustic impedance contrast (Fig. 5d).

The HIL crosscuts the stratigraphic series and rise towards the seafloor at the mound locations without revealing any particular internal organisation compared to stratigraphic boundaries. At the location of complex shaped pockmarks, the interface between the series affected and unaffected by the HIL is continuous and sometimes tends to align with stratigraphic boundaries (Fig. 5e).

Significance of thermogenic seep sites geometry

Seafloor expressions

Defining characteristic features of thermogenic seeps on geophysical data is crucial for the understanding of petroleum system plays, to define the best seafloor coring targets and to seek and Previous work on the LCB reported a large number of clustered pockmarks at roughly 550 metres depth [START_REF] Gay | Geological controls on focused fluid flow associated with seafloor seeps in the Lower Congo Basin[END_REF], i.e. along the feather edge of the BSR at the seabed. The presence of salt is known to deform the temperature field above and around diapirs [START_REF] Lucazeau | High-resolution heat flow density in the lower Congo basin[END_REF], and may affect the salinity of formation water conditions. However, the identification of BSRs above salt diapirs in this area indicates that the BGHSZ remains within the sedimentary interval at salt protrusion areas (Fig. 11). The strict association between submarine mounds and spiky diapirs suggests that the development of mounds depends on the degree of focusing of the hydrocarbon migration system.

Similar shaped submarine mounds (Pingoes) were described and interpreted as composed of carbonate reefs interspersed with hydrates patches in the Norwegian Sea [START_REF] Hovland | Submarine pingoes: Indicators of shallow gas hydrates in a pockmark at Nyegga, Norwegian Sea[END_REF], but also in the Kwanza basin interpreted as imprinting the development of underlying hydrates nodules [START_REF] Serié | Gas hydrate pingoes: Deep seafloor evidence of focused fluid flow on continental margins[END_REF]. Pockmarks associated with pore-water and biogenic methane release occur extensively in minibasins in relation to local-scale features such as polygonal faults (Andresen and Huuse, 2011;[START_REF] Maia | Shallow plumbing systems inferred from spatial analysis of pockmark arrays[END_REF] and/or tectonic faults (Gay et al., 2006 c). Methanogenic carbonate precipitates even in the case of a "slow seep" [START_REF] Ho | Vertical evolution of fluid venting structures in relation to gas flux, in the Neogene, Quaternary of the Lower Congo Bain[END_REF]. The absence of positive high-amplitude anomalies [START_REF] Ho | Vertical evolution of fluid venting structures in relation to gas flux, in the Neogene, Quaternary of the Lower Congo Bain[END_REF] in association with conical pockmarks therefore suggests that the associated fluid is likely pore-water dominated. We have observed that pockmarks located in minibasins are steeper (~15°) and more conformable compared to those located above diapirs. Let us examine the factors that could explain this difference. The mid Plio-Quaternary sedimentation in salt minibasins is dominated by fine-grained sediments that act as an impermeable barrier to fluid migration (Uenzelmann-Neben, 1998; Anka et al., 2013). The sediment type could have a strong influence on pockmark geometry; the recent sedimentary overburden being gently eroded across the fluid ascent path could explain the conformable geometry of minibasin pockmarks. The uplift of older/coarser Miocene/Mid-Pliocene sediments [START_REF] Anderson | Controls on turbidite sand deposition during gravity-driven extension of a passive margin: examples from Miocene sediments in Block 4, Angola[END_REF], combined with salt-induced faulting and seafloor hydrates/carbonates distribution might be good candidates to generate multiple seafloor fluid outlets [START_REF] Jatiault | Geophysical characterisation of active thermogenic oil seeps in the salt province of the lower Congo basin part I: Detailed study of one oilseeping site[END_REF]. In this condition, this could explain the morphological differences between thermogenic seep sites above diapirs and pore-dewatering escape features in minibasin.

Studies

Sub-seafloor expression

Oil-supplying seeps correspond to small (100m diameter) sub-circular high amplitude anomalies on the seafloor amplitude map (d, e and f in Fig. 5; n°3 in Fig. 8A). The presence of a BSR shows that migration is focused above diapirs and that the presence of anomalies above the base of the hydrates indicates that the hydrate seal is locally breached when the fluid overpressure is too high [START_REF] Cathles | The physics of gas chimney and pockmark formation, with implications for assessment of seafloor hazards and gas sequestration[END_REF]. The vertical successions of strong reflections are interpreted as fluid escape pipes (Fig. 8 G; Löseth et al., 2011;[START_REF] Karstens | Seismic chimneys in the Southern Viking Graben-Implications for palaeo fluid migration and overpressure evolution[END_REF] which can be related either to vertical stacking of authigenic carbonate cementations or pavements [START_REF] Hovland | Methane-related carbonate cements in pockmarks of the North Sea[END_REF][START_REF] Hovland | On the self-sealing nature of marine seeps[END_REF][START_REF] Pierre | Authigenic carbonates from methane seeps of the Congo deep-sea fan[END_REF][START_REF] Römer | Methane fluxes and carbonate deposits at a cold seep area of the Central Nile Deep Sea Fan, Eastern Mediterranean Sea[END_REF], vein-filled/ hydrate-rich sediments [START_REF] Sahling | Pockmarks in the Northern Congo Fan area, SW Africa: Complex seafloor features shaped by fluid flow[END_REF] or gas-charged sediments [START_REF] Schroot | Shallow gas and gas seepage: expressions on seismic and otheracoustic data from the Netherlands North Sea[END_REF].

Outcrops of Miocene-Pliocene sediments at the seabed are privileged leakage pathways due to changes in lithologic properties from fine-grained/high cap-rock efficient upper Pliocene and Quaternary silts and clays to coarser, more permeable Miocene series [START_REF] Anderson | Controls on turbidite sand deposition during gravity-driven extension of a passive margin: examples from Miocene sediments in Block 4, Angola[END_REF] and appear as privileged fluid outlets.

The negative impedance contrast resulting in the BSR is known to primarily reflect the presence of free gas beneath the base of the hydrates stability zone, rather than the increase of impedance due to the presence of hydrates above [START_REF] Holbrook | Seismic studies of the Blake Ridge: Implications for hydrate distribution, methane expulsion, and free gas dynamics[END_REF]. The identification of a second BSR was reported in a few case studies [START_REF] Andreassen | A double gas-hydrate related bottom simulating reflector at the Norwegian continental margin[END_REF][START_REF] Foucher | Observation and tentative interpretation of a double BSR on the Nankai slope[END_REF]Popescu et al., 2006;[START_REF] Posewang | The enigma of double BSRs: indicators for changes in the hydrate stability field?[END_REF][START_REF] Zander | On the origin of multiple BSRs in the Danube deep-sea fan, Black Sea[END_REF] with various interpretations (changes in environmental conditions, paleo BSR relics, and heavier gas components). In the LCB, the observations of a second cross-cutting and negative polarity reflections exclusively occur;

(1) at the crest of salt diapirs and (2) in the conditions of proven thermogenic fluid seepage from the identification of recurrent slicks. The second BSR could be associated with heavier gas components in conditions of abundant thermogenic dysmigration (Jatiault et al., 2019, Part I). Stacked BSR 1 and 2 would then involve distinct hydrocarbon compositions between the two layers, which excludes the hypothesis of a mix of hydrocarbon components from BSR1 and BSR2, and therefore suggests distinct migration pathways. In that case, the 100% methane-related BSR would probably follow a lateral path along minibasin stratigraphic interfaces towards the crest of the diapir. Knowing that structure II and H hydrates are stable are greater depths [START_REF] Sloan | Clathrate Hydrates of Natural Gases[END_REF] and considering the limited extent of BSR2, the thermogenic BSRs would follow more vertical advection paths along salt flanks.

Relevance of multi-scale integrated evaluation of the thermogenic seepage potential

Pre-existing methodology

The combination of different geophysical observations provides key features to better understand hydrocarbons seep systems and fluid flow phenomena [START_REF] Serié | Subsurface fluid flow in the deep-water Kwanza Basin, offshore Angola[END_REF]. The integration of multiproxy tools is relevant for confirming source rock maturation and for studying fluid migration across sedimentary series. The technique used in pre-existing methodology has its own advantages and limitations that are listed and explained below:

-For underexplored areas, remote sensing analysis provides a first-order approach for evaluating the seepage potential [START_REF] Williams | The Role of Satellite Seep Detection in exploring the South Atlantic's Ultradeep Water, in Surface exploration case histories: Applications of geochemistry, magnetic, and remote sensing[END_REF]. The main SAR system limitations are its limited spatial resolution (Table 1) and dependency on weather conditions (1.5 to 5.5 m.s -1 ; Jatiault et al., 2017). On SAR images, low backscatter areas can also be look-alikes (such as algae blooms, surface micro-layer; [START_REF] Mityagina | Detection and discrimination of sea surface films in the coastal zone of northeastern Black Sea using SAR data[END_REF] and the natural origin of low radiometric features without a seafloor validation therefore remains questionable, even when recurrent [START_REF] Ryan | Recurrent frontal slicks of a coastal ocean upwelling shadow[END_REF]. The SAR detectability thickness threshold may prevent the recognition of active oil seeps when the oil flow is low.

-Even low-intensity oil seep sites are detectable with seabed geochemistry using fluorescence analysis [START_REF] Abrams | Significant of hydrocarbon seepage relative to petroleum generation and entrapment[END_REF]. The recognition of asphalt inclusions in cores demonstrates the thermogenic origin of fluid migrations. However, the pinpoint nature of geochemical coring means that even with a dense set of cores, the technique potentially misses a large array of active seep sites.

-The seafloor seismic amplitude provides information on lithological modifications, especially above salt diapir where the outcropping Pliocene/Miocene series constitutes privileged pathways for migrating fluids.

-The identification of a negative polarity BSR also helped us locate focused hydrocarbon fluid migration above salt diapirs [START_REF] Taylor | Trapping and migration of methane associated with the gas hydrate stability zone at the Blake Ridge Diapir: new insights from seismic data[END_REF][START_REF] Lucazeau | High-resolution heat flow density in the lower Congo basin[END_REF][START_REF] Serié | Subsurface fluid flow in the deep-water Kwanza Basin, offshore Angola[END_REF]. In the context of a shallow underlying diapir, the associated thermal anomaly creates an upward displacement of the BSR that is suitable for the formation of a hydrate-related trap for hydrocarbon migration. However, the survival period of a BSR following the cessation of fluid migration remains under-documented, limiting the appraisal of seepage activity.

Improvements of the methods

In addition to pre-existing methodology, the following additional data analysis can help to refine the fluid flow phenomena in sedimentary basin (Fig. 12). Let us examine what are the improvements performed by the integration of additional data:

-A detailed analysis of surface slicks better constrains oil seep sites locations and address the present-day seepage expelled volumes. In this study, recurrent surface slicks were successfully linked with seafloor features 98% of the time. This confirms the relevance of combining the SAR system with marine geophysical surface data to constrain hydrocarbon plumbing system during the exploration workflow. The occurrence rates defined as the ratio between the amount of detected slicks and SAR data coverage range from 5 to 80% [START_REF] Jatiault | Monitoring of natural oil seepage in the Lower Congo Basin using SAR observations[END_REF]. Considering that the SAR system efficiency depends on weather conditions and that active seep sites are mostly intermittent, exhaustive inventory of oil seep sites relies on a high multi-temporal satellite data coverage with at least fifty overlapping SAR scenes in our study area. The amount of oil that naturally enters the marine environment is roughly split into two equal parts between natural seepage and human activity [START_REF] Prince | Bioremediation of marine oil spills[END_REF]. For quantitative geological studies, it is necessary to discriminate natural seepage to unnatural oil [START_REF] Macdonald | Natural and unnatural oil slicks in the Gulf of Mexico[END_REF]. Considering average displacement values at the sea surface in the LCB of 17 cm.s -1 due to surface currents and winds [START_REF] Jatiault | Monitoring of natural oil seepage in the Lower Congo Basin using SAR observations[END_REF] and assuming that slicks thinner than 0.1 µm thick are not detectable [START_REF] De Beukelear | Distinct side-scan sonar, RADARSAT SAR, and acoustic profiler signatures of gas and oil seeps on the Gulf of Mexico slope[END_REF], the estimated minimum detectable oil flow varies between 0.76 to 9.2 l.h -1 depending on the spatial resolution of SAR systems (Table 4). Considering that 4380m 3 are expelled annually from one hundred seep sites [START_REF] Jatiault | Monitoring of natural oil seepage in the Lower Congo Basin using SAR observations[END_REF], the average expelled volume of a random oil seep is estimated at 5 l.h -1 in the study area. This demonstrates that the expelled volume is within the order of the detectability threshold. The SAR detectability thickness threshold might therefore be too high to detect the activity of a subset of potential seep sites interpreted from seismic data. -The integration of current profilers (or the use of regional metocean models when the former are not available) helps constrain probable oil migration routes through the water column and quantify the probable lateral deflection [START_REF] Jatiault | Deflection of natural oil droplets through the water column in deep-water environments: The case of the Lower Congo Basin[END_REF].

-The detection of asphalt mounds on HR survey confirms that oil is migrating towards the seafloor and constitutes another proof of an active oil-seep sites [START_REF] Jones | Asphalt mounds and associated biota on the Angolan margin[END_REF]. The presence of asphalt mounds implies that hydrocarbon dysmigration is sufficiently slow for the oil to be heavily degraded and stored on the seafloor [START_REF] Head | Biological activity in the deep subsurface and the origin of heavy oil[END_REF][START_REF] Larter | The controls on the composition of biodegraded oils in the deep subsurface-part 1: biodegradation rates in petroleum reservoirs[END_REF][START_REF] Jones | Crude-oil biodegradation via methanogenesis in subsurface petroleum reservoirs[END_REF][START_REF] Larter | The controls on the composition of biodegraded oils in the deep subsurface: Part II-Geological controls on 52 subsurface biodegradation fluxes and constraints on reservoir-fluid property prediction1[END_REF][START_REF] Peters | The biomarker guide[END_REF][START_REF] Peters | The Biomarker Guide Volume 2, Biomarkers and Isotopes in Petroleum Systems and Earth History[END_REF][START_REF] Jones | Crude-oil biodegradation via methanogenesis in subsurface petroleum reservoirs[END_REF]. In the present case, using specific seismic attributes such as curvature identified a high proportion of the mounds and may be used at least for a first screening.

-The computation of an RMS amplitude map of shallow subsurface (first 100 ms in this study)

quickly pinpointing isolated sub-circular high-amplitude anomalies likely to sign the presence of fluid migration, while reducing the amplitude anomaly associated with the slope inflection. Vertical stacking of anomalies, if observed on cross-sections, is a strong additional indication of hydrocarbon leakage.

However, the technique is irrelevant for assessing the fluid type.

-We observed double BSRs only at the location of oil seeps proven by SAR analysis. Double BSR could mark the basal level of stability of a mix of heavier gas. Additional studies would be needed to know if this is related to compositional or concentration effects.

-The HIL is presumably related to limited-size carbonate slabs with shallow asphalt and The inventory of geophysical features associated with the oil seeps sites in this study would benefit from comparative studies conducted in different geological contexts.

Spatio-temporal variability of oil seepage activity

The combined manifestation of recurrent sea-surface slicks and seafloor evidence of active fluid flow demonstrates that liquid hydrocarbons are currently being expelled from the seafloor in the study area. The combination of seismic data together with SAR imagery and geochemical features validated the natural origin of the seepage [START_REF] Serié | Subsurface fluid flow in the deep-water Kwanza Basin, offshore Angola[END_REF]. We highlighted a series of geophysical features (seafloor and subsurface) associated with oil-seep systems that were extrapolated towards similar fluid flow features that could also be responsible for thermogenic escapes, namely potential oil-seep sites.

Both geochemical and geophysical evidence demonstrate that many potential seep sites were not associated with recurrent sea-surface slicks. The present-day seepage activity observed on SAR data corresponds to 40 % of the total array of potential oil seep sites detected on the seafloor and subsurface (Table 3). The remaining 60 % of geophysical anomalies n°3 present the same characteristics The SAR data collection dates from 1994 to 2016, which is very brief in comparison to geological time scales. The remaining 60% of geophysical anomalies could also be associated with dormant seep sites whose inactive stages: (1) exceed the SAR monitoring period (22 years), or (2) are shorter than SAR acquisition intervals (varying from few hours to few months).

Barely half of seafloor areas characterised by seafloor asphalt presence (seafloor mounds and oilimpregnated cores) are currently supplying oil. Using fluorescence as a proxy for oil gravity [START_REF] Abrams | Significant of hydrocarbon seepage relative to petroleum generation and entrapment[END_REF], seafloor samples in our study area range from 10 to 40°API. High biodegradation leads to an increase in the density of the oil in the reservoirs [START_REF] Head | Biological activity in the deep subsurface and the origin of heavy oil[END_REF][START_REF] Peters | The Biomarker Guide Volume 2, Biomarkers and Isotopes in Petroleum Systems and Earth History[END_REF] but also in the shallow sub-surface (e.g. [START_REF] Zengler | Methane formation from long-chain alkanes by anaerobic microorganisms[END_REF]Wilkes et al., 2001), so that biodegraded oils can reach density values greater than that of ocean water [START_REF] Larter | The controls on the composition of biodegraded oils in the deep subsurface-part 1: biodegradation rates in petroleum reservoirs[END_REF]. For oil when the °API gravity is below that of ocean water (10), asphalts remain stored on the seafloor [START_REF] Macdonald | Asphalt volcanism and chemosynthetic life in the Campeche Knolls, Gulf of Mexico[END_REF][START_REF] Jones | Asphalt mounds and associated biota on the Angolan margin[END_REF]. Bathymetric hummocky features, reportedly composed of a large volume of stored asphalt on the seafloor, have already been observed in other provinces without sea surface manifestations [START_REF] Keller | Tectonic geomorphology and hydrocarbon induced topography of the mid-channel Anticline, Santa Barbara Basin, California[END_REF][START_REF] Valentine | Asphalt volcanoes as a potential source of methane to late Pleistocene coastal waters[END_REF].

Extreme marine ecosystems are highly reactive to hydrocarbon flow fluctuations with a survival period following seepage extinction of about one year [START_REF] Hessler | Temporal change in megafauna at the Rose Garden hydrothermal vent (Galapagos Rift; eastern tropical Pacific). Deep Sea Research Part A[END_REF][START_REF] Shanks | Mid-Ocean Ridges-Rebirth of a Sea-Floor Vent[END_REF][START_REF] Shank | Temporal and spatial patterns of biological community development at nascent deep-sea hydrothermal vents (9 50′ N, East Pacific Rise)[END_REF][START_REF] Mullineaux | Tubeworm succession at hydrothermal vents: use of biogenic cues to reduce habitat selection error?[END_REF]. In-situ records of photo/video from ROV dives would greatly help ascertain the presence of modern chemosynthetic communities and therefore address the seeping mechanism and activity of the remaining 60% dormant geophysical anomalies [START_REF] Charlou | Physical and chemical characterization of gas hydrates and associated methane plumes in the Congo-Angola Basin[END_REF], Gay et al., 2006a[START_REF] Marcon | Fluid flod regimes and growth of a giant pockmark[END_REF].

Comparable studies conducted in similar geological context, yet revealed a much tenuous oil seepage manifestations [START_REF] Serié | Subsurface fluid flow in the deep-water Kwanza Basin, offshore Angola[END_REF]. Recent sedimentation stage of the LCB were particularly high due to the establishment of the Congo DSF during Neogene and upwelling system during Early Quaternary [START_REF] Savoye | Structure et évolution récente de l'éventail turbiditique du Zaïre: premiers résultats scientifiques des missions d'exploration Zaïango1 & 2 (marge Congo-Angola)[END_REF][START_REF] Ferry | Morphogenesis of Congo submarine canyon and valley: implications about the theories of the canyons formation[END_REF]Seranne and Anka, 2005;[START_REF] Marcano | Major controlling factors on hydrocarbon generation and leakage in South Atlantic conjugate margins: A comparative study of Colorado, Orange, Campos and Lower Congo basins[END_REF], which lead to salt-related deformation enhancement. The tectonic regime of seeping provinces, mostly transitional in Kwanza basin and compressive in LCB could play a significant role on expelled volumes.

Summary and Conclusions

We used several tools to assess the seepage potential in the distal salt province of the Lower Congo

Basin. The analysis of sea-surface oil manifestations imaged with satellite imagery, coupled with a dense geochemical core-set and seafloor/ sub-seafloor manifestations of oil presence provides an integrated approach of the present-day seepage efficiency.

• The geophysical features of thermogenic seeps sites previously recognised at local scale (Jatiault et al., 2019, Part I) are validated at the full scale of the study area. Thermogenic seeps sites (1) consist of complex-shaped features (clusters of irregular pockmarks and seafloor mounds), (2) are associated with shallow high-impedance layers on the sub-bottom profiler and (3) are underlain by BSR that (4) are underlain by consistent amplitude and (5) are overlain by clusters of pipe-like features (PHAA).

• The knowledge of the fluid flow phenomena is improved from the integration of detailed slicks map that provide a means to quantify seepage activity. The SBP data that provides a discrimination technique between hydrocarbons and pore dewatering features and also reveals the seafloor distribution of asphalt storage areas.

• About 40% of geophysical anomalies are associated with recurrent oil slicks at the sea surface while the remaining 60% are most likely related to dormant seep sites whose periodicity exceeds the 22-year SAR monitoring or are shorter than SAR scene acquisition intervals.

• A large number of low-amplitude pipes (582) were identified in salt minibasins from the 12.5m seismic resolution that are linked with conical-shaped seafloor pockmarks, interpreted as related to pore-water expulsion.

• Extensive high amplitudes related to the uplift of older/coarser/ more permeable sediments due to the salt tectonics and characterised by extensive near-surface positive amplitude geophysical anomalies appear to correspond to preferential seafloor thermogenic migration outlets.

• Only 20% of BSRs are double, and these occur exclusively in association with thermogenic hydrocarbons seeps.

,

  Serié et al. (2017) set out a methodology to study hydrocarbon fluid flow by combining remote sensing, seafloor coring campaign and industrial seismic data. The present study relies on this pre-existing methodology that we complemented with a denser dataset of sea surface natural slicks together with high resolution sub-seafloor data. More recently, Jatiault et al. (2019, Part I) presented seafloor and sub-seafloor high-resolution characteristics of one active oil seep in the Lower Congo

Fig. 1 :

 1 Fig. 1: a. Location map of the study area showing the main geological provinces of the LCB. The map

  and recurrent oil slicks at the sea surface (black stars, from Jatiault et al., 2017). Continuous black lines represent 1000 m isobaths (Gebco). b. Regional cross section across the LCB displaying the pre-rift, syn-rift and post-rift sequences (modified from Jatiault et al., 2017). c. Chronostratigraphic log and petroleum system plays (modified from Anka et al., 2013).

.

  In the salt minibasins, widespread classical type I pockmarks (Conical; see definition of Type I in Riboulot et al., 2016) related to sediment pore dewatering and biogenic methane venting are associated with the polygonal fault networks (Cartwright 2007; Andresen et al., 2011). Aligned type I pockmarks commonly follow the maximal flexural syncline axis of the minibasins (No. 1 in Fig. 1a; Andresen et al., 2011) or occur as sinuous trends marking the ancient pathways of the Congo canyon (No. 2; Gay et al., 2003). A biogenic origin is also suspected for the giant (~2000m diameter) Regab pockmark, located in the deep abyssal plain at a water depth of 3150m (No. 3; Gay et al., 2006a). The origin of the gas expelled at the salt front remains undetermined (No. 4; Wenau et al., 2014). A thermogenic contribution is highly suspected for isolated pockmarks linked with Miocene turbidite channels (No. 2; Gay et al., 2006 b) and for clustered pockmarks linked with salt diapirs (No. 5; Andresen, 2012). The migration of thermogenic hydrocarbon is proven from the identification of a large number of seafloor asphalt mounds that spread on top of salt diapirs (n°6; Jones et al., 2014). Seafloor asphalt mounds may reach approximately 1m in height and host an abundance of biofauna. From the analysis of spaceborne SAR imagery, Jatiault et al. (2017) calculated that the 100 oil seep sites of the compressive salt domain (black stars in Fig. 1a) emit a volume of 4.38*10 6 litres of oil per year The vertical association of sea-surface slicks and seafloor data show that seafloor seeps correspond to clusters of pockmarks and seafloor mounds (n°7; Jatiault et al., 2019, Part I). One pockmark cluster investigated in detail in Jatiault et al., 2019, part I consists of a complex of heterometric depressions associated with asphalt mounds with diameters of 10 to 15 m in on the seafloor

;

  [START_REF] Jatiault | Monitoring of natural oil seepage in the Lower Congo Basin using SAR observations[END_REF]. The proximal terminations of individual slicks in the oil surfacing area are called Oil Slick Origins (OSO; Garcia-Pineda et al., 2010). When there is no predominant current, the vertical projection of the average location of the OSO on the seafloor provides the most accurate location of the source on the seafloor.

Fig. 2 :

 2 Fig. 2: Geographical localisation of the study area and compilation of available dataset. The red

  concentration area within point clusters were used to identify active oil seep sites. The oil seepage system consists of more than 100 seep sites distributed on the deep part of the Miocene Congo deepsea fan for water depths between 1200 to 2700 m. Observations of multi-temporal oil slicks at the sea surface vary from 5 to 80 % from one seep site to another. The radius of the spatial scatter of OSO for individual sea bottom seepage points has a mode radius of 750 m[START_REF] Jatiault | Deflection of natural oil droplets through the water column in deep-water environments: The case of the Lower Congo Basin[END_REF], suggesting that the lateral deflection due to underwater currents remains low compared to the water depth (Fig.3b; Jatiault et al., 2019, Part I). The vertical projection of OSO locations, the density and the geometric mean centre coincide vertically at 98 % with seabed morphological anomalies, confirming the absence of permanent deflection in the water column, which helps to identify active oil seep sites nearly at the apex of OSO locations (Fig.3c, d, and e).

Fig. 3 :

 3 Fig. 3: a. Compilation map of seepage slick outlines identified from the interpretation of 156

Fig. 4

 4 Fig. 4: a. Graph (logarithmic scale) of the concentration of the high molecular-weight hydrocarbons

Fig. 5 :

 5 Fig. 5: Compilation of major fluid vents morphotypes observed on the seafloor in the Lower Congo

Fig. 7

 7 Fig. 7: a. Seafloor map from 3D seismic, the dynamic range of the seafloor colour scale is 60 m. b.

Fig. 8 :

 8 Fig. 8: A. Map of amplitude root mean square (RMS) over the first 100 ms TWT below seabed,

  ) of acoustic anomaly corresponds to local, sub-circular and Positive High Amplitude Anomalies (PHAA; Ho et al., 2012) that are vertically stacked on successive lithologic interfaces, marking a vertical succession of strong impedance contrasts (bright anomalies; Fig. 8 G). The PHAA diameters vary from 60 to 350m and occur either as multiple vertical pipe clusters (see Jatiault et al., 2019, Part I) or isolated chimneys (Fig. 8 G). We identified 75 individual PHAA on the RMS amplitude stack of the first 100 ms TWT, mostly associated with diapirs (Fig. 9). They tend to congregate along the upslope salt flank or minibasin edges. High-amplitude vertical pipes are mostly associated with strong RMS amplitude in the central area surrounded by a moderate amplitude peripheral anomaly. The seafloor expression of PHAA corresponds essentially to the complex shaped pockmarks (clustered or isolated) and submarine mounds.

Fig. 9 :

 9 Fig. 9: a. Map of the RMS amplitude stack of the first 100 ms TWT below seafloor. Geophysical

Fig. 10 :

 10 Fig. 10: a. Seafloor map superimposed with 5m isobaths, oil slick origins (OSO; Garcia-Pineda et al.,

Fig. 11 :

 11 Fig. 11 : a. Extract of the Bottom Simulating Reflection (BSR) map with basin structure in background

  inventory extreme marine ecosystems[START_REF] Jones | Asphalt mounds and associated biota on the Angolan margin[END_REF]. As demonstrated by previous studies on the LCB[START_REF] Andresen | Fluid flow features in hydrocarbon plumbing systems: What do they tell us about the basin evolution[END_REF] Jatiault et al., 2019, Part I), the location of seafloor thermogenic seeps is controlled by the shallow deformation related to the salt tectonics. Thermogenic pockmarks locate along faults associated with the underlying diapir deformation. Therefore, the location of seafloor seeps, especially clusters of heterometric pockmarks is consistently influenced by the underlying salt tectonics-related fault network. Both clustered and irregular pockmarks occur above fluid-related seismic anomalies, such as high-amplitude vertical pipes (Lᴓseth et al., 2011), shallow HIL, BSR(s), or above salt. The distinction between clustered and irregular pockmarks is facilitated using the HR bathymetry, while the characteristics of the subsurface are similar. The distinction between the two is sometimes tricky and can be subject to data resolution issues or could mark a temporal evolution that depends on seepage persistence over time. The geometrical distinction could correspond to the process evolution, one being the initial stage of the other. In actively deforming areas above salt, the area affected by seepage features is likely to correspond to areas undergoing active fracturing; the pinpoint versus distributed character of seeps may reflect the localised versus distributed character of fracturing.

  conducted at the salt front of the LCB led to differentiating active from inactive sites based on seafloor reflectivity strength[START_REF] Wenau | Cold seeps at the salt front in the Lower Congo Basin II: The impact of spatial and temporal evolution of salt-tectonics on hydrocarbon seepage[END_REF], applying the general criterion defined by Sager et al.(2003) who relate low backscatter area to dormant sites. In our study, we observed major recurrent oil seeps on SAR data that are associated with a "moderate" seafloor response (Fig.3in Wenau et al.,2014).Authigenic carbonate precipitation is presumed to consume methane in priority (Anaerobic Oxidation of Methane; e.g.[START_REF] Barnes | Methane production and consumption in anoxic marine sediments[END_REF][START_REF] Ivanov | Interaction of sulphur and carbon cycles in marine sediments[END_REF][START_REF] Boetius | A marine microbial consortium apparently mediating anaerobic oxidation of methane[END_REF][START_REF] Paull | Re-evaluating the significance of seafloor accumulations of methane-derived carbonates: seepage or erosion indicators?[END_REF][START_REF] Knittel | Anaerobic oxidation of methane: progress with an unknown process[END_REF][START_REF] Boetius | Seafloor oxygen consumption fuelled by methane from cold seeps[END_REF]. The seafloor reflectivity strength used in this study is rather similar between oil-supplying and non-oil-supplying seeps (Fig.5), even if slope artefacts may occur. Recent studies have shown that the type of carbonates is different between systems dominated by oil and gas (e.g.Hudson, 1997;[START_REF] Thomas | Understanding Engineering Challenges Posed by Natural Hydrocarbon Infiltration and the Development of Authigenic Carbonate[END_REF] Smrza et al., 2016), which might involve a difference in precipitation rate.

  potentially with hydrates[START_REF] Jatiault | Geophysical characterisation of active thermogenic oil seeps in the salt province of the lower Congo basin part I: Detailed study of one oilseeping site[END_REF], Part I) also helps locate active hydrocarbon seepage.HIL patches are typically wide, but reach the seabed in a very limited area; consequently, the identification of the point where the HIL reaches the seabed reduces the uncertainty on the identification of the point of seepage. The SBP dataset provide information on sub-seafloor hardness and possibly a discrimination between similar shaped features, i.e. soft mud volcanoes[START_REF] Serié | Subsurface fluid flow in the deep-water Kwanza Basin, offshore Angola[END_REF] compared to harder carbonate and/or hydrate mounds/pingoes (this study; Hovland and Svensen, 2006). The proximity of HIL features with the seafloor also constrains seep outlets location and improve the targeting of cores locations. However, the deployment of such toolsets in a given area implies that the given area has already been extensively explored.The combination of different observations and techniques provides an integrated vision of the hydrocarbon fluid seepage phenomena in the Lower Congo Basin and helped refining the methodology of studying the fluid flow phenomena (Fig.12).

Fig. 12 :

 12 Fig. 12: Synthetic block diagram illustrating the methodology and the main results concerning the characterization of the heavy hydrocarbon fluid flow in

  as active oil seep sites and most of them (75 %) are overlain by a seafloor fluid-flow feature, which shows that most are active at seismic resolution scale and suggest much longer emission frequencies. The active array proportion is concordant with results published in the Gulf of Mexico, where 50% of geophysical anomalies identified on seismic datasets correspond to an active oil seep site at the sea surface (Garcia-Pineda et al., 2010). Oil seep sites are mostly transient with a widespread range of periods of recurrence (Garcia-Pineda et al., 2010; Garcia-Pineda et al., 2015; Jatiault et al., 2017).

Table 1 : Inventory of SAR scenes collection characteristics. Satellite/ Sensor Period of imagery Wavelength Pixel Spacing (m) Amount of SAR data Envisat /WSM 2002

 1

		-2012	C (5.8 cm)	75	46
	ERS/IMP	1994 -2001	C (5.8 cm)	12.5	17
	Cosmo-SkyMed	2009 -2014	X (3.1 cm)	3-15	80
	TerraSAR -X	2011	X (3.1 cm)	20	6
	RADARSAT-2	2005 -2012	C (5.3 cm)	25-50	7

Table 2 : Characteristics of the available datasets including seafloor and sub-seafloor imagery.

 2

	Data type	Line spacing (m)	Maximum Penetration (ms TWT)	Dominant frequency	Horizontal Resolution	Vertical Resolution	Covered area
		3D exploration seismic	12.5*12.5	4500	20 -100 Hz	12.5 m	~4 m	3300 km²
	Sub-seafloor	AUV Sub-bottom profiler	175*1000	100	1.5 -4.5 kHz	1.5 m	~10 cm	5500 km²
		Multibeam	-	-	12.5 kHz	12.5 m	-	16 000 km²
	Seafloor	AUV HR bathymetry	-	-	200 kHz	1.5 -3 m	-	830 km²
		AUV Seafloor Reflectivity	175*1000	Few centimetres	120 -410 kHz	3 m	-	830 km²

Table 3 : Synthesis of fluid flow manifestations as observed throughout the study area compared to detectable oil seeps. Pockmarks inventory are deduced from the interpretation of the 12.5m resolution multi-beam bathymetry. Coupling the features related to oil seep sites with non-seeping geophysical anomalies contributes to the understanding of the seepage potential. Fluid flow manifestations

 3

	Total	Oil supplying sites (SAR data)
	picked-out	Active	Percentage of correlation

Table 4 : Minimum oil output estimation depending on the spatial resolution of different SAR systems. Satellite/ Sensor Envisat /WSM ERS/IMP Cosmo-SkyMed TerraSAR -X RADARSAT-2 Pixel Spacing (m) 75 12.5 3-15 20 25-50 Minimum volume output (l.h -1 ) 9.2 0.76 0.18 -0.92 1.22 3.06 -6.13

 4

Acknowledgements

The authors express their grateful thanks to Total SA, Total Angola (TEPA) and partners for the data disclosure approval of the industrial seismic and sub-bottom profiler data. The authors thank Total team DSO for the geochemical analysis. This work was performed in the frame of a PhD project (CIFRE convention) granted by the ANRT (Agence Nationale de la Recherche et de la Technologie, No. 2014/0308), funded by Total SA in collaboration with University of Perpignan, France. The authors thank the European Space Agency (Envisat WSM, IMP) for the delivering of free-access data. Seismic interpretation was carried out using Total's proprietary software Sismage. We kindly thank Christophe Serié and an additional reviewer who provided critical suggestions in order to improve the impact and benefits of the paper.