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From seep carbonates down to petroleum systems: An outcrop study from the southeastern France Basin
AAPG Bulletin ( IF 3.5 ) Pub Date : 2021-05-12 , DOI: 10.1306/06192019025 Jean-Philippe Blouet , Patrice Imbert , Anneleen Foubert , Sutieng Ho , Gerard Dupont
AAPG Bulletin ( IF 3.5 ) Pub Date : 2021-05-12 , DOI: 10.1306/06192019025 Jean-Philippe Blouet , Patrice Imbert , Anneleen Foubert , Sutieng Ho , Gerard Dupont
Despite the obvious link between hydrocarbon seepage at the surface and the activity of petroleum systems at depth, the majority of studies on seep carbonates concentrate on shallow aspects, whereas they are rarely oriented toward a source-to-sink perspective. Here, seep carbonates are conceptualized as a key element for analyzing petroleum systems at basin scale. Mapping of a 150-m-thick, 200-m-wide outcrop of the Lower Cretaceous slope wedge of the southeastern France Basin reveals two distribution patterns of seep carbonate concretions: (1) aligned in a continuous bed secant to the stratigraphy and (2) clustered in patches vertically stacked over 35 m. The δ13C signatures of the seep carbonates as light as −40‰ Peedee belemnite point toward a biogenic methane–dominated seepage. A set of turbidite channels pinching out right below the seep carbonates is seen as a potential gas trap, whereas biostratigraphic analysis indicates that the channels are coeval with a prominent black shale in the center of the basin. Any biogenic methane generated by the black shale could have used the channels as drains up to their pinch-out. The switch from a continuous seep carbonate level to stacked patches suggests initial widespread capillary leakage, followed by the opening of a preferential migration pathway. The distribution of seep carbonates thus appears dominated by breaching mechanisms of reservoir–cap-rock seals. This case study illustrates how seep carbonate mapping, in combination with the tectono-sedimentary context of each basin, may be a valuable tool for petroleum geologists.Authigenic carbonates with significant δ13C depletion were first reported by Hathaway and Degens (1969), who initiated the hypothesis that anaerobic oxidation of methane (AOM) coupled with sulfate reduction in marine settings could trigger the precipitation of early diagenetic concretions. These carbonates are widely known as “methane-derived authigenic carbonates” (MDACs) (Kulm et al., 1986; Ritger et al., 1987), “methane-related carbonates” (Hovland et al., 1987), or “cold seep carbonates” (Hecker, 1985). Thirty-five years after the hypothesis was proposed for methane, Joye et al. (2004) reported that the oxidation of oil accounts for the majority of sulfate reduction at seep sites in the Gulf of Mexico, where oil may also, in addition to methane, contribute to the precipitation of carbonates. Hydrocarbon oxidation by seawater sulfate can fuel distinctive chemosynthetic communities typically including crustaceans and bivalves (Kiel, 2010a); furthermore, the microbial consortia mediating the redox reactions result in characteristic petrographic textures (Greinert et al., 2002; Reitner et al., 2005). Based on these observations, many publications propose a “seep search strategy” based on five techniques to identify fossil seep carbonates: petrography, stable isotope analysis, paleontology, field observation, and biogeochemistry (Campbell and Bottjer, 1993; Kelly and al., 1995). Oceanographic observations made it clear that hydrocarbon seepage at the seabed is a ubiquitous phenomenon at the global scale (Judd and Hovland, 2007), and seep-related authigenic carbonates are the best witnesses of this elusive phenomenon in the geological record.The terms MDAC and seep carbonates are commonly used indistinctly. In the frame of this petroleum geology paper, it should be noted that although the term MDAC explicitly indicates the nature of the hydrocarbons, it tells nothing about their source. As such, MDAC may not be linked to the activity of a petroleum system (sensu Magoon and Dow, 1994) because biogenic methane may be generated and oxidized in the very shallow subseafloor sediment (e.g., Lasch, 2018). In contrast, the term “seep carbonate” more clearly puts the emphasis on a notion of advection of fluids up to the seabed (water, gas, oil, or a combination), suggesting the activity of a hydrocarbon source rock and the existence of a migration pathway, two basic components of a petroleum system. The term seep carbonate is thus viewed as more precise when dealing with methane-derived carbonates related to cross-stratal migration from a deeper source in a petroleum geology context. Although the potential of seep carbonates in petroleum geology is obvious, Talukder (2012) reported that the great majority of studies concentrate on superficial or shallow aspects (e.g., seabed structures and benthic faunas), whereas they are rarely oriented toward a source-to-sink perspective, that is, the seep carbonates being regarded as exit points of an underlying petroleum system. Lateral variations of seep carbonate facies have been related to changes in hydrocarbon flux (Roberts, 2001). The vertical succession of amplitude anomalies on seismic data, interpreted as buried stacks of seep carbonates, allowed Ho et al. (2012) to reconstruct the seepage history of a particular site. Seep carbonates have ultimately been linked to hydrocarbon migration pathways in the shallow seabed (permeable layers, faults, fluid chimneys, hydraulic fractures) (Hovland, 1984; Hovland et al., 1994; Gay et al., 2003; Mazzini et al., 2003; Ho et al., 2016, 2018a, b; Casenave et al., 2017). Only a few studies attempt to connect seep carbonates to a migration pathway, down to possible reservoirs, and ultimately to the hydrocarbon feeding source rock (Agirrezabala, 2009, 2015; Agirrezabala et al., 2013; Blouet et al., 2017).In this study, we propose to adapt the methodology developed on seismic data by Ho et al. (2012) to an outcrop case study of the southeastern France Basin. Whereas classic petrographic and geochemical techniques will allow the inference of the precipitation mechanisms of the seep carbonate concretions, lateral and vertical distribution patterns mapped at meter to hundred-meter scales will permit an interpretation in terms of evolution of seepage style over time and space. Seepage style will, in turn, be discussed in relation to the rupture mechanisms of an underlying turbidite channel, being interpreted as a potential gas trap. A complete model of a petroleum system will eventually be proposed using the input of the regional geological context.The studied outcrop is located at the locality Les Blaches, close to the village of Gigors, at the northeastern margin of the Vocontian Basin in southeastern France (Figure 1A, B).The southeastern France Basin results from the Triassic rifting of the Alpine Tethys (Lemoine et al., 2000; Masini et al., 2013). Triassic transgressive deposits above the peneplaned magmatic and metamorphic Hercynian basement and Permian–Carboniferous basins consist of shallow-water siliciclastics and evaporites. The latter acted as a decollement level during the structuration of the basin. Deepening of the depositional environment during the Early Jurassic coupled with large lateral thickness variations reveal synsedimentary tilted block tectonics linked to the paroxysmal phase of rifting (Lemoine et al., 1986). The more homogeneous character of the shaly limestone calcareous Dogger, then shale-dominated Terres Noires Formation, whose total thickness reaches several kilometers, is attributed to the onset of spreading of the ocean and associated thermal subsidence of the margin (Figure 1C). During the Triassic and Jurassic, the southeastern France Basin was elongated along a northeast-southwest axis inherited from Hercynian structures, whereas during the Cretaceous, prograding carbonate platforms narrowed the basin, giving it an east-west elongation. This change in geometry led regional geologists to distinguish the so-called “Dauphinois” and “Vocontian” phases of the southeastern France Basin. From the Aptian, the compression of the Pyreneo–Provencal orogeny led to east-west folding of the basin and generalized emersion during the Santonian, whereas from the Oligocene onward, the Alpine orogeny superimposed a north-south structuring.The studied concretions are developed in the Aptian–Albian Marnes Bleues Formation, a maximum 500-m-thick interval, composed of silty marl in the center of the basin, enriched in sand along the basin slope (Flandrin, 1963) (Figure 1C, D). Many slumps and turbidite channels within the axes of present-day synclines attest to recurrent slope instability and early structuring of the basin (Friès and Beaudoin, 1987; Friès and Parize, 2003). Several black shale levels have been used as basin-scale markers (Bréhéret, 1994) (Figure 1D).A synsedimentary normal fault trending N45° (Gigors fault), located less than 1.5 km to the north of Les Blaches outcrop, made the boundary between the Vercors platform domain to the northwest, where the thickness of the Marnes Bleues is approximately a few meters, and the basin domain to the southeast (Friès, 1986) (Figure 1B). Ferry and Flandrin (1979) mapped in detail three major lowermost Aptian mass transport complexes (MTCs), whose head scarps were located along the Gigors fault. The reworked material flowed toward the south-southeast, down to the center of the basin over several tens of kilometers (Figure 1A).Potential hydrocarbon source rocks of the southeastern France Basin have been studied by Institut Français du Pétrole (1982a, b, c) and later by Mascle and Vially (1999) and Wannesson and Bessereau (1999). In stratigraphic order, these potential source rocks are as follows.Carboniferous coal-rich basins have been exploited in the twentieth century in post-Hercynian grabens that crop out around the southeastern France Basin. Although no drilling ever hit the pre-Mesozoic series below the Vocontian Basin, their presence in the study area is possible.Marly Early Jurassic marine deposits are potentially bearing type II organic matter. The Domerian pro parte–Aalenian pro parte interval, known as Lias Marneux Formation (Marly Lias) (Pieńkowski et al., 2008), has been partly penetrated in a single place in the central part of the Dauphinois Basin (Aurel borehole) (Compagnie Française des Pétroles France-Afrique [COPEFA], 1967). The 1.5-km-thick section penetrated by the borehole in the Toarcian–Aalenian consists of laminated, black, pyrite-bearing, and micaceous shale. Numerous gas shows were encountered during the drilling. Based on 50 total organic carbon (TOC) measurements covering the entire interval drilled at Aurel, Wannesson and Bessereau (1999) reported an average TOC of 0.6 wt. % (maximum of 0.9 wt. %; minimum of 0.3 wt. %). Vitrinite reflectance ranges from 3% to 4% (Robert, unpublished results), which means that the organic matter is overmature and that the original TOC content was significantly higher (Bruneau et al., 2017).Although the Bathonian–Callovian Terres Noires Formation (Artru, 1972) crops out in many places scattered over the basin, relatively few TOC data are available. Based on 75 measurements from the lower Oxfordian section of the formation, Tribovillard (1988) reported a TOC between the range of 0.5 and 1 wt. %, dominated by type III organic matter. Despite this very moderate petroleum potential, Wannesson and Bessereau (1999) consider that the total thickness (approximately 2 km) of the formation could have generated significant amounts of hydrocarbons. This assumption is reinforced by the presence of numerous oil-filled septarian concretions (Montenat and Patillet, 1968; Barlier et al., 1974; Touray and Barlier, 1975).Two black shale intervals with a metric thickness have been reported by Bréhéret (1994) in the upper part of the lower Barremian with a TOC of approximately 4 wt. %.The average TOC of the Aptian–Albian Marnes Bleues Formation is 0.5 wt. %; it can locally reach 4 wt. % in black shale intervals. The 4-m-thick “Niveau Goguel” lies at the very bottom of the Marne Bleues Formation and is the thickest of the series (Bréhéret, 1994). Niveau Goguel has been correlated to global oceanic anoxic event 1a (Bralower et al., 1994).Individual concretions have been described from a millimeter scale to object scale, and their distribution has been mapped from an outcrop (200-m) to regional (10-km) scale.Forty thin sections have been examined with optical microscopy using plane-polarized light, crosspolarized light (CPL), and cathodoluminescence (CL). The CL was generated by a Cambridge Image Technology Ltd system (Hatfield, United Kingdom), model CCL 8200 ink4 (12 kV; 450 mA).Scanning electron microscope (FEI XL30 Sirion FEG, Oregon) observations were carried out on thin sections coated with carbon. Energy-dispersive spectrometry (EDS) was used to determine the qualitative elemental composition of samples.The interpretation of the foraminiferal assemblages is based on Moullade et al. (2005), describing the foraminifera succession in the lower Aptian section of Cassis–La-Bédoule (lower Aptian regional stratotype). The chronostratigraphic attribution is based on the chart of Ogg et al. (2008).A total of 180 samples of carbonate powders were selected for oxygen and carbon stable isotope analyses. Micrite and microsparite were sampled as bulk powders, containing all matrix minerals, whereas sparite cementing cavities were sampled as one single phase. They were taken from polished blocks and on rock chips using a handheld microdrill. Samples were analyzed using a Kiel III automated carbonate preparation device coupled to a Finnigan MAT 252 isotope-ratio mass spectrometer (IRMS) (Bremen, Germany). The system digests carbonate material with 100% phosphoric acid for 10 min at 70°C and purifies the CO2 product, which is then passed to the IRMS for mass 44, 45, and 46 measurements. Instrumental precision is monitored by analysis of National Bureau of Standards (NBS) 18, NBS 19, and in some instances, lithium-carbonate reference material. Precision for carbon is ±0.05‰ and for oxygen ±0.14‰. Isotope results are given relative to the Vienna Peedee belemnite standard.Thirteen outcrops of the Marnes Bleues Formation have been visited around the Suze and St. Pancrace synclines (Figure 1B). Concretions have been observed only in two places (Baumes Rousses and Les Blaches). Because exposure quality is notably better in Les Blaches outcrop, the study will focus on this site.The outcrop exposes the Marne Bleues pro parte up to the Turonian. A fault swarm oriented northwest-southeast defines a western and an eastern compartment, respectively, devoid of and rich in concretions (Figure 2). The bluff is oriented 160°N with an average topographic slope of 30°N, and the structural dip of the strata is 135°N /12°S; therefore, the outcrop corresponds approximately to a section inclined 45° with respect to stratification.The Marnes Bleues Formation can be subdivided into four lithological units with transitional boundaries (Figure 3). The base of the outcrop taken is as a datum.From 0 to 30 m is an alternation of decimeter-thick beds of silty marls and siliceous spiculite beds. At least one erosive turbidite channel (probably two), more than 5 m deep and filled with spiculite-rich silty beds, is visible between 13 and 19 m (Figure 4).From 30 to 130 m is marl with intercalations of centimeter-thick silty to sandy beds. This unit contains a turbidite channel less than 5 m deep.From 130 to 225 m is sandy marl progressively enriching upward in limestone beds. The studied concretions are restricted to the lower part of this interval. Macrofossils are rare; pelagic organisms include ammonites, belemnites, and fish teeth; benthic organisms include the bivalve Inoceramus sp., claws of crustaceans, and one specimen of an irregular echinoid found in life position.From 225 to 233 m are limestone beds increasingly enriched in detrital and carbonate sand grains to form a grainstone characterized by current ripples. Flandrin (1974) noticed the lithological similarity with the Cenomanian deposits in other parts of the basin, but no precise dating is available.Except at the outcrop of Les Blaches, no turbidite channels have been observed around the syncline of St. Pancrace, in agreement with the observations of Ferry and Flandrin (1979).From 0 to 30 m is an alternation of decimeter-thick beds of silty marls and siliceous spiculite beds. At least one erosive turbidite channel (probably two), more than 5 m deep and filled with spiculite-rich silty beds, is visible between 13 and 19 m (Figure 4).From 30 to 130 m is marl with intercalations of centimeter-thick silty to sandy beds. This unit contains a turbidite channel less than 5 m deep.From 130 to 225 m is sandy marl progressively enriching upward in limestone beds. The studied concretions are restricted to the lower part of this interval. Macrofossils are rare; pelagic organisms include ammonites, belemnites, and fish teeth; benthic organisms include the bivalve Inoceramus sp., claws of crustaceans, and one specimen of an irregular echinoid found in life position.From 225 to 233 m are limestone beds increasingly enriched in detrital and carbonate sand grains to form a grainstone characterized by current ripples. Flandrin (1974) noticed the lithological similarity with the Cenomanian deposits in other parts of the basin, but no precise dating is available.Eight samples have been selected for foraminiferal analysis between 133 and 205 m, including the entire concretion-rich interval (Appendix 1). Two samples were taken in the immediate vicinity of a patch of concretions.Planktonic foraminifera can be ascribed to the Ferreolensis biozone (middle Aptian), as defined in Moullade et al. (2005) (Figure 1D). The following criteria were retained for the lower part of the section (133 m up to 190 m).Co-occurrence of Globigerinelloides ferreolensis (typical forms as well as morphs with a very large terminal chamber: “macrocameratus”) and subspecies G. ferreolensis heptacameratus;Absence of Schackoina cabri – protuberans, which disappears at the base of the Ferreolensis biozone;Absence of Globigerinelloides bizonae; andThe relative low abundance of Praehedbergella globulifera (2% to 4% in the four samples of the lower part of the outcrop) is regionally considered to characterize the upper part of the Ferreolensis biozone. An abundance over 10% is considered to indicate the lower part of the biozone.From 190 to 205 m (623–672 ft), the persistence of both G. ferreolensis and G. ferreolensis heptacameratus, along with the absence of Globigerinelloides barri, indicates that the upper part of the section is still below the base of the Barri biozone.Co-occurrence of Globigerinelloides ferreolensis (typical forms as well as morphs with a very large terminal chamber: “macrocameratus”) and subspecies G. ferreolensis heptacameratus;Absence of Schackoina cabri – protuberans, which disappears at the base of the Ferreolensis biozone;Absence of Globigerinelloides bizonae; andThe relative low abundance of Praehedbergella globulifera (2% to 4% in the four samples of the lower part of the outcrop) is regionally considered to characterize the upper part of the Ferreolensis biozone. An abundance over 10% is considered to indicate the lower part of the biozone.Within the benthic fauna, both Conorotalites aptiensis and Gavelinella brielensis indicate a middle to upper Aptian age, whereas the absence of Gavelinella barremiana suggests that these samples cannot belong to the lower Aptian.Microsparitic limestone beds are characterized by a greenish to gray color, transitional boundaries with the encasing marls, and sparse presence of tubular burrows. Lithification varies gradually within a given limestone bed: weathering highlights lithified domains called “limestone bodies” (Figure 5A). Limestone bodies have an oblate shape, of a few tens of centimeters to 1 m in diameter, with their equatorial plane parallel to the bedding. In places, columnar limestone bodies are centered on vertical burrows. The columns extend up to 1.5 m in height and a few tens of centimeters in diameter (Figure 5B). Continuous limestone beds may evolve laterally in discontinuous alignments of limestone bodies.Two morphologies of micritic concretions have been recognized:“Nodules” form tuberculiform bodies and are a few to approximately 10 cm in diameter (Figure 5C). No internal structure is distinguishable, neither under natural nor ultraviolet light.“Tubes” are distinguished by the presence of a central conduit. Their average external diameter is approximately 2.7 cm (16 measurements); the central circular channel is approximately 1 cm in diameter, partially occluded with micrite or cements. Tubes are randomly oriented and interconnected to form a complex ramified network that may extend over several meters (Figure 5D). Several individual tubes more than 1 m long have been collected.Nodules are more common than tubes; these two types of concretions are always found in association.“Nodules” form tuberculiform bodies and are a few to approximately 10 cm in diameter (Figure 5C). No internal structure is distinguishable, neither under natural nor ultraviolet light.“Tubes” are distinguished by the presence of a central conduit. Their average external diameter is approximately 2.7 cm (16 measurements); the central circular channel is approximately 1 cm in diameter, partially occluded with micrite or cements. Tubes are randomly oriented and interconnected to form a complex ramified network that may extend over several meters (Figure 5D). Several individual tubes more than 1 m long have been collected.Several scales of concretion sets have been defined (Figure 6) and mapped (Figure 7).The lowest concretions in the studied outcrop are aligned in a continuous bed ranging from a single line of concretions to more than 1 m in thickness. Taking the limestone beds as reference for the stratigraphic bedding, the concretions’ alignment undulates with a wavelength of less than 100 m and an amplitude of less than 10 m (Figure 8A). The thicker domains of the wavy concretion bed coincide with the highest points in the stratigraphy. Those points may be associated with tubes having an exceptionally large external diameter (exceeding 10 cm), with a preferred vertical orientation.Concretions concentrate in places, forming aggregates a few tens of centimeters up to a few meters in diameter. Aggregated concretions are commonly cemented together by microsparite so that the aggregate protrudes as an individual limestone body (Figure 8B). Concretions cemented by microsparite commonly have more complex shapes and more diffuse boundaries than concretions encased in marl (Figure 8C).Aggregates of concretions are aligned parallel to the limestone beds (probably forming flat patches in three dimensions [3-D]). The number of aggregates outcropping in one patch ranges from a few units to several tens of units, over an apparent diameter ranging from a few up to 60 m. This suggests that a patch can comprise in 3-D several hundreds of aggregates. The largest patches show the following structural trends:The density of aggregates is higher in the central part of the patches, with these aggregates being the richest in concretions. At the periphery of the patches, aggregates appear to develop preferentially on the upper side of limestone beds.Laterally to the patches, limestone beds progressively undulate, turn into discontinuous bodies, and may completely disappear toward the center of the patches.The patches tend to stack vertically. Two stacks are entirely visible in the mapped area, and others are most probably present laterally (Figure 7). The best exposed stack is 35 m thick and widens upward from less than 10 m to 60 m.The density of aggregates is higher in the central part of the patches, with these aggregates being the richest in concretions. At the periphery of the patches, aggregates appear to develop preferentially on the upper side of limestone beds.Laterally to the patches, limestone beds progressively undulate, turn into discontinuous bodies, and may completely disappear toward the center of the patches.The wavy bed and the stacked patches of concretions are truncated by an erosive surface. Nodules and tubes are scattered horizontally to form small conglomeratic patches located right above in situ concretions (Figure 9). The matrix between the reworked concretions is rich in detrital and bioclastic sand grains (fragments of crinoids, bryozoans, and mollusks) and in belemnite rostra and is commonly cemented into flat microsparitic limestone bodies (Figure 10A, B). Reworked concretions may be encrusted by oysters. In particular, oysters have been observed encrusting the tip of an in situ tube protruding by approximately 15 cm above the erosion surface (Figure 10C, D). Ten-centimeter-long tube fragments consisting of bioclastic grains are scattered on the erosive surface. These structures most likely originate from biologic constructions, possibly reworked Diopatrichnus burrows (Figure 10E, F). Away from the patches or reworked concretions, the erosive surface contains neither lithoclasts nor bioclasts, making it impossible to trace laterally within the background marl.The wavy bed of concretions is unique and corresponds to the lowest interval in which concretions appear. No spatial relationship exists between the bumps of the wavy bed and the overlying stacked patches. All stacked patches originate from the same stratigraphic level but culminate at different horizons. The uppermost patches are shifted abruptly to the northwest relative to the underlying subvertical stacks. The erosive surface crosscuts the vertical stacks without causing any shift in the stacking axis.The events of the paragenetic sequence have been grouped into early and late diagenetic stages depending on whether they have been affected by synsedimentary reworking.Limestone beds and bodies are composed of silt to fine sand–grade detrital grains with occasional sponge spicules and glauconite grains in a calcite microsparite matrix (Figure 11A). Concretion cortices have the same grain content in a micrite matrix (Figure 11B). Micrite luminescence under CL is homogeneous red, darker than that of microsparite.Tube conduits are commonly partially occluded by strongly concave-up geopetal deposits lining the side of the channels (Figure 11C, D). The texture of each geopetal layer ranges from mudstone to silty packstone, with well-defined laminae particularly enriched in grains. The orientation of infill laminae is random in tubes lying on the erosion surface.The voids remaining in the conduits of in situ tubes are occluded by blocky sparite showing a homogeneous dark red CL followed by megaquartz, whereas voids in reworked tubes are filled with marl or microsparite.Septarian cracks filled with sparite radiate from the center of the tubes and commonly line the boundary between geopetal layers (Figures 11D, 12A). Septarian cracks may be lined with aggregated spherules, 10 μm in diameter, extinct in CPL. The EDS analysis revealed the composition of silica oxides, indicating that the spherules are made of amorphous silica. Silica spherules are coprecipitated with euhedral crystals appearing almost extinct in CPL (extremely low birefringence). The EDS analysis yielded an atomic composition of 60% O, 30% Si, 7% Al, 2% K, 2% Ca, and 0.3% Mg, whereas Raman spectroscopy confirms identification as heulandite (Figure 12B–D). Micritic geopetal layers may be partially replaced by silica spherules or microsparite. The size of microsparite crystals in recrystallized geopetal layers gradually increases toward the contact with sparite in septarian cracks. In crooked segments of tubes further bent during compaction, septarian cracks are cut by compaction cracks partially filled with a second generation of sparite.All previous cements are crosscut by a third generation of sparite in subvertical faults, commonly a few centimeters in thickness and with a vertical throw less than a few tens of centimeters. Both the geopetal infills and the sparite of the septarian cracks may be replaced by chalcedony.The sampling strategy was established at three scales to cover heterogeneity observed at the outcrop.At individual concretion scale, two nodules were sampled from core to rim, with steps of approximately 5 mm (Figure 13A, B). One of these was taken from the wavy bed (sample 51) and the other from an aggregate of concretions (sample 2). In three representative tubes, each geopetal layer, the blocky sparite in the central conduit, and the micritic cortex have been sampled (Figure 13C–E). One tube was collected from the wavy bed (sample 170) and the other two from aggregates of concretions (samples 173 and 174).In two representative aggregates of concretions cemented by a limestone body, 21 and 14 samples have been analyzed, respectively, for a vertical and a stratigraphic profile, both in the concretions and in the encasing limestone matrix. The sampling point for each nodule was always chosen in its central part.From the bottom to the top of the outcrop, a total of 64 samples have been analyzed in each limestone bed and marl interbed. Additionally, 36 samples have been collected in three particular limestone beds evolving laterally from continuous beds toward discontinuous bodies (located 157, 166, and 180 m above the reference level).At individual concretion scale, two nodules were sampled from core to rim, with steps of approximately 5 mm (Figure 13A, B). One of these was taken from the wavy bed (sample 51) and the other from an aggregate of concretions (sample 2). In three representative tubes, each geopetal layer, the blocky sparite in the central conduit, and the micritic cortex have been sampled (Figure 13C–E). One tube was collected from the wavy bed (sample 170) and the other two from aggregates of concretions (samples 173 and 174).In two representative aggregates of concretions cemented by a limestone body, 21 and 14 samples have been analyzed, respectively, for a vertical and a stratigraphic profile, both in the concretions and in the encasing limestone matrix. The sampling point for each nodule was always chosen in its central part.From the bottom to the top of the outcrop, a total of 64 samples have been analyzed in each limestone bed and marl interbed. Additionally, 36 samples have been collected in three particular limestone beds evolving laterally from continuous beds toward discontinuous bodies (located 157, 166, and 180 m above the reference level).Isotope Signature of the Carbonate Phases—The results of the stable carbon and oxygen isotope analyses are summarized in Appendix 2 (supplementary material available as AAPG Datashare 131 at www.aapg.org/datashare) and plotted in Figure 14. Three clusters can be recognized. A first cluster has δ13C and δ18O values close to marine values (respective averages of 0.8‰ and −1.5‰). It comprises the background marl, the limestone beds, the limestone bodies, and the blocky sparite in the conduit of tubes (except for two samples). A second cluster encompasses a continuum from the marine pole to a pole with the lowest δ13C values, approximately −40‰, and the highest δ18O values, approximately 0.25‰. This cluster comprises the cortices of the concretions, without distinction between the tubes and the nodules or between those encased in the marl and those occurring in the limestone beds. Nonrecrystallized geopetal infill layers in the conduits of tubes have widely spread values, whereas recrystallized such layers have values close to the marine pole. A third cluster is characterized by slightly depleted δ13C values (0.86‰ to −8.75‰) and the lowest δ18O values (−3.41‰ to −5.43‰); it comprises the sparite from faults and one sample of blocky sparite from the conduit of a tube. The analyzed sample of sparite from compaction cracks has intermediate δ18O values between this cluster and the marine pole and a δ13C value of 4.13‰.Spatial Variability of the Stable Isotopic Signatures—Within one nodule, the isotopic values are constant within a few per mil in the central part but show clear 13C enrichment coupled to 18O depletion along the rim, reaching values close to the marine pole (Figure 14). Within one aggregate of concretions and over the outcrop as whole, there is no significant spatial variation of the isotopic signature for any type of carbonate body. In particular, concretions have identical values in the wavy bed and in distinctive aggregates. The isotopic signature of limestone beds is not influenced by the proximity of concretions, neither does it vary along the lateral transition from one continuous limestone bed to discontinuous bodies.Sedimentological analysis makes it possible to characterize the depositional environment of each lithological unit; the vertical stratigraphic section shows a regular regressive trend.From 0 up to 130 m: Turbidite channels associated with hemipelagic sediments indicate deposition in a lower offshore slope environment.From 130 up to 225 m: The progressive increase in density of the number of limestone beds associated with the transition from individual turbidite beds to homogenized sandy marl may result from more intense bioturbation or hydrodynamic agitation, which is interpreted to indicate shoaling of the area.From 225 up to 233 m: The continuous increase in the content of detrital and bioclastic grains up to cross-stratified sandstone indicates a shoreface environment at the top of the section.Micropaleontological data make it possible to specify the depositional environment in the concretion-bearing interval. The planktonic and benthic microfauna reflect a depositional environment of a few hundred meters water deep:persistence of the following association: Falsogaudryinella, Spiroplectinata, Dorothia, and Textularia bernardii;fairly low representation of arenaceous foraminifers (maximum 18%); andrelatively frequent Gavelinella at the base of the studied interval, which are known for their adaptation to relatively poorly oxygenated seafloor conditions (Friedrich, 2010).The foraminifera content reflects a regressive trend through the following:the regular decrease in the proportion of planktonic foraminifera from the base to the top of the interval;the decrease in the proportion of arenaceous forms, from 10%–18% at the base to 8% and then 1% in the upper part; andthe increasing development of diversified calcareous benthic fauna (30%–50% in the basal part, up to 50%–60% in the upper part).From 0 up to 130 m: Turbidite channels associated with hemipelagic sediments indicate deposition in a lower offshore slope environment.From 130 up to 225 m: The progressive increase in density of the number of limestone beds associated with the transition from individual turbidite beds to homogenized sandy marl may result from more intense bioturbation or hydrodynamic agitation, which is interpreted to indicate shoaling of the area.From 225 up to 233 m: The continuous increase in the content of detrital and bioclastic grains up to cross-stratified sandstone indicates a shoreface environment at the top of the section.persistence of the following association: Falsogaudryinella, Spiroplectinata, Dorothia, and Textularia bernardii;fairly low representation of arenaceous foraminifers (maximum 18%); andrelatively frequent Gavelinella at the base of the studied interval, which are known for their adaptation to relatively poorly oxygenated seafloor conditions (Friedrich, 2010).the regular decrease in the proportion of planktonic foraminifera from the base to the top of the interval;the decrease in the proportion of arenaceous forms, from 10%–18% at the base to 8% and then 1% in the upper part; andthe increasing development of diversified calcareous benthic fauna (30%–50% in the basal part, up to 50%–60% in the upper part).The very low δ13C signatures of the concretions indicate that they result from the biochemical AOM combined with sulfate reduction in the sulfate methane transition zone (SMTZ); they can thus be identified as MDACs (Boetius et al., 2000; Peckmann and Thiel, 2004). The synsedimentary reworking of the MDACs along the erosive surface, with both the cortex and the geopetal infill of tube conduits already lithified, provides evidence that precipitation occurred during very early diagenesis (Figure 15).Sparite precipitated after the synsedimentary reworking of the MDACs has a marine isotopic signature. Therefore, it marks the start of a second phase of diagenesis, after deactivation of AOM. Sparite within the fault zones is characterized by the lowest δ18O values, indicating precipitation from connate water mixed with meteoric fluids or at relatively high temperature.The precipitation of the tuberculiform nodules could originate from local colonies of bacteria (Reitner et al., 2005; Hendry et al., 2006). The branching morphology of the tubes suggests that they are cemented burrows, and the presence of claws in the background sediment could indicate that the burrowing organism was a crustacean. The numerous detrital grains and the preservation of sedimentary structures, including trace fossils, within tube cortices evidence that they precipitated below the seafloor; tubes protruding above the erosion surface result from an exhumation process, such as observed for modern seep carbonate “chimneys” (Magalhães et al., 2012; Wetzel, 2013).The cementation of Thalassinoides burrows by MDACs has been attributed by Wiese et al. (2015) to the function of burrows as preferential gas migration pathways. Near complete obstruction of tube channels by geopetal deposits suggests that sediment accumulated after the burrows were abandoned by organisms. The alternation of mudstone and grain-rich layers in the geopetal infill is probably linked to variations in the transport capacity of the fluid migrating through the tubes. The U-shaped geometry of the geopetal layers could have been directly shaped by the concave-up meniscus at the lower part of gas bubbles in a liquid medium. If tubes facilitated the advection of gas bubbles, it is probable that free gas was emitted at the seafloor, constituting an actual gas venting site. The MDACs could thus be interpreted as seep carbonates.Neither anomalies in fossil density nor endemic species have been found associated with the seep carbonates. It is difficult to assess whether bioturbation was more intense around patches of concretions than in background sediments because of the difference in preservation of burrows. Kiel (2010b) noticed an increase in the proportion of hydrocarbon seep obligate taxa with water depth, without endemic seep communities present at outer shelf and slope depths during the Mesozoic. The location of the studied outcrop at the edge of the basin might be one of the factors explaining the absence of seep communities.The wavy bed of seep carbonates is interpreted as a record of the position of the SMTZ (i.e., a widespread front of methane migrating upward). The highest points of the wavy bed could correspond to zones where methane flux from below was locally enhanced, pushing the front upward. The higher amount of seep carbonates observed here results from the locally enhanced supply of methane that increased the rate of AOM (Regnier et al., 2011).In contrast with the continuous wavy bed, the areal restriction of the stacked patches of seep carbonates indicates a focused mode of methane seepage. Could the 35-m-thick stack have been produced abruptly by a single burst of methane through a 35-m-thick sulfate-rich sediment column? This assumption is incompatible with the erosive surface cutting across the stacks, demonstrating that seep carbonates below this level precipitated before those above. The vertical stacking of seep carbonates is thus interpreted as the result of localized methane seepage that remained active over a substantial time period (within the G. ferreolensis biozone).On the outcrop section, the best exposed stack of seep carbonates broadens upward. The progressive widening of the seepage domain over time could result from the partial blocking and associated dispersion of the rising flow of methane by the buried sediment and seep carbonate concretions (self-sealing concept developed by Hovland, 2002). The lateral offset of the uppermost patches of seep carbonates indicates a displacement of the seepage domains preceding the complete cessation of seepage.Based on U/Th dating of samples from Hydrate ridge (Oregon), Teichert et al. (2003) tentatively correlated variations of seep carbonate precipitation rate to sea-level changes, with the pressure of the water column possibly controlling the outflow of methane (Boles et al., 2001). Can the preferential development of patches of seep carbonates in limestone bed intervals at Gigors be related to any environmental factor? First, the transition from widespread seepage (wavy bed of seep carbonates) to focused seepage (stacked patches of seep carbonates) is not associated with any sedimentological change. Second, the erosive event had no impact on the precipitation of the seep carbonates. In conclusion, seep carbonate precipitation mechanisms were not primarily controlled by the external environmental factors that modulate sedimentation. Conversely, it is obvious that the intrinsic properties of the host rock have a preponderant impact in fluid circulation. De Boever (2010) showed that seep carbonates may preferentially develop in the most porous intervals, which was likely the case for the limestone beds at very shallow burial. The function of limestone beds as drains and of marl beds as barriers could explain why seep carbonates are, in places, preferentially observed on top of the limestone beds, that is, where methane would have been preferentially accumulated. Later compaction and recrystallization of the sediment mean that this hypothesis cannot be tested through discrete permeability measurements.The morphology of the limestone beds, evolving laterally from continuous beds to discontinuous alignments of rounded or columnar bodies, cannot be explained by depositional processes. Morphologic changes more likely originate from the transport of bicarbonate and calcium ions through interstitial fluids over meter distances during the transformation of the original micritic matrix into microsparitic cement (Westphal et al., 2015). Heterogeneities in the host sediment probably acted as favorable nuclei during the recrystallization process, explaining the frequent inclusion within limestone bodies of aggregates of seep carbonates or of burrows at the center of columnar bodies. Because most of the limestone available in the vicinity of the groups of seep carbonates has been mobilized to form individual limestone bodies, limestone beds have gradually disappeared around patches of seep carbonates.Several phases of methane seepage can be distinguished (Figure 16):Evolution from widespread to focused migration: The absence of a relationship between the location of the high points of the wavy bed and the apex of the overlying stacks of seep carbonates support that the transition from widespread seepage (Figure 16A) to focused seepage (Figure 16B) was abrupt. The onset of focused migration implies the opening of preferred migration pathway, such as fractures.A substantial period of localized seepage permitting the vertical stack of patches of seep carbonates (Figure 16C, D).Lateral displacement: The lateral offset of the last patches of seep carbonates is probably linked to reactivation of the fractures or a shift of the preferred channeling position within some fractures (Tsang and Neretnieks, 1998) (Figure 16E).Demise of the seepage: In the absence of any abrupt lithologic change upward, disposition of seep carbonates is most likely related to demise of methane seepage in this region.Evolution from widespread to focused migration: The absence of a relationship between the location of the high points of the wavy bed and the apex of the overlying stacks of seep carbonates support that the transition from widespread seepage (Figure 16A) to focused seepage (Figure 16B) was abrupt. The onset of focused migration implies the opening of preferred migration pathway, such as fractures.A substantial period of localized seepage permitting the vertical stack of patches of seep carbonates (Figure 16C, D).Lateral displacement: The lateral offset of the last patches of seep carbonates is probably linked to reactivation of the fractures or a shift of the preferred channeling position within some fractures (Tsang and Neretnieks, 1998) (Figure 16E).Demise of the seepage: In the absence of any abrupt lithologic change upward, disposition of seep carbonates is most likely related to demise of methane seepage in this region.By comparing the δ13C of methane and associated seep carbonates at 11 seep sites, Peckmann and Thiel (2004) observed an average δ13C positive shift of 19‰, with a maximum of 38‰. This is explained by the incorporation of bicarbonate ions into the crystal lattice, which is not derived from hydrocarbons but from another carbon reservoir having a heavier isotopic composition, for example, seawater-dissolved inorganic carbon (δ13C ≈ 0‰), sedimentary organic matter (δ13C ≈ −20‰), or CO2 degassing from the magmatic (δ13C ≈ −6‰) and metamorphic crust (δ13C ≈ 0‰).The isotopic signatures of the seep carbonates at Gigors show a clear mixing trend with marine limestones, either caused by variable incorporation of seawater-derived carbonates or resetting during diagenetic recrystallization; it is likely that a pristine sample entirely sourced from AOM would exhibit a δ13C of −40‰, the lowest measured value, or lower. This likely points toward a methane δ13C at least as light as −60‰. According to a review on natural gases by Milkov and Etiope (2018), such values most likely indicate biogenic methane as the source of carbon, although the potential contribution of early mature thermogenic methane cannot be excluded.Bacterial methane generation in marine sediment occurs below the sulfate depletion level by CO2 reduction (Reeburgh, 2007). One of the major parameters that controls biogenic methane generation is temperature (Rice and Claypool, 1981); the optimum interval ranges from 30°C to 50°C and sterilization occurs above 70°C to 80°C (Wilhelms et al., 2001; Clayton, 2010; Sandu and Bissada, 2012; Stolper et al., 2014). However, many other factors have an impact, such as sedimentation rate, TOC, and sediment permeability (Clayton, 1992; Sivan et al., 2007). The depth of the methanogenic zone and the amount of gas generated are highly variable in modern seafloor sediments (Regnier et al., 2011). During the end of the early Aptian, the Barremian black shale was buried ∼150 m and the Goguel black shale was buried ∼50 m (not decompacted) in the central part of the basin (Figure 1C). This burial depth corresponds most probably to temperatures below the optimal range of bacterial methanogenesis but permitted significant generation of gas.However, according to Wannesson and Bessereau (1999), the Lias Marneux and the Terres Noires Formations were within the thermogenic gas window in the Aurel area, approximately 15 km to the southwest of Gigors, during the Aptian. Those two formations can be considered as potential thermogenic source rocks, whereas hypothetical coal-bearing Carboniferous would have been largely overmature.Mixing of biogenic methane generated by the Goguel black shale or Barremian black shales with thermogenic gas sourced from the Terres Noires Formation or the Lias Marneux may account for moderate measured δ13C negative values.Leakage of hydrocarbons from a reservoir through its overburden can result from three processes (Hao et al., 2015): (1) failure by capillary leakage (corresponding to the so-called loss of retention of a membrane seal) occurs where the pressure exerted by the buoyant hydrocarbon column at the base of the seal exceeds its capillary entry threshold (Berg, 1975; Koestler and Hunsdale, 2002; Jain and Juanes, 2009); (2) failure by hydrofracturing (corresponding to the so-called loss of integrity of hydraulic seals), which occurs when the pressure at the top of the reservoir exceeds the sum of the minimum horizontal stress and the resistance to horizontal traction (Grunau, 1987; Watts, 1987; Clayton and Hay, 1994; Biteau and Baudin, 2017); and finally (3) opening of preexisting faults, which is more complex, because faults can act alternatively as seals and reactivated migration pathways (Sibson, 1990).Capillary leakage is a defocused mechanism that will tend to occur above large parts of a hydrocarbon-filled trap. Seen on seismic data, capillary leakage may result in patches of “bottom simulating reflection” (Hyndman and Davis, 1992), indicating the presence of gas hydrate over an area (Clennell et al., 1999, 2000), or positive high-amplitude anomalies (Roberts, 2001) linked to a pervasive cementation by seep carbonates (Naeth et al., 2005). Conversely, hydrofracturing occurs where the pressure exerted by the buoyant hydrocarbon column reaches a maximum, typically at the highest point of the reservoir or at the weakest point of the seal (Løseth et al., 2009). Hydrofracturing is typically imaged by “seismic chimneys,” either expressed by seismic reflection discontinuities in narrow zones (Heggland, 2005; Løseth et al., 2009; Hustoft et al., 2010) or vertical stacks of high-amplitude anomalies (Hustoft et al., 2010; Petersen et al., 2010; Andresen, 2012; Ho et al., 2012, 2016), interpreted as seep carbonate stacks capping the chimney (Petersen et al., 2010; Plaza-Faverola et al., 2011; Ho et al., 2012, 2016).Regional exploration of outcrops (Figure 1B) and numerous logs described in regional studies (Friès, 1986; Bréhéret, 1995) clearly show that the presence of seep carbonates in the Marnes Bleues Formation is exceptional at both regional and basin scale. The isolated character of the seep carbonates implies the collecting function of a drain followed by focusing into a unique migration pathway up to the seafloor. Conversely, pervasive in situ generation of methane in the Marnes Bleues Formation would have produced uniformly distributed MDAC beds (e.g., Lash and Blood, 2004). Only a few turbidite channels have been recognized in the lower Aptian of the study area (Ferry and Flandrin, 1979); the coexistence of turbidite channels and seep carbonates in Les Blaches outcrop points toward a possible function of the channels as gas carriers. The following section discusses this hypothesis.The spiculite beds making the base of Les Blaches outcrop sealed the surface of rupture of the MTCs whose head scarp was located along the Gigors fault (Ferry and Flandrin, 1979; Friès, 1986) (Figure 1B). It is likely that the turbidite channels pinch out immediately to the south of the fault scarp, less than 1.5 km north of the seep carbonate outcrop, where the thickness of the Marnes Bleues decreases drastically. Because of the topographic slope along the syncline of St. Pancrace, the main turbidite channels, located stratigraphically 120 m below the first seep carbonates, crop out a few hundreds of meters to the north of the seep carbonate’s exposure. Given the south-southeast paleodirection of flowing, it is very probable that the turbidite channels extend right below the outcrop of seep carbonate. The proximal pinch-out of turbidite channel-fill deposits appears as a plausible trap for methane having eventually leaked vertically up to the seafloor where seep carbonates precipitated.The stratigraphy of the Marnes Bleues Formation has been studied in detail all over the Vocontian Basin: every major turbidite body has been mapped, dated, and named (see a review in Friès and Parize, 2003), particularly in each available outcrop in the study area (Ferry and Flandrin, 1979; Friès, 1986; Friès and Beaudoin, 1987; Bréhéret, 1995). It is thus possible to propose an identification of the turbidite channels visible at the outcrop of Les Blaches. The first package of turbidites in the Marne Bleues Formation, identified as Plaquettes 1 (P1), is dated from the S. cabri biozone. This is younger than the seep carbonates and contemporaneous of the deposition of the Goguel black shale (Bréhéret, 1995; Friès and Parize, 2003; Moullade et al., 2015) (Figure 1D). The interlayering of bioclastic and siliciclastic centimeter-thick P1 turbidite beds with black shale extends vertically over several meters and covers an area of several thousands of square kilometers in the central part of the Vocontian Basin (Friès, 1986). The second package of turbidite, Plaquettes 2 (P2), is dated from the base of the Globigerinelloides algeriana biozone (i.e., older than the seep carbonates). Because the turbidite channels of the studied outcrop are younger than the seep carbonates, they are a fortiori younger than P2 and can only be correlated with P1. In places, P1 contains numerous sponge spicules (Bréhéret and Delamette, 1987), but it is not an almost pure spiculite such as the infill of the channels at Les Blaches. Ferry et al. (2005) explain this apparent inconsistency by a process of erosion and bypass of the coarse sediment in the proximal part of the channels up to the deposition area in the deeper part of the basin. The eroded channels were left “empty” and afterward filled with spiculites and marl beds.In the hypothesis that the Goguel black shale generated biogenic methane, the intimate interbedding of the P1 turbidite lobes with the source rock produced a very extensive drain. Additional input of gas from deeper biogenic or thermogenic source rocks was possible (Figure 17). From the turbidite lobes, gas migrated updip toward the proximal pinch-out of permeable channel-fill deposits. Facies transition toward the basin margin results in an enrichment of the channel infill in clay, whereas the host marls are considerably enriched in sand and spicules. The degradation of reservoir and seal properties, respectively, permits a lateral spread of the gas from the channel pinch-out into the surrounding formation (Schowalter and Hess, 1982; Downey, 1984).The wavy bed of seep carbonates, resulting from unfocused methane seepage, may correspond to an initial phase of gas charging of the channel pinchout area associated with capillary leakage. In a second phase, tectonic activity or ongoing filling of the channel by additional gas may have resulted in the fracturing pressure being exceeded at the base of the seal, opening a series of hydrofractures. These preferred migration pathways carried gas to the stacked patches of seep carbonates. Lateral shifting was recorded toward the end of seep activity and may document reactivation of some fractures or a shift in the preferred gas channeling directions within fracture planes (Saffer and Tobin, 2011; Seebeck et al., 2014). Finally, cessation of seepage may have various causes, including the breaching of the seal at another location or shutdown of the methane supply to the reservoir.The detailed mapping of seep carbonates at outcrop scale, combined with the study of the geological framework at regional and basin scale, allow proposing the following model for methane migration and associated seep carbonate precipitation. Earliest Aptian turbidity currents sourced from the Vercors platform flowed along slope channels toward the south-southeast and were deposited in thin lobes interbedded with the Goguel black shale in the central part of the Vocontian Basin. After burial of several tens of meters, the turbidite lobes acted as drains collecting biogenic methane generated by the Goguel black shale or gas from deeper thermogenic source rocks. Methane migrated updip along the channels and accumulated at their pinch-out along the edge of the basin. From those traps, a widespread front of methane migrated upward, probably by capillary leakage, through the uncompacted Marnes Bleues Formation and precipitated a continuous bed of seep carbonates subparallel to the seafloor. During a second phase, seepage style switched abruptly toward a highly focused mode, giving rise to local patches of seep carbonates. Preferred focused migration pathways have probably been induced by the opening of fractures, possibly because of hydrofracturing of the seal. As seepage carried on over a substantial period of time, patches of seep carbonates stacked vertically over several tens of meters. This case study demonstrates that seep carbonate features may be predominantly impacted by subsurface processes; as such, seep carbonates can be incorporated as a valuable tool into basin analysis to constrain the timing of emplacement and location of hydrocarbon accumulations and to shed light on the evolution of petroleum systems.We thank Total Exploration and Production for the financing, as well as granting permission to publish this research, and for many insightful discussions with geoscientists of the sedimentology and geochemistry research teams. Thibaut Renard and Martin Livet greatly helped during the field reconnaissance, and Andreas Wetzel identified heulandite at the University of Basel. The International Association of Sedimentologists provided financial support to organize a field trip to outcrops including Gigors. The Thin Section Lab (Toul, France) and its personnel are thanked for providing access to their facilities. The authors have no conflict of interest to declare.Appendix 2 is available in an electronic version on the AAPG website (www.aapg.org/datashare) as Datashare 131.Jean-Philippe Blouet is an exploration geologist affiliated with the University of Brussels. He worked at Total’s headquarters, then completed a Ph.D. at the University of Fribourg (Switzerland), exploring the petroleum significance of seep carbonates based on outcrop studies. He currently pursues a similar problematic based on reactive transport modeling. Additionally, he collaborates paleontology exhibitions with natural history museums worldwide. He is the corresponding author of this paper.Patrice Imbert is currently an associate researcher at the University of Pau (France) after a 39-year career with Total Exploration and Production. In the last decade there, he promoted the use of seismic evidence for fluid leakage, present and past, to de-risk basin and prospect evaluation. His current research is focused on mud volcano processes.Anneleen Foubert has been an associate professor of carbonate sedimentology at the University of Fribourg (Switzerland) since February, 2013. Previously, she worked as researcher at the University of Leuven (Belgium) and as carbonate sedimentologist at Total (Pau, France). Trained as a marine geologist at Ghent University (Belgium), she specializes in the study of carbonate mound systems through space and time.Sutieng Ho is a geoscientist with 12 years of experience specializing in seismic analysis of hydrocarbon leakage systems and is a member of Fluid Venting Research Group and National Taiwan University. After years of training in France, she worked at Total’s headquarters from 2007 to 2014. She developed alternative methods to assess fault–seal integrity and discovered new varieties of hydrocarbon indicators.Gérard Dupont was a geologist at Total Exploration and Production, exploration division. He received his Ph.D. from the University of Lyon-1 in 1981. He specialized in micropaleontology at Total’s headquarters and Gabon exploration laboratories. He occupied the position of biostratigraphy team manager in Pau in 2010. His research interests include stratigraphy and paleoenvironmental reconstructions based on various microfossil groups.
更新日期：2021-05-14
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