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Vertical evolution of fluid venting structures in relation to gas flux, in the Neogene-Quaternary of the Lower Congo Basin, Offshore Angola
... This has promoted interest in the study of complex geological structures known to either host or are indicative of gas/ fluid migration. The expression of fluid and gas migration through the sedimentary column is shown by a variety of features such as: gravity collapse structures, mounds, mud volcanoes, surface and buried pockmarks, diapirs, cold seeps and polygonal faulting Hartwig et al. 2012;Ho et al. 2012). These features are often associated with seismic anomalies such as positive high amplitude anomalies (PHAAs), bottom simulating reflectors (BSRs), and pipe and chimney venting structures (Løseth et al. 2011;Gay et al. 2003;Cartwright et al. 2007;Hustoft et al. 2010;Ho et al. 2012). ...
... The expression of fluid and gas migration through the sedimentary column is shown by a variety of features such as: gravity collapse structures, mounds, mud volcanoes, surface and buried pockmarks, diapirs, cold seeps and polygonal faulting Hartwig et al. 2012;Ho et al. 2012). These features are often associated with seismic anomalies such as positive high amplitude anomalies (PHAAs), bottom simulating reflectors (BSRs), and pipe and chimney venting structures (Løseth et al. 2011;Gay et al. 2003;Cartwright et al. 2007;Hustoft et al. 2010;Ho et al. 2012). Natural gas/ fluid escape features identified offshore South Africa and Namibia in the shallow Orange Basin include seafloor and buried pockmarks (Hartwig et al. 2012), seismic chimneys (Ben-Avraham et al. 2002;Paton et al. 2007;Kuhlmann et al. 2010;Boyd et al. 2011), mud diapers and volcanoes (Ben-Avraham et al. 2002;Viola et al. 2005), reflecting the basin's underlying hydrocarbon system. ...
... Since pockmarks and mud volcanoes are found in a wide variety of settings, from passive to active margin settings, in compressional zones such as accretionary prisms, fold and thrust belt systems, deltaic settings and deep sedimentary basins related to active plate margins (e.g., Judd and Hovland 1988;Gay et al. 2003Gay et al. , 2006Gay et al. , 2007Loncke et al. 2004;Hustoft et al. 2010;Andresen and Huuse 2011;Ho et al. 2012;Hartwig et al. 2012;Anka et al. 2014), the source of their hydrocarbon system varies greatly. From the synthesis of past studies in the shallow Orange Basin (Jungslager 1999;Ben-Avraham et al. 2002;Kuhlmann et al. 2010;Hartwig et al. 2012), we postulate the source of hydrocarbons in this study to also be of both thermogenic and biogenic (or microbial) origin. ...
Many features indicative of natural gas and oil leakage are delineated in the deep-water Orange Basin offshore South Africa using 3D reflection seismic data. These features are influenced by the translational and compressional domains of an underlying Upper Cretaceous deep-water fold-and-thrust belt (DWFTB) system detaching Turonian shales. The origin of hydrocarbons is postulated to be from both: (a) thermogenic sources stemming from the speculative Turonian and proven Aptian source rocks at depth; and (b) biogenic sources from organic-rich sediments in the Cenozoic attributed to the Benguela Current upwelling system. The late Campanian surface has a dense population of > 950 pockmarks classified into three groups based on their variable shapes and diameter: giant (> 1500 m), crater (~ 700–900 m) and simple (< 500 m) pockmarks. A total of 85 simple pockmarks are observed on the present-day seafloor in the same area as those imaged on the late Campanian surface found together with mass wasting. A major slump scar in the north surrounds a ~ 4200 m long, tectonically controlled mud volcano. The vent of the elongated mud volcano is near-vertical and situated along the axis of a large anticline marking the intersection of the translational and compressional domains. Along the same fold further south, the greatest accumulation of hydrocarbons is indicated by a positive high amplitude anomaly (PHAA) within a late Campanian anticline. Vast economical hydrocarbon reservoirs have yet to be exploited from the deep-water Orange Basin, as evidenced by the widespread occurrence of natural gas/fluid escape features imaged in this study.
... For instance, observations of seep carbonate at the sea floor, complemented with acoustic (1-10 kHz) and high-resolution seismic data (1-50 Hz) can be used to reconstruct fluid circulation in the shallow seabed and are predominantly used in geohazard prevention, i.e. in the study of seafloor instability (Riboulot et al., 2019). On a million-year timescale, the vertical succession of seep carbonate bodies observed in seismic data has been used to reconstruct the history of hydrocarbon leakage (Imbert et al., 2007;Ho et al., 2012Ho et al., , 2016Ho et al., , 2018. When hydrocarbon seepage is linked to the activity of an underlying petroleum system (sensu Magoon and Dow, 1994), information about seep carbonates can be combined with knowledge about the tectonosedimentary context of their host basin, to help track migration pathways down to hydrocarbon source rocks, potentially identifying reservoirs and accumulations (Agirrezabala, 2009(Agirrezabala, , 2015Blouet et al., 2017Blouet et al., , 2021a. ...
... The porosity profile is constant for all our simulations and fitted to values measured on the Angola passive continental margin (φ z0 =0.92; φ ∞ =0.8; attpor = 0.15; Eric Cauquil, personal communication), a prolific hydrocarbon province rich in seep sites (e.g. Ho et al., 2012Ho et al., , 2018aHo et al., , 2018b. The molecular diffusion coefficients D i for each specie i are adjusted for temperature, salinity and sediment tortuosity (Boudreau, 1997): ...
... The model results presented in this study provide a quantitative framework that potentially allows for a more holistic and quantitative assessment of the major processes controlling seep carbonate precipitation. We illustrate this holistic approach on the basis of a classical example of a vertical stack of amplitude anomalies, interpreted as seep carbonates, observed on seismic data in hemipelagic sediments in the Plio-pleistocene offshore Angola (1300 m water depth) by Ho et al. (2012) (Fig. 9). Eustatic variations merely reach an order of magnitude of 100 m over the Plio-pleistocene (Haq et al., 1987) and thus represent only ca. ...
Seep carbonates tell us where and when CH4-charged fluids escaped from the subsurface, thus providing qualitative information to reconstruct the activity of petroleum systems. The potential of seep carbonates as quantitative proxies for the amount of CH4 leaked, however, remains largely unexplored, which limit their applicability as exploration tools. This paper tackles the quantification of the CH4 flux - seep carbonate relationship by simulating the coupled sedimentary carbon (C) – sulfur (S) cycles in a reaction-transport modeling (RTM) framework. We first establish a theoretical basis demonstrating that the stoichiometry of diagenetic reactions and the ambient pH of pore waters are the main drivers of the rate of change in the saturation state of carbonate minerals (ΩCal), while the concentrations of total dissolved inorganic carbon and sulfide are only of secondary importance. It results that anaerobic oxidation of methane (AOM) is the main driver of carbonate precipitation, while organoclastic sulfate reduction (SR) has a minor impact. We further show that SR mostly drives carbonate dissolution, but can also contribute to precipitation when pH is low (<7–7.1). The RTM simulations reveal that an increase in upward fluid flow triggers an intensification of peak AOM rates, associated to a shallowing and thinning of the zone of carbonate precipitation. Such behavior leads to an almost linear relationship between the amount of carbonate precipitated and flux of CH4 (nCH4 = 3.3–5.2 * nCaCO3), until, eventually, full cementation occurs. We thus define a “quantitative domain” at moderate fluid flow and a “threshold domain” at high fluid velocities, where full cementation solely provides a lower bound estimate of the amount of CH4 leaked. We also show that in contrast to a traditional view of seep carbonate formation mainly controlled by venting activity, sedimentation rate and water depth also play major roles, via their control on residence time and saturation concentration of CH4, respectively. The interpretation of vertical seep carbonate stacks should thus not solely focus on changes in fluid flow, but also consider changes in sedimentation rate and/or water depth.
... 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) Friès and Parize, 2003). ...
... 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. ...
... 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 Petersen et al., 2010;Andresen, 2012;Ho et al., 2012Ho et al., , 2016, interpreted as seep carbonate stacks capping the chimney (Petersen et al., 2010;Plaza-Faverola et al., 2011;Ho et al., 2012Ho et al., , 2016. ...
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 distri-bution 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 d
13
C 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.
... Therefore, trap sealing is supposed to cause overpressure development and eruption [77]. During overpressure eruption, the gas hydrate mass and authigenic carbonate mass are blown out to the seafloor and form the gas hydrate outcrops and carbonate stacks, respectively [68,70,[78][79][80][81]. Seepages are discovered along with the fractures in the carbonate reef, indicating that the fractures exist as the pathway for seepages [58,82]. ...
... The nodular high-Mg carbonate forms under low methane content conditions, and the aragonite carbonate crust forms under high methane or high Ca content conditions [48,82]. Microbial mats and benthos are discovered beside the active seepage [38,60,74,82,91], related to anaerobic oxidation of methane and sulfate reduction [48,79,82,92], which make a significant contribution to the formation of the authigenic carbonates. ...
... Carbonate ridges and stacks are found beside pockmarks, and many researchers have indicated that authigenic carbonates occur as trap layers, leading to overpressure eruptions [79,87]. The authigenic carbonates are considered as products related to methane oxidation and sulfate reduction [63] that are import indications of hydrocarbon gas and fluid migration. ...
Geological structure changes, including deformations and ruptures, developed in shallow marine sediments are well recognized but were not systematically reviewed in previous studies. These structures, generally developed at a depth less than 1000 m below seafloor, are considered to play a significant role in the migration, accumulation, and emission of hydrocarbon gases and fluids, and the formation of gas hydrates, and they are also taken as critical factors affecting carbon balance in the marine environment. In this review, these structures in shallow marine sediments are classified into overpressure-associated structures, diapir structures and sediment ruptures based on their geometric characteristics and formation mechanisms. Seepages, pockmarks and gas pipes are the structures associated with overpressure, which are generally induced by gas/fluid pressure changes related to gas and/or fluid accumulation, migration and emission. The mud diapir and salt diapir are diapir structures driven by gravity slides, gravity spread and differential compaction. Landslides, polygonal faults and tectonic faults are sediment ruptures, which are developed by gravity, compaction forces and tectonic forces, respectively. Their formation mechanisms can be attributed to sediment diagenesis, compaction and tectonic activities. The relationships between the different structures, between structures and gas hydrates and between structures and authigenic carbonate are also discussed.
... Our previous studies addressed the relationship between a unique system of chimneys and a Neogene tier-bound fault system in the deepwater sediments offshore West Africa. Detailed seismic attribute maps of layers intersected by chimneys and tier-bound faults, first shown in Ho et al. (2012), reveal an intriguing, geometric parallelism between these two structural elements which strongly suggests their interrelated development. Statistical data on the position of chimneys intersecting faults from Ho (2013) are used to perform a quantitative analysis of fault seal integrity. ...
... This geometry contrasts with the chimneys commonly reported in the literature, which typically exhibit a circular or elliptic planform and a cylindrical or columnar 3D morphology. Linear Chimneys comprise a new type (Ho et al., 2012). They have an aspect ratio > 4:1 in map view. ...
... Linear Chimneys in the study area offshore West Africa occur along and parallel to tectonic faults and/or tier-bound faults. Their planar morphology has been interpreted to result from hydraulic fractures propagating under the influence of anisotropic stress fields surrounding the adjacent faults and the local salt tectonic structures (Ho et al., 2012(Ho et al., , 2018. Like the common chimneys reported in the literature (e.g. ...
An innovative and low-cost method for assessing fault seal integrity by using the geometry of intersection between faults and gas conduits (so called chimneys, or pipes) is proposed, exemplified by 3D seismic data from offshore West Africa. A new type of gas chimney with uncommon linear planform (in contrast to the more common sub-circular chimneys), has been observed intersecting downward different parts of faults or originating from fault surfaces. Statistical analysis shows that 73% of these chimneys originate from the lower part of footwall or the lower half of fault surface. This suggests that, in ¾ of the cases, overpressured gas did not migrate all the way up the fault, but rather created preferentially vertical hydraulic fractures along the entire fault strike, or reactivated pre-existing fractures to leak upward. Thus, at least the upper half of the fault planes did not act as efficient migration pathways and was impermeable during expulsion of gas. As a result, hydraulic fractures formed wall-like structures termed "Linear Chimneys". The top of a Linear Chimney is commonly characterized by aligned seep carbonates that formed in depressions on the palaeo seafloor and which are seismically expressed as linear positive high-amplitude anomalies. These structures are parallel to adjacent tectonic and/or tier-bound faults (i.e. polygonal faults). These faults show preferential orientation parallel to salt-related structures. The relationship between fault anisotropy and local structures suggests a control of the regional and local stress states on fault initiation and propagation. Chimneys with a linear planform only develop in areas exhibiting anisotropic arrangement of (polygonal) faults. In areas having an isotropic (polygonal) fault network, the chimneys and overlying amplitude anomalies display a circular planform. Using spatial relationships between faults and chimneys to investigate fault seal integrity forms the base of an innovative method. So far, no other study has used fact-based statistical data demonstrating that chimneys principally emanated from below the topmost tip/the upper segment of faults to support the hypothesis that fault impermeability induces chimney formations via propagation of vertical fractures. Seep carbonates with linear planform precipitated above Linear Chimneys record the timing of gas leakage. Both Linear Chimneys and the carbonates can be used to reconstruct orientations of palaeo stresses states.
... Vertical stacking of buried pockmarks are caused by reactivation of fluid leakage (Hovland 1981;Mazzotti et al. 1987;Çifçi et al. 2003;Moss 2010). Therefore, variations of pock-mark size and morphology along a vertical succession can be used to reconstruct the evolution of fluid venting intensity through time (Ho et al. 2012a). In contrast to purely aggradational pockmark succession, slant stacked, or lateral-shifted pockmarks, both buried and modern, have been recognised in a few studies (e.g. ...
... The Neogene to Quaternary sedimentary succession within the study area consists of well-bedded hemipelagic sediments (Broucke et al. 2004;Vignau et al. 2000). Two late-Miocene turbidite channel complexes oriented NE-SW incise Upper Miocene strata (Ho et al. 2012a). Channel Complex 1 (CC1) bends around the NW flank of Diapir-1. ...
... Fig. 9i, j; cf.Petersen et al. 2010;Plaza-Faverola et al. 2011;Ho et al. 2012a). The dome-like appearance very likely developed due to increased methane flux that pushed the SMTZ towards the seabed(Paull and Ussler 2008; Hovland et al. 1987, Fig. 3). ...
Pockmarks in Pliocene-Quaternary continental slope deposits offshore Angola show features related to: (1) fluid leakage craters that formed repeatedly, (2) authigenic methane-derived carbonates that indicate the (former) presence of hydrocarbons and (3) erosional–depositional structures that are clearly related to current activity. Depending on topography, the pockmarks show differing development: “Advancing Pockmarks” preferentially developed on regional slopes or inclined topography (> 2.5°–3°). They arranged in a chain-like pattern and mimic the outline of buried turbidite channels below. These pockmarks and their infill migrated downslope in response to shifting vents. “Nested Pockmarks” occur in gently sloping areas (< 2°). Their isolated conical infill records slope-parallel migration within a specific depth range pointing to the influence of contour currents. Both pockmark types are long-lived and they record preferential fluid migration along specific pathways, which developed at the downcurrent sidewalls of pockmarks due to flow separation initiating “cavity flow” within the pockmarks. The durable specific migration paths include pockmark sidewalls, vertically stacked erosional-interface of sediment waves, or entire pockmark bodies. The vertical extent of both pockmark types from End Miocene to the present-day seafloor documents various intensities of episodic fluid bursts followed by periods of quiescence and fill.
... The study area is located in the Lower Congo Basin (offshore western Africa; Fig. 1), which was created by the Early Cretaceous opening of the South Atlantic Ocean (Ho et al., 2012). Vast quantities of clastics were delivered into this basin by the Zaire River, with a drainage catchment of 3.8 × 10 6 km 2 and a sediment load of 4.3 × 10 7 t/yr ( Fig. 1), giving rise to the areally extensive Zaire (Congo) fan (Ho et al., 2012). ...
... The study area is located in the Lower Congo Basin (offshore western Africa; Fig. 1), which was created by the Early Cretaceous opening of the South Atlantic Ocean (Ho et al., 2012). Vast quantities of clastics were delivered into this basin by the Zaire River, with a drainage catchment of 3.8 × 10 6 km 2 and a sediment load of 4.3 × 10 7 t/yr ( Fig. 1), giving rise to the areally extensive Zaire (Congo) fan (Ho et al., 2012). The studied UCs are Quaternary in age, and occur on the southeastern margin of the Quaternary Zaire fan (Fig. 1). ...
... Contour currents generally involve a significant volume of water mass in a large area, and persist over very long time intervals, most likely causing the documented UCs to migrate northward consistently in the direction of the modern northward-flowing South Equatorial Current throughout their life span ( Figs. 1 and 2). It should be noted that channels on the West African slope with water depth greater than the effective depth of the South Equatorial Current do not have unidirectional migration trajectories (Ho et al., 2012). ...
Inspired by the two-layer model of a stratified lake forced by wind stress, we introduce the concept of Wedderburn number (W) to quantify, for the first time, how turbidity and contour currents interacted to determine sedimentation in unidirectionally migrating deep-water channels (UCs). Bankfull turbidity flows in the studied UCs were computed to be supercritical [Froude number (Fr) of 1.11–1.38] and had velocities of 1.72–2.59 m/s. Contour currents with assumed constant velocities between 0.10 and 0.30 m/s flowing through their upper parts would result in pycnoclines between turbidity and contour currents, with amplitudes of up to 7.07 m. Such pycnoclines, in most cases, would produce Kelvin-Helmholtz (K-H) billows and bores that had velocities of 0.87–1.48 m/s and prograded toward the steep channel flanks by 4.0° to 19.2°. Their wavefronts with the strongest shocks and deepest oscillations would, therefore, occur preferentially along the steep flanks, thereby promoting erosion; on the other hand their wavetails with the weakest shocks and shallowest oscillations would occur preferentially along the gentle flanks, thereby promoting deposition. Such asymmetric intra-channel deposition, in turn, forced individual channels to consistently migrate toward the steep flanks, forming channels with unidirectional channel trajectories and asymmetrical channel cross-sections.
... Sur les données sismiques, les monts carbonatés apparaissent majoritairement comme des évènements ponctuels très réflectifs (des patchs), pouvant avoir une forme en dôme. Les MDAC sont principalement identifiables de par leur signature sismique de très forte amplitude (Bayon et al., 2009 ;Savini et al., 2009 ;Ho et al., 2012 ;Römer et al., 2014), contrastant fortement avec les réflecteurs de faible amplitude, continus, subparallèles, définissant les dépôts courants sur le fond de l'eau. (Buerk et al., 2010).. Trois processus interagissent dans la morphologie des MDAC : (1) les fuites de fluides enrichis en méthane (en bas à gauche). ...
... The envelope display of a 2D-AUV profile across Morphotype B shows closely spaced patches of high amplitude buried within the first 60 ms TWT below the main seafloor depression of Morphotype B (Figure 49.A&B). In fluid venting zones, on siliciclastic mud-dominated slopes, positive high amplitude patches on seismic data are generally interpreted as the response of buried carbonate concretions (Bayon et al., 2009;Ho et al., 2012;Römer et al., 2014;Savini et al., 2009).These patches thus seem to correspond to buried seep carbonates that mainly concentrate along four laterally discontinuous levels under the depression of Morphotype B (Figure 49). ...
... Moss et al., 2012) ou encore le long de cheminées sismiques (e.g. Ho et al., 2012). Ces anomalies d'amplitude sont similaires à celles observées au coeur des pockmarks allongés des structures en araignée ( Figure 45) où les patches de forte amplitude ont été corrélés à la présence de carbonates méthanogènes se mettant en place au coeur des pockmarks (Casenave et al., in press). ...
Les structures d’échappement de fluides et leurs mécanismes de migration à travers la pile sédimentaire sont un phénomène connu sur les marges continentales. Elles ont été largement étudiées depuis une vingtaine d’années, notamment en raison de l’amélioration de la résolution des données sismiques, et de l’abondance des données dans ces zones, du fait de la prospection pétrolière. Le bassin du Bas Congo, au large de l'Afrique de l’ouest, est une province pétrolière prolifique qui a été largement étudiée et qui est exploitée depuis plus de 30 ans. La zone d’intérêt est située au-dessus d'un champ pétrolier producteur (la zone de Moho), dans laquelle les hydrocarbures sont piégés dans des chenaux turbiditiques. Le travail est principalement basé sur l'analyse de données géophysiques comprenant de la sismique 3D et 2D-THR, de la bathymétrie multifaisceaux et la rétrodiffusion correspondante, ainsi que des données de fond (échantillons prélevés, photos ROV, analyses géochimiques). L'analyse de cet important jeu de données sismiques a révélé de nombreuses évidences de migration de fluides dans la pile sédimentaire mio-pliocène. Ces dernières correspondent principalement à des indices de migration focalisée de fluides, incluant des structures actuelles d’expulsion de fluides sur le fond de l’eau et des structures enfouies, interprétées comme fossiles, et indiquant une activité ancienne du système d’expulsion de fluides. Les indices de migrations de fluides étudiés correspondent principalement à des pockmarks (dépressions) et à des cimentations carbonatées. Ces deux types d’indices peuvent s’empiler sur plusieurs centaines de mètres, mettant en évidence la pérennité des échappements et suggérant le développement de véritables conduits.Un nouveau type de pockmarks a été mis en évidence, les structures en araignée, qui sont localisées au-dessus d'un réservoir turbiditique, et qui résultent d'une migration focalisée des hydrocarbures thermogéniques. Leur fonctionnement est lié aux hydrates de gaz dans le contexte d’un BSR penté (Bottom Simulating Reflector), du fait de la présence du biseau des hydrates. Un modèle dynamique de leur fonctionnement est développé, montrant que ces structures se développent vers l'amont de la pente, du fait de la migration du gaz sous le BSR penté. Une étude du BSR, dans le contexte particulier du biseau des hydrates, permet de proposer un modèle des échappements de gaz liés à la dissociation des hydrates, lors d'une baisse du niveau marin. Ce modèle met en scène une dissociation des hydrates de gaz d'échelle régionale (associée à du gaz biogénique), localisée au niveau du biseau des hydrates du dernier bas niveau marin. Enfin, le réseau de structures d’échappements de fluides de l’intervalle Mio-Pliocène a été investigué dans le but de comprendre son architecture et les mécanismes de migration de fluides dans la zone d’étude. Les hydrocarbures semblent migrer principalement le long de certaines portions de failles et verticalement à travers la pile sédimentaire sous forme de « pipes » ou de cheminées. La base du Pliocène, associée à une baisse du niveau marin, marque la formation des premières paléo-araignées ainsi que d'un niveau contenant de nombreux indices de présence de gaz. Un modèle de ce réseau de migrations de fluides est proposé, intégrant les chemins de migrations des hydrocarbures à travers la pile sédimentaire, et l’événement majeur de la base Pliocène. Cette étude semble indiquer que les phases de baisse du niveau marin constituent des déclencheurs pour la migrations des fluides dans les bassins.Ce travail marque ainsi le point de départ d'une investigation à plus grande échelle qui consiste d'une part à rechercher des structures similaires (araignées et bandes de pockmarks) dans d'autres bassins et d'autres part à comparer les événements d'échappement de fluides à la courbe eustatique.
... The resulting mounded seafloor morphology, following such events of fluid flow, will then influence the sediment deposition (cf. Plaza-Faverola et al., 2011;Ho et al., 2012). ...
... The seismic-scale internal structure of fluid-escape pipes has been associated with seismic artefacts, fracture systems, collapsed sediments, pockets of free gas, as well as authigenic minerals and hydrates (Hustoft et al., 2010;Moss and Cartwright, 2010;Løseth et al., 2011;Plaza-Faverola et al., 2011;Ho et al., 2012;Koch et al., 2016). In this study, the up-bending geometry of the stratigraphic reflections at the margins and along the pipes (Figs. 4, 8) indicates the presence of two possible processes, capable of modifying the original sediment structure: ...
... (2) The enhanced 'hard' reflection along Pipe-α, at 30-40 mbsf, likely representing a buried hydrate/carbonate mound (Fig. 8c, e) (cf. Chun et al., 2011;Ho et al., 2012). (3) The extensive deformation along the pipes observed where free gas doming would be ineffective, i.e. well-below the first tens of metres of overburden (Figs. ...
Fluid-escape pipes represent seismic evidence for the focused cross-stratal migration of fluids. In natural gas hydrate systems, these features serve both as conduits for methane-rich fluids and as preferred locations for the formation of gas hydrates. In this study, 3D seismic, well-log and core data from offshore Sabah (NW Borneo) are used to investigate the controls on the occurrence of fluid-escape pipes and their impact on hydrate distribution in a system dominated by the vertical leakage of thermogenic hydrocarbons.
The pipes are observed within a gas hydrate stability zone (GHSZ) that extends 100 m below a bottom simulating reflector (BSR), located at 155 m below the seafloor (mbsf). Pipes are restricted to an area with evidence of free gas-bearing sediments, suggesting a causative link where the free gas promotes the build-up of critical fluid pressures. The stacking of the upper terminus of fluid-escape pipes at discrete stratigraphic intervals suggests that fluid flow to the seabed has been episodically enhanced. Possible triggers for cyclical increases of pore fluid pressures are sea-level and temperature fluctuations, tectonic activity and gas leakage from deep reservoirs.
This fluid flow system further impacts the gas hydrate distribution. The fluid-escape pipes can be locations where hydrates occur at high concentrations up to the seafloor if the pipe is presently active. Therefore, the observed up-bending of the stratigraphic reflections along the pipes are interpreted as a combination of a net volume increase of the host sediment owing to hydrate formation and seismic velocity pull-up effects. Away from the pipes, hydrates do not occur until 65–152 mbsf and are present only at low to moderate concentrations. At this site of focused fluid flow, fluid-escape pipes constitute, by volume, only 7–11% of the gas hydrate occurrence zone. Nevertheless, we predict that they could host between 20 and 50% of the whole hydrate volume. It is therefore likely that, in similar systems, a volumetrically significant portion of the total hydrate reservoir is hosted within fluid-escape pipes. The distribution of these features should thus be considered as a critical parameter for hydrate volume estimates.
... In some cases, we find a central vertical conduit brings fluid, including dissolved and gaseous methane, up from depth in the submarine sediment to the seafloor (Paganoni et al., 2018), where it may disperse into the water column; this can be associated with a submarine mud volcano, a seep, or a seabed pockmark (Cartwright & Santamarina, 2015;Maestrelli et al., 2017). In some cases, however, the exit to the ocean of such a central conduit may become blocked (Ho et al., 2012), owing to what has been termed self-sealing processes (Hovland, 2002). In sea-floor plumbing systems with a blockage of the exit of the central conduit, there may be a layer of methane hydrate blocking the exit and evidence of pressure-induced doming of the seabed (Barry et al., 2012;Koch et al., 2015). ...
... Submarine "spider structures" have been observed and described in the literature as pockmark-like seabed venting structures ; see also Ho et al. (2012) and Maia et al. (2016). These are composed of large quasi-circular depressions, the "body" of the spider, surrounded by more elongated depressions, (the "legs" of the spider, as shown in Figure 1. ...
Methane‐rich water moves through conduits beneath the seafloor whose surfaces are formed through precipitation reactions. To understand how such submarine fluid conduit and venting systems form and grow, we develop a detailed mathematical model for this reaction‐advection system and we quantify the evolution of an ensemble of similar filaments. We show that this growth can be described by a superposition of advection and dispersion. We analyze analog laboratory experiments of chemical‐garden type to study the growth of a single filament undergoing a precipitation reaction with the surrounding environment. We apply these findings to geological fluid conduit and venting systems, showing that their irregular trajectories can lead to very effective spreading within the surrounding seabed, thus enhancing contact and exchanges of chemicals between the conduit and external fluids. We discuss how this methane venting leads to the formation of marine authigenic carbonate rocks, and for confirmation, we analyze two field samples from the Gulf of Cadiz for composition and mineralogy of the precipitates. We note the implications of this work for hydrate melting and methane escape from the seabed.
... Ascending fluids that disturb or destroy otherwise parallel geological layers can result in locally "pulled-up" or "bent-up" seismic reflections. Alternatively, pulled-up reflections can be velocity artefacts due to anomalously high seismic velocities associated with, for example, gas hydrates or authigenic carbonate cement within the fluid-escape conduit (Ecker et al., 1998;Ho et al., 2012;Hustoft et al., , 2007. Additionally, gas hydrate in pore spaces may reduce impedance contrasts between geologic layers causing acoustic blanking (Fraser et al., 2016;Lee et al., 1996;Lee and Collett, 2001;Westbrook et al., 2008). ...
... Boetius et al., 2000;Ritger et al., 1987). These carbonates are often associated with high-amplitude, positivepolarity reflections and a high backscatter signal in pockmarks (Böttner et al., 2019;Dandapath et al., 2010;Ho et al., 2012;Judd and Hovland, 2009). Similarly, the 5.4 Discussion 122 suspension of fine-grained material due to fluid escape, leaving the coarser-grained material behind as well as enhancing biological activity due to seeping fluids (skeleton remains, dead and living shells, etc.) can create a strong impedance contrast and a rougher surface area and therefore result in strong backscatter values in pockmarks (Hovland, 1989;Hovland et al., 2002;Reusch et al., 2015). ...
Fluid migration from deep sedimentary basins towards Earth’s surface has various implications for hydrocarbon accumulations and influences slope stability, climate and ecological systems. Fluids seeping from the seafloor can give insight into deep crustal and tectonic processes and can significantly change the seafloor morphology as well as the chemical composition of the overlying ocean. (...)
... In recent years, most attention has been paid to seepage processes within hydrocarbon basins that typically contain thick sedimentary sequences and the potential for hydrocarbon trapping. Research focused on the geological setting of fluid migration pathways Cartwright et al., 2007;Serié et al., 2016) and the formation of seafloor seepage features in particular (Cathles et al., 2010;Ho et al., 2012;Sultan et al., 2010Sultan et al., , 2014. In association with hydrocarbon migration, the interaction of sand injectites with fluid flow systems has been investigated specifically (Cartwright et al., 2008;Hurst et al., 2011;Monnier et al., 2014;Shoulders & Cartwright, 2004). ...
... Especially the Lower Congo Basin (LCB) has been investigated extensively with respect to the fluid flow systems in its upper and middle slope setting Gay et al., 2003Gay et al., , 2004Gay et al., 2007;Ho et al., 2012;Lucazeau et al., 2004;Maia et al., 2016;Serié et al., 2016). The lower slope and the salt front, however, received less attention although hosting abundant fluid flow phenomena (Olu et al., 2009;Ondréas et al., 2005;Sahling et al., 2008;Wenau et al., 2015aWenau et al., , 2015b. ...
Seafloor seepage is a widespread phenomenon within salt-influenced basins as the deformation provides pathways for hydrocarbons to reach the seafloor. However, only minor attention has been given to the distal parts of such systems where the impact of salt-tectonic deformation is relatively unpronounced. The stress put on the sedimentary column by moving salt on a continental margin may influence fluid flow systems even outside of the salt province. This stress may lead to overpressure formation within reservoirs and determine the orientation of overpressure-induced fractures.
Seepage in the Congo Fan has been discovered in such a distal position at the Regab pockmark, about 35 km west of the salt front and its geology and biology have been studied extensively in recent years. We present high-resolution multichannel seismic data from the Regab pockmark that reveal the underlying migration pathways from a buried channel flank 300 mbsf to the seafloor via hydraulic fractures in the sealing overburden. Local doming of the reservoir and the remobilization and uplift of sedimentary strata along the migration pathways are interpreted as the result of overpressure within the reservoir. The orientation of the hydraulic fractures is WSW-ENE and the fracture outline corresponds to the area of most intense seepage activity within the seafloor pockmark. Along with a similar orientation of other fractures in the vicinity, we propose that this alignment is due to the stress imposed on the sedimentary column in the fan by the seaward moving salt and rafting sedimentary packages of the salt province further east.
... We define paleo-pockmarks as pockmarks that have been buried and preserved within the subsurface rock record. Though difficult to observe due to decreasing spatial resolution in seismic data with increasing depth, the identification and study of paleo-pockmarks may provide insights into ancient fluid migration and expulsion events (Andresen et al., 2008;Ho et al., 2012). ...
... The observation of amplitude anomalies suggestive of hydrocarbon accumulations, above a large number of the buried pockmarks indicates that these features, and their underlying feeder pipes, have influenced subsequent fluid migration. Studies by Ho et al. (2012) and similarly document examples in offshore Angola where fluid escape pipes and paleo-pockmarks were reutilized as fluid migration conduits by thermogenic hydrocarbons and methane subsequent to their formation. While the origin of this phase of fluid migration is unclear, it is interpreted to be due to further maturation of Triassic source rocks, a result of continuous burial after the paleopockmarks and fluid escape pies had formed. ...
This study records 315 paleo-pockmarks with associated focused fluid flow pipes within the Jurassic and Triassic sediments over three study areas on the Exmouth Plateau, offshore Northwest Australia. Paleo-pockmarks are identified along a surface that represents the top of Jurassic sediments, while the fluid flow pipes extend into Triassic sediments from the base of these pockmarks. The pockmarks and pipes form in linear trends that are parallel to and laterally offset from the tops of extensional faults intersecting an interval from the top of Jurassic sediments into Triassic sediments, where they are seen to terminate. Bases of the fluid flow pipes are observed to intersect and terminate along these extensional faults within the Triassic sediments. The pockmarks and associated fluid flow pipes are interpreted to have formed when extensional faults developed that intersected an overpressured unit within Triassic sediments. This caused a localized reduction of lithostatic pressure along the overpressured sequence at the intersection which then acted as a focal point for fluid escape and vertical migration. The source of the fluid overpressure could not be confirmed in this study. The Triassic sequence is a known hydrocarbon source and 1D modelling shows that at the time of fluid flow and pockmark formation, these Triassic sediments were entering the hydrocarbon generation window. However, no evidence of hydrocarbons associated with the pockmarks has been observed. Our findings identify fluid migration pathways that are seal risks for hydrocarbon reservoirs, but could also potentially be fluid migration pathways that were previously untested.
... More recently, several studies have highlighted that hydrocarbons can migrate vertically across even deeply buried lowpermeability deposits by exploiting seal bypass systems such as faults, pipes, and intrusions Andresen and Huuse, 2011;Moss et al., 2012;Ostanin et al., 2013;Bertoni and Cartwright, 2015). Evidence of vertical and focused fluid flow through sealing sequences using these cross-stratal pathways has accumulated in the past decade largely through the greater resolution and imaging quality of modern three-dimensional (3-D) seismic surveys (Mazzini et al., 2003;Van Rensbergen et al., 2003;Berndt, 2005;Cartwright et al., 2007;Hustoft et al., 2009;Ho et al., 2012). Seismic data can be interpreted to reconstruct timing and pathways of migrating fluid in a basin (Andresen, 2012;Ho et al., 2012). ...
... Evidence of vertical and focused fluid flow through sealing sequences using these cross-stratal pathways has accumulated in the past decade largely through the greater resolution and imaging quality of modern three-dimensional (3-D) seismic surveys (Mazzini et al., 2003;Van Rensbergen et al., 2003;Berndt, 2005;Cartwright et al., 2007;Hustoft et al., 2009;Ho et al., 2012). Seismic data can be interpreted to reconstruct timing and pathways of migrating fluid in a basin (Andresen, 2012;Ho et al., 2012). However, the onset, duration, and fluxes of the leakage elements in these particular systems remain poorly constrained. ...
We describe a spectrum of seismic high-amplitude anomalies from the Great South and Canterbury Basins (offshore New Zealand) that have a number of characteristics that make them distinct from previously described hydrocarbon-related amplitude anomalies. We propose a new classification scheme that is based on the specific vertical stacking of the anomalies, and thereby reflects their genetic inter-relationships.
We demonstrate by combining AVO (Amplitude Versus Offset) and other attribute analyses that the anomalies are the product of gas migration across thick sequences of low permeability sediments and identify specific units where migration is focused through discontinuities (e.g. faults), and storage units where the gas spreads laterally. Based on our observations we argue that fluid flow phenomena in which Darcy flow must have occurred can be hosted within low permeability layers that would normally be regarded as high quality sealing sequences.
The documentation of a wide range of gas-related anomalies developed in a relatively uniform lithostratigraphy and in similar basinal contexts allows us to infer a migration sequence based on the morpho-structural and geophysical characteristics of the anomalies. We suggest that the shape of the composite anomalies is directly controlled by gas flux and by probably quite subtle variations in physical properties of the host sediments. The distribution of the anomalies is generally correlated with maximum burial regions of the most prospective source rock intervals that are currently in the gas maturation window. This spatial coincidence strongly suggests that the anomalies result from vertical migration of thermogenic gas from these underlying source kitchens.
... In addition to fractures, faults and magmatic activity, any sufficiently strong overpressure in the source layers can promote hydraulic fracturing (Andresen, 2012;Haacke et al., 2007;Ho et al., 2012). Given the complex geological background and the combination of multiple elements on the Shenhu slope, only two aspects have been acknowledged so far. ...
Gas chimneys are key pathways for geofluid vertical migration; therefore, deciphering their formation and evolution is crucial for hydrocarbon exploration and geohazard risk assessment. However, the influences of ambient conditions (e.g. bathymetry, tectonics, sediment supply flux) on gas chimney development have not been thoroughly investigated.
Using high-resolution 3D seismic data, we have identified 59 gas chimneys beneath the Shenhu Slope (a gas hydrate test production area on the northern South China Sea margin), 35 of which intersect faults. Above fault interfaces, internal seismic structures are dominated by chaotic discontinuous reflections. Internal structures below interfaces display more continuous reflections, which are also apparent in gas chimneys not intersecting faults. A higher degree of chaotic or discontinuous seismic reflections may indicate more fragmented networks. This may occur due to increased fluid flow along faults and concomitant fluid overpressure.
The present-day undulating seafloor comprises the inter-canyon (IT) region, intra-canyon (IN) region, and flat slope (FD) region downstream of canyons. Gas chimneys beneath IT and IN regions exhibit elongated elliptical shapes in the plane, with the long axis azimuth (sub-) parallel to the main strike of canyons. Chimneys beneath the IT region have larger heights than those beneath the IN and FD regions. Thicker sediment in the IT region corresponds to a higher overburden pressure, which may induce stronger overpressure in the subsurface reservoir region. This overpressure may promote chimneys gathering in the IT region. Canyons’ main directions are likely to limit hydraulic fracturing due to maximum gradient boundaries between overlying sediment stress fields.
This study provides insights into gas chimney distribution, morphology and structure evolution in relation to bathymetry and fault conditions. It contributes to an improved understanding of how geofluids migrate in marginal ocean basins.
... Pockmarks are generally smaller, crater-like depressions that document fluid escape on the seafloor from shallower subsurface depths (King and Maclean, 1970;Hovland and Judd, 1988;Judd and Hovland, 2007;Riboulot et al., 2011;Sultan et al., 2014;Bayon et al., 2015;Wei et al., 2015;de Prunelé et al., 2017;Marsset et al., 2018). On seismic imagery, pockmarks are commonly underlain by pipes, seal bypass systems (Cartwright et al., 2007), recognised as columnar zones of disrupted reflections (Heggland and Nyggard, 1998;Løseth et al., 2011;Hustoft et al., 2007;Moss and Cartwright, 2010;Ho et al., 2012;2018). Pipes display circular to elliptical geometries, with amplitude anomalies that may be due to the presence of either gas pockets (negative amplitude anomalies, NAAs), or carbonate cement (positive amplitude anomalies, PAAs) (Løseth et al., 2001;Hustoft et al., 2007;Gay et al., 2007;Sultan et al., 2014;Bayon et al., 2015;Wei et al., 2015;de Prunelé et al., 2017;Marsset et al., 2018). ...
The offshore Niger Delta provides a spectacular example of gravity collapse tectonics, but the timing of shale mobilisation remains poorly understood. Here we present new information from the western Niger Delta, based on a detailed interpretation of a 3D seismic volume, calibrated with biostratigraphic data from exploration wells. The study area is underlain by mobile shales containing thrust-fold anticlines, overlain by >5 km thick Late Eocene to present succession of folded and faulted sediments with fluid migration features. Sedimentation rates estimated at one deep well increased during two phases, more than doubling during the Late Eocene to Serravallian (39.5–12.5 Ma), and by up to ten times during the Tortonian (9.5 Ma) to present. Thinning and onlapping geometries within the lowermost, Late Eocene-Burdigalian (39.5–18.5 Ma) interval are interpreted to record syn-depositional deformation. This indicates that shale tectonics in the offshore western Niger Delta initiated early in the evolution of the region, supporting previous interpretations that the active compressional zone of the Niger Delta was in the present continental slope before prograding to the outer-fold-thrust belt in the Pliocene. Stratal thinning above the crests of thrust-fold anticlines in the northeast and southwest of the study area suggests that shale tectonics persisted throughout the Neogene and Quaternary. A chaotic column that rises to seafloor from the crest of thrust-fold anticline, interfingering with the stratified succession is interpreted as a mud volcano edifice that has been active since at least the Burdigalian. This supports early shale mobilisation in response to generation of overpressure and also the fact that mud volcano system once formed, may act as conduits for pressure release throughout the history of a gravity-driven collapse system. The spatial association of seafloor mud volcanoes and pockmarks with deeper thrust-fold anticlines and normal faults suggests fluid migration from depth. In contrast, pockmarks overlying mass-transport deposits and submarine channels suggest fluid-flow from dewatering of more recent sediments. Our findings provide new insights into the Cenozoic tectono-stratigraphic evolution of the offshore western Niger Delta and the relation of shale mobility to fluid migration and escape during continental margin collapse.
... Other processes contributing to pockmark formation include bottom current scour Picard et al., 2018), permafrost thawing (Woo, 2012), ice rafting (Paull et al., 1999), carbonate dissolution (Betzler et al., 2011), salt tectonics (Michaelovitch de Mahiques et al., 2017;Serié et al., 2017) and seismic activity (Hasiotis et al., 1996). Pockmark morphology is controlled by the seafloor sediment type and thickness (Chand et al., 2009), fluid flux and concentration (Ho et al., 2012) and bottom currents velocity and direction (Bøe et al., 1998). ...
Shallow seabed depressions attributed to focused fluid seepage, known as pockmarks, have been documented in all continental margins. In this study we demonstrate how pockmark formation can be the result of a combination of multiple factors – fluid type, overpressures, seafloor sediment type, stratigraphy, and bottom currents. We integrate multibeam echosounder and seismic reflection data, sediment cores and pore water samples, with numerical models of groundwater and gas hydrates, from the Canterbury Margin (off New Zealand). More than 6800 surface pockmarks, reaching densities of 100 per km2, and an undefined number of buried pockmarks, are identified in the middle to outer shelf and lower continental slope. Fluid conduits across the shelf and slope include shallow to deep chimneys/pipes. Methane with a biogenic and/or thermogenic origin is the main fluid forming flow and escape features, although saline and freshened groundwaters may also be seeping across the slope. The main drivers of fluid flow and seepage are overpressure across the slope generated by sediment loading and thin sediment overburden above the overpressured interval in the outer shelf. Other processes (e.g. methane generation and flow, a reduction in hydrostatic pressure due to sea‐level lowering) may also account for fluid flow and seepage features, particularly across the shelf. Pockmark occurrence coincides with muddy sediments at the seafloor, whereas their planform is elongated by bottom currents.
... Methane-derived authigenic carbonates form close to the seabed through the production of cold seep communities Judd and Hovland, 2007). As the biogenic methane escapes through the pockmarks, it favours the development of chemosynthetic communities, which could result in the hard anomalies (Netzeband et al., 2005;Bayon et al., 2007;Betzler et al., 2011;Ho et al., 2012). Therefore, we suggest that the hard anomalies within the pockmarks could represent methane-derived authigenic carbonates. ...
... Methane-derived authigenic carbonates form close to the seabed through the production of cold seep communities Judd and Hovland, 2007). As the biogenic methane escapes through the pockmarks, it favours the development of chemosynthetic communities, which could result in the hard anomalies (Netzeband et al., 2005;Bayon et al., 2007;Betzler et al., 2011;Ho et al., 2012). Therefore, we suggest that the hard anomalies within the pockmarks could represent methane-derived authigenic carbonates. ...
Pockmarks are morphological expressions of fluid escape along continental margins. Identifying the underlying controls on their formation and spatial distribution is crucial for understanding the substrate fluid plumbing systems and has important implications for hydrocarbon exploration, seafloor stability and global warming. Here, we use 3D seismic reflection dataset and a machine learning approach to present the first evidence for paleo-pockmarks in the Bass Strait, southeast Australia. The paleo-pockmarks are identified in the Pelican Trough of the Bass Basin, within an interval between 130 and 300 m below present-day seafloor, corresponding to the Miocene carbonate-dominated Torquay Group. The paleo-pockmarks have depths ranging from 15 to 74 m and areas between 0.006 to 0.8 km 2 , with diameters varying between 0.1 and 1.1 km. The absence of an underlying seal-bypass system such as pipes and faults associated with these paleo-pockmarks discounts a deeper thermogenic source or a potential magmatic-driven fluid system. Rather a biogenic fluid system derived from the degradation of organic-rich layers is hypothesised to drive the paleo-pockmark formation. The seismic interval comprising the paleo-pockmarks demonstrates a distinctive seaward progradation and stepping-down configuration, indicating a forced regression. We propose this resulted in destabilisation of hydrostatic pressure triggering the creation of the paleo-pockmarks. The findings from this study shed light on pockmark formation mechanisms in shallow water sedimentary systems.
... Seep carbonate systems are commonly observed in seismic data as vertical or subvertical stacks of lenticular anomalies, which document that seep sites can be long-lived features (Kauffman et al., 1996;Hovland and Judd, 1988;Plaza-Faverola et al., 2011). In particular, the morphology and vertical variation of amplitude anomalies are useful to qualitatively reconstruct the history of fluid leakage intensity (Ho et al., 2012). ...
The mechanisms that govern the vertical growth of seep carbonates were deciphered by studying the sedimentary architecture of a 15 m thick, 8 m wide column of limestone encased in deep-water marl in the middle Callovian interval of the Terres Noires Formation in the SE France Basin. The limestone body, also called “pseudobioherm”, records intense bioturbation, with predominant traces of the Thalassinoides/Spongeliomorpha suite, excavated by decapod crustaceans. Bioturbation was organized in four tiers. The uppermost tier, tier 1, corresponds to shallow homogenization of rather soft sediment. Tier 2 corresponds to pervasive burrows dominated by large Thalassinoides that were later passively filled by pellets. Both homogenized micrite and burrow-filling pellets are depleted in 13C in the range from −5 ‰ to −10 ‰. Tier 3 is characterized by small Thalassinoides that have walls locally bored by Trypanites; the latter represent tier 4. The diagenetic cements filling the tier-3 Thalassinoides are arranged in two phases. The first cement generation constitutes a continuous rim that coats the burrow wall and has consistent δ13C values of approximately −8 ‰ to −12 ‰, indicative of bicarbonate originating from the anaerobic oxidation of methane. In contrast, the second cement generation is dominated by saddle dolomite precipitated at temperatures >80 ∘C, at a time when the pseudobioherm was deeply buried. The fact that the tubes remained open until deep burial means that vertical fluid communication was possible over the whole vertical extent of the pseudobioherm up to the seafloor during its active development. Therefore, vertical growth was fostered by this open burrow network, providing a high density of localized conduits through the zone of carbonate precipitation, in particular across the sulfate–methane transition zone. Burrows prevented self-sealing from blocking upward methane migration and laterally deflecting fluid flow. One key aspect is the geometric complexity of the burrows with numerous subhorizontal segments that could trap sediment shed from above and, hence, prevent their passive fill.
... These depressions tend to be circular and measure a few to hundreds of meters in diameter and between 1 m and 20 m in depth (Judd and Hovland, 2007). Gas seepage influences the seafloor topography by creating pockmarks and sea mounds, which are seafloor depression features on seabed caused by sediment removal and dome-shaped features caused by precipitation of seep carbonates or methane-derived authigenic carbonates (Davis, 1992;Judd and Hovland, 1992;Hovland et al., 2002;Ho et al., 2012;Blouet et al., 2017). Another class of acoustic anomalies associated with shallow gas on seismic data is 'bright spots' characterized by high amplitude reverse polarity reflectors (i.e., Sheriff and Geldart, 1995). ...
High resolution multi-channel seismic reflection data (∼1000 km) and multibeam echosounder bathymetry from the southeastern Korean continental shelf of the East Sea (Japan Sea) reveal numerous shallow gas indicators and seepage-related features, such as bright spots, enhanced reflections, seismic chimneys, acoustic blanking, pockmarks, and bathymetric mounds. Bright spots, indicating gas-charged layers, appear as local negative-polarity reflection anomalies (up to 5 km wide) and occur at various stratigraphic levels within a subsurface depth of ∼320 m. Bright spots covering an area of ∼60 km² are clustered at the tip of NE-SW–trending reverse faults in the northeastern and southeastern part of the investigated region, suggesting gas entrapment. Enhanced reflections (ca. 20-km-long) are developed along erosional unconformities and tilted sedimentary layers below them. This suggests that unconformities formed during sea-level low stands in the study area are potential reservoirs and may have acted as potential conduits for lateral migration of gas-rich fluids due to their permeable nature. Some enhanced reflections are formed along interfluves of channels where channel walls cut them, and thus they may potentially act as fluid reservoirs. Seismic chimneys, expressed as vertical disturbances in seismic data, are interpreted as the upward movement of fluids (i.e., either in liquid or gaseous form). Lack of faulting in some seismic chimneys suggests higher permeability in the sedimentary interval, which would allow the migration of deeper-sourced fluids. Pockmarks (up to 500 m in diameter) are typically associated with seismic chimneys in the sub-seabed, suggesting that they were formed by the explosive emission of gas or gas fluids. Some exhibit mound-like features near their crests that are interpreted as carbonate mounds. The locations of mounds above uplifted fault blocks in the central part suggest a structural control on the formation of these seabed features.
... In such cases, high rates of AOM strongly foster carbonate precipitation by the production of alkalinity and consumption of protons (Blouet et al., 2021a). Such authigenic carbonates are commonly referred to as methane-derived authigenic carbonates (MDAC; Ritger et al., 1987;Hovland et al., 1987;Ho et al., 2012), or seep carbonates (Hecker, 1985;Juniper and Sibuet, 1987). They are often associated with highly specialised chemosymbiotic animals, such as worms and clams (Kiel, 2010;Fischer et al., 2012). ...
... Unlike the fault and source rock controlled fluid flow feature distribution in other passive margins, such as the Barents Sea offshore Norway (Vadakkepuliyambatta et al. 2013) and the Lower Congo Basin (Ho et al. 2012), where erosion does not affect their distribution much, the development of the gas chimneys in the SCS Slope is likely controlled by the sedimentary history and magnitude of gas accumulation. For instance, seafloor erosion removes the mass of overburden. ...
We analyzed original and published seismic data to investigate the distribution and characteristics of subsurface fluid flow features in the gas hydrate provinces along the northeastern South China Sea (SCS) continental slope, including Jiulong Ridge, Horseshoe Ridge, Pointer Ridge, and Formosa Ridge. Numerous features indicating the presence/migration of hydrocarbons, such as bottom simulating reflections (BSRs), bright spots, gas chimneys, and fluid flow through faults, are identified. The results reveal that the hydrofracturing-induced gas chimneys act as the primary conduits for the overall focused fluid flow in the study area, though fluid flow along permeable faults is also observed at Pointer Ridge. Thirty-three gas chimneys identified are categorized into two types depending on their capability for focused fluid flow migrating into the gas hydrate stability zone (GHSZ). The type-I chimneys that transport fluids into the GHSZ contribute to gas hydrate formation and may even lead to seafloor seepage. Buried by a considerable thickness of sediments, the type-II chimneys cannot directly contribute to generating gas hydrate and surface seepage. Our results suggest that the sedimentary processes and fluid accumulation significantly control the development of gas chimneys in the study area. Since the focused fluid flow conduits that indicate overpressured fluids are critical pathways that feed gas into the GHSZ and form gas hydrates, utmost attention should be paid to them during hydrocarbon exploration.
... Trace fossils are common in seep-impacted sediments, where background (non-seep obligate) taxa such as shrimps and crustaceans can graze on chemosynthetic microbial mats and sediment Vanreusel et al., 2009;Cordes et al., 2010;Niemann et al., 2013;Vrijenhoek, 2013;Grupe et al., 2015;Karanovic and Brandão, 2015;Levin et al., 2015Levin et al., , 2016. In the geological record, however, studies on trace fossils associated with seep-impacted deposits are scarce compared to the more abundant literature on fluid expulsion structures (pockmarks, craters, mud volcanoes) (De Boever et al., 2006;Andresen et al., 2008;Krause et al., 2009;Ho et al., 2012;Agirrezabala et al., 2013;Reitner et al., 2015;Feng et al., 2018). In particular, the activity of crustacean is underestimated (Klompmaker et al., 2018) and only a few papers describe crustacean traces and burrows as structures favoring methanerich fluid expulsion at fossil cold seeps (Bishop and Williams, 2000;Peckmann et al., 2007;Cunningham and Sukop, 2012;Wetzel, 2013). ...
The Marnoso-arenacea turbidite succession outcropping in northern Italy is one of the most studied deep-water systems worldwide due to its excellent exposure. Despite the well-established sedimentology of the arenaceous succession, the origin of fine-grained intervals within the Marnoso-arenacea Fm is reconsidered in the light of methane-rich fluid expulsion, in particular as seep-carbonates are present. Fine-grained intervals are marked by limited vertical and lateral extent, in the order of 10² m and 10³ m respectively, and are often associated with small-scale sediment deformation. In this study, we conduct a detailed sedimentological and stratigraphic investigation of the well-exposed Prati Piani fine-grained interval (Serravallian in age) with particular focus on the characterization of trace fossils and seep-carbonates. Trace fossils interpreted as Thalassinoides are widespread throughout the interval and form a complex network testifying long-lasting burrowing activity of decapod crustaceans. Seep-carbonates crop out at different stratigraphic levels and form meter-sized blocks with δ¹³C from -33.2‰ to -18.2‰. The occurrence of aragonite cements and fossil chemosymbiotic fauna indicates episodic carbonate precipitation close to the paleo-seafloor. Although crustaceans have been frequently reported grazing microbial mats at modern seeps and leaving a ¹³C-depleted isotopic signature in trace fossils, in our study we found only one slightly depleted δ¹³C value (-9.5‰,) allowing us to exclude preferential authigenesis; moreover burrows are filled by detrital fine-grained sediments and hemipelagic micrite that may have diluted the methane related signature. The association between in situ seep-carbonates and well-preserved trace fossils are of use to interpret the interval as formed by fine-grained sedimentation draping a thrust-related anticline. In this model, the stratigraphically- and laterally-confined deformed zones affecting the Prati Piani interval reflect in-situ soft-sediment deformation and small-scale slumping induced by fluid seepage along the ridge, consistent with present-day observations from several continental margins worldwide.
... In such cases, high rates of AOM strongly foster carbonate precipitation by the production of alkalinity and consumption of protons (Blouet et al., 2021a). Such authigenic carbonates are commonly referred to as methane-derived authigenic carbonates (MDAC; Ritger et al., 1987;Hovland et al., 1987;Ho et al., 2012), or seep carbonates (Hecker, 1985;Juniper and Sibuet, 1987). They are often associated with highly specialised chemosymbiotic animals, such as worms and clams (Kiel, 2010;Fischer et al., 2012). ...
At a Pliocene methane seep site in Taiwan, an about half meter-long dolomitic concretion hosting a lucinid bivalve Anodontia goliath, ∼11 cm in diameter reveals the impact of chemosymbiotic bivalves on fluid migration through the shallow sediment. The concretion consists of a central channel filled with sparite and varying amounts of brecciated material derived from the channel walls. The studied bivalve is preserved with the hinge upward. When restored to life position, the central channel of the upper, ∼10 cm-long concretion segment is connected to the shell above the anterior side of the umbos. For the lower, ∼30 cm-long concretion segment, the central channel is connected to the shell underneath the posterior sector of the bivalve. The channels are interpreted as anterior tube for supply of respiration water and posterior tube, mining sulphide generated in the sulphate-methane transition zone of the sediment, respectively. The dolomitic cortex surrounding the central channel is characterized by δ¹³C values as low as -29‰ VPDB, indicating that the rate of anaerobic oxidation of methane was enhanced around the conduit. At the base of the central channel, upward soft sediment deformation structures likely resulted from the dragging effect when fluids migrated out of the sediment and entered the channel. Brecciation of the channel walls indicates pulsating seepage during the incipient stages of cementation. The burrow, thus, acted as a highly permeable conduit channeling upstreaming methane-charged fluid for a substantial period of time. This example illustrates that burrows are efficient in pre-determining the formation of preferential fluid migration conduits at seep sites, most likely with an important impact on fluid mixing and diagenetic reaction rates in the sediment, and ultimately the rate of methane release to the ocean.
... According to the chronostratigraphic study of Clift et al. (2015), the studied canyons mainly developed in the late Pliocene to Quaternary in response to the late tectonic phase mentioned above (Steinke et al., 2003;Li et al., 2018). Previous work has revealed that turbidity flows and debris flows formed deposits in the canyons on the deepwater slope (Anderson et al., 2000;Ho et al., 2012;Gong et al., 2013) and that contourite currents modified the original deposits and contributed to the migration of the canyons (Chen, 2005;Lüdmann et al., 2005;Tian et al., 2009;Gong et al., 2013). There is also vertical flow due to gas hydrate leakage near the canyon walls, especially in the canyon area in the Shenhu area (Sun et al., 2012). ...
In the deep-sea slope areas, canyons provide an ideal space to preserve sediments which provide a window to explore the deep-water deposition process, such as the turbidity flow and contourite currents. In this study, we present results of the study of the northern continental slope of the South China Sea characterized by the presence of mostly straight canyons. After describing core samples and interpreting the corresponding seismic data, we static the sedimentary parameter and identified two types of canyons with different migrating trajectories: “convex downward curve-shaped” trajectories and “convex upward curve-shaped” trajectories. The convex downward curve-shaped canyon trajectory is distinguished by a lower layer of coarse-grained sediment and an upper layer of fine-grained sediment, whereas the convex upward curve-shaped canyon trajectory features a lower layer of fine-grained sediment and an upper layer of coarse-grained sediment. Combining the grain size of the core sample and the scale of the sedimentary structure, we restore the turbidity flow rates and the corresponding turbidity flow behaviors. Coarse-grained turbidity flows are characterized by lower vertical erosion rates and higher lateral abrasion rates, while fine-grained turbidity flows exhibit the opposite characteristics. Thus, the convex downward curve-shaped migration trajectory is mainly formed by coarse-grained turbidity flow erosion in the first stage (late migration stage) and fine-grained turbidity flow deposition in the second stage (vertical aggradation stage). In contrast, the convex upward curve-shaped trajectory forms through the opposite pattern of sedimentary evolution.
... Also called "bright spots" in these studies, these peculiar seismic reflections usually show a limited lateral extent but a long vertical extent, alternately intercalated with high and weak amplitudes (Zeng et al. 2011a, b;Tian et al. 2016;Lu et al. 2017;Basso et al. 2018;Yang et al. 2018). In the karst-related interval, bright spots often appear because of the dramatic difference between the low velocity and low density in a karst interval, and the high velocity and high density in the host units, which produces a high impedance anomaly that is recorded in the seismic data (Zeng et al. 2011a, b;Ho et al. 2012;Tian et al. 2016;Yu et al. 2016;Basso et al. 2018). While the geologic origins of bright spots associated with a karst reservoir are complicated, it can be concluded that are products of syngenetic-penecontemporaneous karstification , weathering unconformity-related karstification (Zeng et al. 2011a, b;Gao et al. 2018), fault-related karstification (Tian et al. 2015(Tian et al. , 2016Yu et al. 2016;Lu et al. 2017), or volcanic hydrothermal-related karstification . ...
Bright spot seismic reflections in carbonate strata are often correlated with effective reservoirs. Widely distributed bright spot reflections have been found in the Permian Changhsing Formation in the Longgang area, Northeast Sichuan Basin, China. However, there is still a lack of systematic understanding of the types, distribution, and geological origin of these bright spot reflections in the Longgang area. Integrated subsurface analysis has revealed that bright sport characteristics represent a combination of strong troughs and peaks, with thicknesses up to 30 ms of two-way time and have a shape like "a string of pearls" in the seismic section. The bright spots located in the platform margin show a NW-trending intermittent strip-like distribution, while in the open platform they are scattered. Bright spots are mainly distributed at the top of the Changhsing Formation and form in the platform margin and open platform environments. The distribution of bright spots has a good coupling with reefs and shoals located in the high paleogeographic regions of the Upper Changhsing Formation. The integrated analysis based on cores, thin sections, and geophysical data reveals that the "pearl-like" bright spots are indicators of needle-like dissolution pores, small karst caves, fracture-vug cystic caves and piebald karst fracture cave systems resulting from eogenetic karstification. Large-scale regression of sea level, which occurred at the end-Permian section, is the main control for reefs and shoals located in the exposed paleohighs suffering from eogenetic karstification. Exploration based on the location of bright spot seismic reflections may be a favorable target for hydrocarbons.
... When the gas reaching the seafloor escapes into the seawater, via seepages, it forms gas bubbles, so that the amplitude of seawater containing such fluid can be distinguished from the surrounding seawater and the corresponding anomalies in the water column are known as gas flares (Judd and Hovland, 1992;Naudts et al., 2009). Gas seepage influences the topography of the seafloor by creating seafloor depressions (pockmarks) and seabed domes, caused by sediment removal and by precipitation of seep carbonates or methane-derived authigenic carbonates that overall develop from a microbially-mediated reaction between seawater sulfate and methane (Blouet et al., 2017;Davis, 1992;Ho et al., 2012;Hovland et al., 2002;Judd and Hovland, 1992). ...
This study investigates the evidence of shallow gas from a newly collected dataset comprising 2D multi-channel seismic (MCS), single-channel seismic (SCS), Chirp sub-bottom profiler (SBP), and multi-beam echo sounder (MBES) data from the southwestern continental shelf of the Ulleung Basin, East Sea, Korea. Various indicators of shallow gas were identified in this part the shelf, including seismic chimneys, acoustic flares in the water column, pockmarks, enhanced reflections, and acoustic blanking. Seismic chimneys, which are related to fluid leakage within the subsurface, are characterized weak to high amplitude, upturned or concave-upward internal reflection with polarity reversal in MCS section, whereas they are faint or transparent on SCS section with 700 Hz and Chirp SBP sections with 3.5 kHz frequency. Acoustic flares, which are diagnostics of active gas venting, were detected only on the Chirp SBP sections. There is also evidence of small-scale depressions, immediately below the acoustic flares; these are interpreted as formed by the sudden explosion of gas. Enhanced reflections (∼3 km-long) concordant with stratification are observed at different levels. Acoustic blanking anomalies, caused by absorption of acoustic energy due to the presence of gas, are seen in the range 50–200 ms TWT below seabed. Amplitude versus offset (AVO) analysis indicates the presence of free gas ca. 350 ms TWT below sea surface using the MCS dataset recorded by a 600 m-long streamer cable. The AVO cross-section shows that the lower repetitive signals in the MCS data are due to the gas plume, not lithological contrast. Our study highlights that AVO analysis is an extremely useful tool for identifying free gas, helping to discriminate water-gas contacts and bright events among the chaotic signals on the MCS data.
... Seeping fluids frequently result in authigenic carbonate precipitation due to the microbially driven anaerobic oxidation of methane once methane enters the SMTZ (e.g., Boetius et al., 2000;Ritger et al., 1987). These carbonates are often associated with high-amplitude, positive polarity reflections and a high backscatter signal in pockmarks (Böttner et al., 2019;Dandapath et al., 2010;Ho et al., 2012;Judd & Hovland, 2009). Similarly, the suspension of fine-grained material due to fluid escape, leaving the coarser-grained material behind as well as enhancing biological activity due to seeping fluids (skeleton remains, dead and living shells, etc.), can create a strong impedance contrast and a rougher surface area and therefore results in strong backscatter values in pockmarks (Hovland, 1989;Hovland et al., 2002;Reusch et al., 2015). ...
Submarine groundwater discharge (SGD) into coastal areas is a common global phenomenon and is rapidly gaining scientific interest due to its influence on marine ecology, the coastal sedimentary environment and its potential as a future freshwater resource. We conducted an integrated study of hydroacoustic surveys combined with geochemical porewater and water column investigations at a well-known groundwater seep site in Eckernförde Bay (Germany). We aim to better constrain the effects of shallow gas and SGD on high frequency multibeam backscatter data and to present acoustic indications for submarine groundwater discharge. Our high-quality hydroacoustic data reveal hitherto unknown internal structures within the pockmarks in Eckernförde Bay. Using precisely positioned sediment core samples, our hydroacoustic-geochemical approach can differentiate intra-pockmark regimes that were formerly assigned to pockmarks of a different nature. We demonstrate that high-frequency multibeam data, in particular the backscatter signals, can be used to detect shallow free gas in areas of enhanced groundwater advection in muddy sediments. Intriguingly, our data reveal relatively small (typically <15 m across) pockmarks within the much larger, previously mapped, pockmarks. The small pockmarks, which we refer to as “intra-pockmarks”, have formed due to the localized ascent of gas and groundwater; they manifest themselves as a new type of ‘eyed’ pockmarks, revealed by their acoustic backscatter pattern. Our data suggest that, in organic-rich muddy sediments, morphological lows combined with a strong multibeam backscatter signal can be indicative of free shallow gas and subsequent advective groundwater flow.
... This distribution suggests a fluid-migration process during the release of overpressure from heated organic carbon around sill intrusions (Berndt et al., 2000;Svensen et al., 2004;Planke et al., 2005;Aarnes et al., 2015;Kjoberg et al., 2017). The second type of fluid migration comprises positive high-amplitude anomalies (PHAAs) and (sub) circular depressions (pockmarks), both of which indicate relatively slow fluid seeps (Fig. 13); Ho et al., 2012). It can be observed that most of these subcircular depressions are located where faults are linked (Fig. 13B). ...
Submarine landslides have affected the mid-Norwegian margin since the last glacial maximum. However, the role of tectonic movements, and most especially fault reactivation, in generating landslides offshore Norway is largely unconstrained. This study uses high-quality 3D seismic and borehole data to understand how landslide development was partly controlled by faults propagating within the uplifted South Modgunn Arch. Variance and structural maps above the South Modgunn Arch show that: a) local scarps of recurrent landslides were formed close to the largest faults, and mainly above strike-slip faults; b) distinct periods of fault generation were associated with tectonic events, including the breakup of the Northeast Atlantic Ocean, and those events forming the South Modgunn Arch; c) important fluid-flow features coincide with faults and sill intrusions. A total of 177 faults were analyzed to demonstrate that fault throw-values vary from 10 ms to 115 ms two-way travel time (8 m to 92 m). We propose that the long-term activity of faults in the study area has contributed to fluid migration, weakened post-breakup strata and controlled the development of submarine slope instability. In particular, strike-slip faults coincide with the location of several Quaternary landslide scars near the modern sea floor. Similar processes to those documented in Norway may explain the onset of large-scale landslides on continental margins.
... These amplitude anomalies are similar in size and acoustic character to hydrocarbon-related diagenetic zones that have been interpreted above many leaky hydrocarbon fields in the NW Shelf 265 petroleum province(Cowley and O'Brien, 2000). They are also very similar in geometry and acoustic 266 character to methanogenic carbonate cemented layers within vertical gas migration zones(Ho et al. 2012). 267 ...
We present a seismic and well-based interpretation of a large ‘leakage zone’ above the Scarborough Gas Field, Exmouth Plateau, NW shelf of Australia. This leakage zone, well imaged on 3-D seismic, extends over a region of 100 km2 (38.6 mi2) encompassing both the crest and flanks of the anticlinal trap, and is termed here as Distributed Crestal Leakage. The present-day gas-water contact is 85 m (278 ft), and the spill point is 110 m (328 ft) below the crest, implying that the trap is underfilled at present. The leakage zone comprises over 500 pockmarks at the present-day seabed with no crosscutting or cannibalization, suggesting that they formed in a short interval of time. These are underlain by sediment remobilization features and amplitude anomalies, consistent with a relatively high flux leakage of gas from the underlying Cretaceous deep-water sand-rich reservoir. By analyzing the geometrical relationship between the leakage zone, the top seal properties, and the gas-water contact, we conclude that the mode of leakage in this specific setting is not the result of gradual addition of gas charge but is instead consistent with a sudden increase of aquifer overpressure. We suggest two alternative models for seal failure in this case study: a conservative model consistent with a modest but rapid increase in aquifer overpressure leading to membrane seal failure, and a model dominated by high aquifer overpressure leading to leakage through hydraulically dilated faults and fractures.
... Shallow gas hydrates are generally associated with authigenic 13 C-depleted carbonates [14,15]. Gas hydrate-associated carbonates, called clathrites [16], have been sampled from present-day seafloor or a few meters below [17][18][19][20][21][22][23]. These carbonates may form bodies of remarkable dimension and show peculiar structures such as vacuolar or vuggy-like fabrics, association of pure aragonitic and gas hydrate layers (zebra-like structures) and breccias produced by the destabilization of gas hydrates [18,[24][25][26]. ...
The occurrence of seep-carbonates associated with shallow gas hydrates is increasingly documented in modern continental margins but in fossil sediments the recognition of gas hydrates is still challenging for the lack of unequivocal proxies. Here, we combined multiple field and geochemical indicators for paleo-gas hydrate occurrence based on present-day analogues to investigate fossil seeps located in the northern Apennines. We recognized clathrite-like structures such as thin-layered, spongy and vuggy textures and microbreccias. Non-gravitational cementation fabrics and pinch-out terminations in cavities within the seep-carbonate deposits are ascribed to irregularly oriented dissociation of gas hydrates. Additional evidences for paleo-gas hydrates are provided by the large dimensions of seep-carbonate masses and by the association with sedimentary instability in the host sediments. We report heavy oxygen isotopic values in the examined seep-carbonates up to +6 that are indicative of a contribution of isotopically heavier fluids released by gas hydrate decomposition. The calculation of the stability field of methane hydrates for the northern Apennine wedge-foredeep system during the Miocene indicated the potential occurrence of shallow gas hydrates in the upper few tens of meters of sedimentary column.
... It has been noted that modern analogues reveal only partial stages of seepage activity (e.g. Plaza-Faverola et al., 2011;Crutchley et al., 2015;Klaucke et al., 2015;Mohammedyasin et al., 2016), while the identification of buried authigenic carbonates may be difficult on seismic data due to resolution limits (Andresen, 2012;Ho et al., 2012). On the other hand, seismic and seafloor data from modern analogues allow observations at a basinwide scale and the recognition of fluid migration pathways that are rarely identified in the geological record (Nyman et al., 2010;Nelson et al., 2017). ...
We present new field data from three outcrops of Miocene methane-derived authigenic carbonates in the foredeep of the northern Apennines that contain chemosynthetic fauna and record a long history (∼1 Ma) of shallow fluid seepage linked to seafloor anaerobic oxidation of methane. The studied outcrops show similar features in terms of carbonate morphology, facies, spatial distribution and lateral and vertical contacts with the enclosing sediments. Methane-derived carbonates occur in two structural positions: 1) on the slope of the ac-cretionary wedge in hemipelagites draping buried thrust-related anticlines, and 2) at the leading edge of the deformation front in the inner foredeep, within fault-related anticlines standing above the adjacent deep seafloor as intrabasinal ridges. We compare fossil seeps with two extensively investigated modern analogues: the Hikurangi Margin, offshore New Zealand and Hydrate Ridge, on the Cascadia margin, offshore the U.S.A. These analogues share a similar compressive structural setting and are marked by the presence of variably extensive and voluminous methane-derived carbonate bodies and chemosynthetic fauna on the present-day seafloor. The comparison allows us to propose a model for the evolution of fluid seeps on thrust-related ridges. At the deformation front, uplift and geometry of the anticlinal ridges are controlled by the growth of splay faults, mostly blind, connected to the basal detachment, favoring the migration of fluids toward the incipient anticline. Fold development generates extensional stresses in the hinge zone of the anticline, promoting the development of normal faults; fluid migration pathways and seafloor seeps shift from the forelimb toward the crest of the ridge as the structures evolve. In the slope setting, far from the deformation front, thrust faults and extensional faults in buried anticlines remain the main fluid migration pathways to sustain seepage at the seafloor. After reaching a mature stage within the wedge, the structure is less active and buried in the slope environment of the evolved prism.
... It has been noted that modern analogues reveal only partial stages of seepage activity (e.g. Plaza-Faverola et al., 2011;Crutchley et al., 2015;Klaucke et al., 2015;Mohammedyasin et al., 2016), while the identification of buried authigenic carbonates may be difficult on seismic data due to resolution limits (Andresen, 2012;Ho et al., 2012). On the other hand, seismic and seafloor data from modern analogues allow observations at a basinwide scale and the recognition of fluid migration pathways that are rarely identified in the geological record (Nyman et al., 2010;Nelson et al., 2017). ...
We present new field data from three outcrops of Miocene methane-derived authigenic carbonates in the foredeep of the northern Apennines that contain chemosynthetic fauna and record a long history (∼1 Ma) of shallow fluid seepage linked to seafloor anaerobic oxidation of methane. The studied outcrops show similar features in terms of carbonate morphology, facies, spatial distribution and lateral and vertical contacts with the enclosing sediments. Methane-derived carbonates occur in two structural positions: 1) on the slope of the accretionary wedge in hemipelagites draping buried thrust-related anticlines, and 2) at the leading edge of the deformation front in the inner foredeep, within fault-related anticlines standing above the adjacent deep seafloor as intrabasinal ridges. We compare fossil seeps with two extensively investigated modern analogues: the Hikurangi Margin, offshore New Zealand and Hydrate Ridge, on the Cascadia margin, offshore the U.S.A. These analogues share a similar compressive structural setting and are marked by the presence of variably extensive and voluminous methane-derived carbonate bodies and chemosynthetic fauna on the present-day seafloor. The comparison allows us to propose a model for the evolution of fluid seeps on thrust-related ridges. At the deformation front, uplift and geometry of the anticlinal ridges are controlled by the growth of splay faults, mostly blind, connected to the basal detachment, favoring the migration of fluids toward the incipient anticline. Fold development generates extensional stresses in the hinge zone of the anticline, promoting the development of normal faults; fluid migration pathways and seafloor seeps shift from the forelimb toward the crest of the ridge as the structures evolve. In the slope setting, far from the deformation front, thrust faults and extensional faults in buried anticlines remain the main fluid migration pathways to sustain seepage at the seafloor. After reaching a mature stage within the wedge, the structure is less active and buried in the slope environment of the evolved prism.
... Linear high permeability conduits, such as neotectonic faults and fractures, have long been recognised as pathways for fluid migration that is commonly initiated by changes in pore water pressure and/or increased seismicity (e.g. Milkov, 2000;Revil, 2002;Jonsson et al., 2003;Ho et al., 2010;Dupre et al., 2015;Franek et al., 2017;Etiope, 2015). Additionally, a modelling study performed by Klusman and Saeed (1996) suggests gaseous hydrocarbons preferably migrate vertically as buoyant microbubbles along fault systems. ...
Multiple proxies in the geological record offshore NW Svalbard track shallow subseafloor diagenesis and seafloor methane seepage during the Last Glacial Maximum (LGM) extent and the disintegration of the Svalbard Barents Sea Ice Sheet (SBIS). Vestnesa Ridge, located at 79°N and in 1200 m water depth, is one of the northernmost known active methane seep sites and is characterised by a subseafloor fluid flow system, numerous seafloor pockmarks and gas flares in the water column. In this study, we develop a Late Pleistocene and Holocene stratigraphic framework, use stable oxygen and carbon isotope signatures (δ¹⁸O, δ¹³C) of benthic and planktic foraminifera, the mineralogical and carbon isotope composition of methane-derived authigenic carbonate (MDAC) and sediment geochemical data of ten sediment cores to assess methane seepage variability on Vestnesa Ridge. The studied cores cover the age range between 31.9 and 10 cal ka BP and record 32 negative δ¹³C excursions in benthic and planktic foraminifera with amplitudes down to −29 ‰ VPDB. These δ¹³C excursions are often associated with elevated Ca/Ti and Sr/Ti elemental ratios in sediments and MDAC nodules. The precipitation of MDAC overgrowth on foraminiferal tests explains most of the negative δ¹³C excursions. In this dataset, the oldest recorded methane emission episodes on Vestnesa Ridge occurred between the LGM (24–23.5 cal ka BP) and Heinrich Event 1 (HE 1; 17.7–16.8 cal ka BP). Geological indicators for past subseafloor methane cycling and seafloor methane seepage, such as negative foraminiferal δ¹³C excursions, MDAC nodules, and elevated Sr/Ti elemental ratios recorded in post-LGM sediments, possibly represent vertical migration of the sulphate-methane transition zone (SMTZ) and post-date sedimentation by up to 13.4 ka. However, it is important to note that indications of post-LGM seafloor methane seepage at Vestnesa Ridge also correspond to the established methane efflux chronology for the adjacent Barents Sea shelf, implying that glacio-isostatic adjustments and associated re-activation of pre-existing deep-seated faults after disintegration of the SBIS are likely important controlling factors on fluid migration towards the seafloor.
... Submarine fluid leakage or seepage is pervasively discovered in various marine environments, from passive continental margins (Ho et al., 2012;Judd and Hovland, 2009;Sun et al., 2011Sun et al., , 2017a to active continental margins (Moore et al., 2013;Pichon et al., 1987;Suess et al., 1985). They represent a fundamental process of the earth's material recycling, which transfer material from geosphere to the biosphere, hydrosphere and atmosphere (Suess, 2014). ...
The active Haima cold seep sites were discovered by the remotely operated vehicle on the western part of the northern slope of South China Sea in 2015 and 2016. However, the subsurface structures in the seep area are not well studied, which prevents the further understanding of the evolution of fluid venting system in the active cold seep area. In 2017, a high-resolution 2D seismic line transecting the two main seep sites was acquired and processed. In this study, the subsurface structures were well resolved on the seismic profile. The new observations include the widespread subsurface fluid flow features (i.e., chimney structures and pockmarks) and the pervasively distributed magmatic activities. Seismic indicators for hydrothermal vents were also observed on the seismic section. This new discovery confirms the contribution of magmatic activities to the evolution of subsurface fluid flow in the study area. Minor faults under the recognizable resolution of seismic profile, which can act as conduits for the subsurface fluid flow, were also identified in the fine-grained sediment sequences. Overall, these results provide new insights into the evolution of subsurface fluid flow in the study area.
... The seismic survey was acquired in a direction parallel to the northwest striking continental shelf and is not aligned with the long axis of fluidventing structures, or with any karst networks. Therefore, no spatial alias ing occurred when imaging shallow features (Ho et al., 2012). The data set follows the Society of Exploration Geophysicists (SEG) European polarity convention, i.e., an increase in acoustic impedance equals a red reflection of negative amplitude (RosleffSoerensen et al., 2016). ...
We investigated pinnacle features at the base of late Oligocene–Miocene isolated carbonate buildups using three-dimensional seismic and borehole data from the Browse
Basin, Northwest Australia. Brightened seismic reflections, dim spots, and other evidence
of fluid accumulation occur below most pinnacle features. An important observation is
that all pinnacles generated topography on successive late Oligocene–Miocene paleoseafloors, therefore forming preferential zones for the settlement of reef-building organisms by raising the paleo-seafloor into the photic zone. Their height ranges from 31 m to 174 m, for a volume varying from 33 km3 to 11,105 km3. Most of the pinnacles, however, are less than 2000 km3 in volume and present heights of 61–80 m. As a result of this work, pinnacles are explained as the first patch reefs formed in association with mud volcanoes or methanogenic carbonates, and they are considered as precluding the growth of the larger isolated carbonate buildups. We postulate that pinnacle features above fluidflow conduits demonstrate a valid seep-reef relationship, and we propose them to be refined diagnostic features for understanding fluid flow through geological time.
... The structures in Group A are characterized as narrow, steep sided gas migration features, with an overall simple geometry, somewhat similar to those observed offshore Norway (Karstens and Berndt, 2015), Namibia (Moss and Cartwright, 2010), northern Australia (Rollet et al., 2009), and in the Congo Basin (Gay et al., 2006;Ho et al., 2012). Several such structures are observed beneath the S2 slump, where they appear to correlate with locations where the overlying unit of stratified sediments thins out (Fig. 8), suggesting that upward gas migration exploits the thinned, weakened overburden at such points. ...
A large geophysical dataset, including bathymetry, and 2D and 3D P-cable seismic data, revealed evidence of numerous gas flares near the S2 Canyon in the Danube Fan, northwestern Black Sea. This dataset allows us to investigate potential relationships between gas migration pathways, gas vents observed at the seafloor and submarine slope failures. Vertical gas migration structures as revealed in the seismics appear to be concentrated near submarine slope failure structures. Where these seismically defined features extend upwards to the seafloor, they correlate with the location of gas flares. However, not all these structures reach the seafloor, in some cases because they are capped by overlying sediments. A strong correlation is inferred between gas migration pathways, heterogeneous mass transport deposits and contacts between adjacent units of contrasting lithology. Although missing age constrains prevent a final judgement, we discuss the potential relationship between submarine slope failures and gas migration in order to determine if gas migration is a precursor to failure, or if the presence of slope failures and associated mass transport deposits facilitates the migration of gas. Our observations indicate that lithological heterogeneity, mass transport deposits and minor sediment deformation control gas migration pathways and the formation of gas chimney-like features. Gas migration is focused and gradual, resulting in gas flares where the chimney-like features extend to the seafloor, with no evidence of erosive features such as pockmarks.
... It was acquired perpendicularly to the northwest-striking continental shelf and is not aligned with the long axis of fluid-venting structures or any karst networks. Therefore, no spatial aliasing issues are expected when imaging shallow features (Ho et al., 2012;Howarth and Alves, 2016). ...
The geometric and depositional responses of isolated carbonate build-ups to Miocene sea-level change and regional tectonics was investigated using a combination of 3D seismic and borehole data from the Browse Basin, North West Australia, and outcrop information from the Cariatiz Reef, southeast Spain. The interpreted seismic volume documents five (5) Miocene sequence boundaries and five (5) main seismic facies. Seismic attribute analyses proved a highly effective tool for interpreting carbonate facies but, when compared with outcrop information from southeast Spain, data are limited to large-scale features of scales beyond 16.4 m vertically and 18.75 m horizontally. Hence, this work clearly shows that estimations of reservoir potential are significantly underestimated if based on seismic data alone. As a corollary of the structural analysis in this work, growth patterns suggest Messinian structural partitioning across the Browse Basin, with deformation associated with plate collision focused in preferentially orientated faults - thus only influencing carbonate build-up evolution at a local scale.
... The decreasing trend in fluid activity from stage 1 to stage 2 is coherent with the decrease in the density of pockmarks from UH6 to UH1. Two processes can explain this change in fluid activity: the sealing of carbonate mounds, i.e. self sealing in the sense of (Fig. 19); this hypothetical scenario integrates the genetic relationship shown through geometric analysis on seismics coupled with the interpretation of amplitude variations in the same way as proposed by other conceptual models (Betzler et al., 2011;Gay et al., 2012;Ho et al., 2012;Sultan et al., 2007;Casenave et al. 2017). ...
The seismic characteristics of focused fluid-related features such as pockmarks and carbonate build-ups on the Nigerian continental slope have been investigated using complementary seismic data sets (reprocessed 3D exploration data and 2D Very High Resolution hull-mounted and near-bottom seismic data) coupled with the results of previous studies (sedimentological, geotechnical and geochemical analyses).
The results show different types of fluid-related features within the hemipelagic phase of long duration (∼1 Myr) Turbidite/Hemipelagic cycles. They are the product of the disturbance of the hosting sediments following two main distinct processes: (1) no or low sediment deposition caused by fluid escape associated with seafloor settlement (benthic fauna, carbonate precipitation, gas hydrate formation) and (2) post-depositional erosion, caused either by fluid release (dewatering from underlying features such as channels and upward migration from a deep source) or by collapse (destabilisation of gas hydrates). The temporal evolution reveals a decrease in fluid flow during the hemipelagic deposition as witnessed by the decrease in methanogenic activity and by gas hydrate destabilisation. The spatial distribution of fluid-related features seems to be controlled by the type of fault (small fault or structural lineament).
... Cette ellipse se situe à 1 km à l'est de l'édifice. Un zoom sur la carte d'amplitude du fond marin ( Figure 4-80D) représente ces pockmarks comme de petits îlots brillants, ressemblant à ceux décrits comme des « anomalies à haute-amplitudes positives» par Ho et al. (2012). Ces auteurs interprètent ces patchs brillants comme des patchs de carbonates dérivés de sorties de fluides (MDAC) dans un contexte similaire à notre zone d'étude, i.e. une pente continentale dans un bassin à hydrocarbures. ...
Fault vertical sealing evaluation is significantly important for revealing hydrocarbon migration and accumulation processes and reducing exploration risks. Based on the analysis of the stress normal to the fault plane, the pore-fluid pressure and the compressive strength of fault rock, a new parameter, namely, the fault vertical sealing index (FVSI), is proposed for improved quantitative evaluation. In this article, static faults distributed in the eastern Pinghu Slope Belt (EPSB) of the Xihu Depression are selected as examples. The vertical sealing properties of four major fault systems that control hydrocarbon distribution are analyzed and evaluated. The FVSI value is positively correlated with the vertical sealing capacity of the fault zone, and the heterogeneous spatial distribution of the FVSI along the fault planes indicates that a fault cannot be uniformly regarded as a conduit or a barrier. According to the relation between the FVSI and the corresponding natural gas and oil shows, a fault zone with an FVSI value less than 0.4 commonly acts as a vertical conduit and cannot accumulate hydrocarbons, while a fault zone with an FVSI value greater than 0.4 has the ability to seal hydrocarbons. However, whether hydrocarbons can continue to migrate vertically depends on the maximum hydrocarbon column height that the corresponding FVSI can seal. In this case study, fault zones with FVSI values greater than 1 completely act as barriers to vertical hydrocarbon migration and no longer transport hydrocarbons. The changing FVSI of the fault zone controls the process of hydrocarbon migration and accumulation, thereby affecting the distribution of hydrocarbons. In the structurally lower position of the EPSB with sufficient hydrocarbon sources, the weak vertically sealed fault zone commonly causes a vertical multilayered distribution of hydrocarbons. Conversely, in the structurally higher position of the EPSB with insufficient hydrocarbon charging, the strong vertically sealed fault zone results in a more concentrated hydrocarbon distribution in the lower strata. However, there are no natural gas and oil shows in the fault zone that completely act as vertical conduits. Therefore, the FVSI can be an effective quantitative method to analyze vertical hydrocarbon migration pathways and accumulation locations controlled by faults and has great application potential in reducing the risks in petroleum exploration projects.
Seafloor mounds are enigmatic features along many continental margins and are often interpreted as gas hydrate pingoes, seep deposits, mud volcanoes, or hydrothermal mounds. When such mounds occur in basins with past volcanic activities, they have the potential to host seafloor metalliferous deposits, which is generally overlooked. Using geophysical datasets, we document the fluid plumbing systems that promoted the formation of seafloor mounds in the Great South Basin (GSB), offshore New Zealand. We also investigate these mounds as potential seafloor metalliferous deposits. Our results reveal 9 seafloor mounds (~ 137 m high) above gigantic (~ 5.4 km high) fluid escape pipes that are associated with deeper crystalline rocks. The structural make-up of the mounds, their geospatial relationships with the pipes and intrusive rocks, and geophysical properties suggest a primary volcanic or hydrothermal origin for the culpable fluids and mounds respectively. Fluids derived from deeper coal beds and shallow foraminiferal oozes in the basin constitute secondary fluid sources focused along polygonal faults and lateral flow cells. A main sub-vertical and minor lateral fluid plumbing patterns are proposed. The relationship between the mounds, pipes, underlying intrusive rocks, and upward routing of mineral-rich fluids could have implications for the formation of ore-grade mineral deposits on the seafloor in the GSB.
Hydrocarbon plumbing systems have been extensively documented in the past two decades using high resolution 3D seismic data, exploiting the ability of seismic imaging techniques to reveal the subsurface geometry of gas charged sediments. In this paper, we present a detailed study of a hydrocarbon plumbing system from the South China Sea, that involves both vertical and lateral (stratal) hydrocarbon migration in Miocene to Recent clastic sediments that comprise multilayer stacking of thinly layered clays, silts and sands. We show that a transtensive fault system that provides lateral seal for fault‐dip traps of deep Miocene reservoirs, and offers a vertical pathway for migration to shallower silty units. These silty units in turn form a ‘spillway’ in a regional, northward migration path. This path involves filling each shallow fault‐dip trap to spill point towards the fault tips, with stratal migration forced around the outer flanks of the fault‐related folds. Successive fill‐to‐spill leads to a continuous trail of amplitude anomalies that merge into a continuous, larger, gas‐charged anomaly pattern. The migrating gas finally accumulates within a zone bounded by a large boundary fault with full juxtaposition seal. The pattern of anomaly distribution suggests that the hydrocarbon migration has been active in the Late Pleistocene and is probably continuing at the present day. Hence this plumbing system may be one of very few examples described to date in which dynamic hydrocarbon migration pathways have been directly imaged by seismic data.
Circular to elliptical topographic depressions, isolated or organized in trails, have been observed on the modern seabed in different contexts and water depths. Such features have been alternatively interpreted as pockmarks generated by fluid flow, as sediment waves generated by turbidity currents, or as a combination of both processes. In the latter case, the dip of the slope has been hypothesized to control the formation of trails of downslope migrating pockmarks. In this study, we use high-quality 3D seismic data from the offshore Ceará Basin
(Equatorial Brazil) to examine vertically stacked and upslope-migrating trails of depressions visible at the seabed and in the subsurface. Seismic reflection terminations and stratal architecture indicate that these features are formed by cyclic steps generated by turbidity currents, while internal amplitude anomalies point to the presence
of fluid migration. Amplitude Versus Offset analysis (AVO) performed on partial stacks shows that the investigated anomalies do not represent hydrocarbon indicators. Previous studies have suggested that the accumulation of permeable and porous sediments in the troughs of vertically stacked cyclic steps may create vertical pathways
for fluid migration, and we propose that this may have facilitated the upward migration of saline pore water due to fluid buoyancy. The results of this study highlight the importance of gravity-driven processes in shaping the morphology of the Ceará Basin slope and show how non-hydrocarbon fluids may interact with vertically stacked
cyclic steps.
We have identified a deeply buried fluid escape pipe province in Cretaceous - late Paleocene sediments of the Great South–Canterbury Basin (NZ). The seismic observations and interpretations point to an unusually vast fossil system of pipes. These features are exceptional in number (>2000 edifices) and appear to have formed from a common root zone. The areal extent of the analysed pipe system (2500 km²) is among the largest systems of fluid expulsion features ever observed in three-dimensional seismic data.
The unclustered distribution of the pipes suggests no specific link to faults or buried sedimentary features and, at their maximum vertical development, the pipes are equally distributed above depocentres or structural highs. The majority of pipes terminate at two discrete levels in the late Paleocene. Based on the geometrical relationship of the pipe edifices to the overburden, and the basinal setting of the hosting units, we interpret these horizons as representing the seabed at the time of pipe formation. This interpretation allows us to date the timing of pipe formation as prominently late Paleocene. We envisage that the pipes originated during discrete episodes of fluid venting in this time interval, disrupting the typical progressive basinal compaction-driven pore fluid expulsion.
The pipes are associated with biogenic gas expulsion. We discuss their triggers, mechanical processes, and global significance for understanding fluid flow processes in sedimentary basins. Creation of the initial overpressure could be related to in situ biogenic gas generation in the stratigraphic interval coincident with the root zone of the pipes. We hypothesise that the main episode of gas release and pipe formation was associated with a sea-level fall during the contemporaneous late Paleocene Carbon Isotope Maximum.
This case study shows a unique example of a preserved Paleogene fluid migration system and highlights how the observation and the interpretation of massive fossil systems of fluid-escape features provides invaluable information in terms of overpressure generation and basin fluid expulsion history.
A new type of gas chimney exhibiting an unconventional linear
planform is found. These chimneys are termed Linear Chimneys, which have been observed in 3-D
seismic data offshore of Angola. Linear Chimneys occur parallel to
adjacent faults, often within preferentially oriented tier-bound fault
networks of diagenetic origin (also known as anisotropic polygonal faults,
PFs), in salt-deformational domains. These anisotropic PFs are parallel to
salt-tectonic-related structures, indicating their submission to horizontal
stress perturbations generated by the latter. Only in areas with these
anisotropic PF arrangements do chimneys and their associated gas-related
structures, such as methane-derived authigenic carbonates and pockmarks, have
linear planforms. In areas with the classic isotropic polygonal fault
arrangements, the stress state is isotropic, and gas expulsion structures of the
same range of sizes exhibit circular geometry. These events indicate that
chimney's linear planform is heavily influenced by stress anisotropy around
faults. The initiation of polygonal faulting occurred 40 to 80 m below the
present day seafloor and predates Linear Chimney formation. The majority of
Linear Chimneys nucleated in the lower part of the PF tier below the
impermeable portion of fault planes and a regional impermeable barrier within
the PF tier. The existence of polygonal fault-bound traps in the lower part
of the PF tier is evidenced by PF cells filled with gas. These PF gas traps
restricted the leakage points of overpressured gas-charged fluids
along the lower portion of PFs, hence controlling the nucleation sites of
chimneys. Gas expulsion along the lower portion of PFs preconfigured the
spatial organisation of chimneys. Anisotropic stress conditions surrounding
tectonic and anisotropic polygonal faults coupled with the impermeability of PFs
determined the directions of long-term gas migration and linear geometries of
chimneys. Methane-related carbonates that precipitated above Linear Chimneys
inherited the same linear planform geometry, and both structures record the
timing of gas leakage and palaeo-stress state; thus, they can be used as a tool
to reconstruct orientations of stress in sedimentary successions. This study
demonstrates that overpressure hydrocarbon migration via hydrofracturing may
be energetically more favourable than migration along pre-existing faults.
The resolution of data acquired over modern seafloors does not allow imaging of the inner features of a fluid seep structure, particularly in the shallow subsurface. In the South-East Basin of France (Drôme), fossil cold seep structures comprising fossil-rich carbonate lenses were identified about 30 years ago within the Oxfordian (Late Jurassic) Terres Noires Formation. These structures were first interpreted as pseudo-bioherms related to hydrothermal activity, but comparison with active seep sites on modern margins, together with isotopic analyses led to a re-interpretation involving cold fluids instead. To date, all seep sites have generally been studied individually without considering any link to neighboring or more distant sites. Based on 23 high-resolution stratigraphic logs within the structure coupled to mosaicked aerial photographs from a drone survey, the 19 fluid seep events were correlated in the area, including two new sites exposed due to weathering. We have shown that each identified sub-site is composed of sub-vertically stacked fossil-rich carbonate lenses interbedded with marls, which developed in smooth, 4 to 6 m deep depressions beneath the local seabed. The nodules present within the depressions are of primary importance as they mark the area of active seeping. This general organization is very similar to the modern Regab giant pockmark in the Lower Congo Basin where only a few sub-sites are active at the same time. A spatio-temporal 3D reconstruction of the position of these sub-sites shows that the carbonate lenses are organized into clusters comprising up to 7 sub-sites grouped together in the same stratigraphic interval and the same geographic zone. A sandbox experiment where gas is injected at constant flow rate at the base of a box filled with a matrix of water-saturated grains displays a pattern consisting of disturbed sediments inside a parabolic-shaped area. This parabolic shape was also identified on a seismic profile across the Regab giant pockmark, suggesting that the processes are similar for the Beauvoisin and Regab seep areas. The laboratory experiments also show that the seeping conduit is stable during a given period of time and suddenly shifts laterally. It is mainly due to the collapse of the conduit, the lateral migration and the reopening at a new position. The general log obtained in the Beauvoisin seep area suggests a similar pattern with periods of seeping alternating with periods of quiescence, each of which is approximately 200 ka. Even if a pockmark seems to have been inactive for a long period of time, it could be due to the lateral shift of the feeder conduit meaning that the sub-seafloor is still charged in gas. This is of primary importance for risk assessment, hydrocarbon exploration and general understanding of geobiology at seafloor seeps.
Fluid seepage features on the upper continental slope offshore Congo are investigated using multi-disciplinary datasets acquired during several campaigns at sea carried out over the last 15 years. This datasets includes multibeam bathymetry, seismic data, seafloor videos, seafloor samples and chemical analyses of both carbonate samples and of the water column. Combined use of these datasets allows the identification of two distinctive associations of pockmark-like seabed venting structures, located in water depths of 600–700 m and directly above a buried structural high containing known hydrocarbon reservoirs. These two features are called spiders due to the association of large sub-circular depressions (the body) with smaller elongate depressions (the legs). Seismic reflection data show that these two structures correspond to amplitude anomalies located ca. 60–100 ms below seabed. The burial of these anomalies is consistent with the base of the methane hydrate stability domain, which leads to interpret them as patches of hydrate-related bottom-simulating reflection (BSR). The morphology and seismic character of the two structures clearly contrasts with those of the regional background (Morphotype A). The spider structures are composed of two seafloor morphotypes: Morphotype B and Morphotype C. Morphotype B makes flat-bottomed depressions associated with the presence of large bacterial mats without evidence of carbonates. Morphotype C is made of elongated depressions associated with the presence of carbonate pavements and a prolific chemosynthetic benthic life. On that basis of these observations combined with geochemical analyses, the spider structures are interpreted to be linked with methane leakage. Methane leakage within the spider structures varies from one morphotype to another, with a higher activity within the seafloor of Morphotype C; and a lower activity in the seafloor of Morphotype B, which is interpreted to correspond to a domain of relict fluid leakage. This change of the seepage activity is due to deeper changes in gas (or methane) migration corresponding to the progressive upslope migration of fluids. This phenomenon is due to the local formation of gas hydrates that form a barrier allowing the trapping of free gas below in the particular context of the wedge of hydrates.
Some pockmarks on the Norwegian continental shelf contain patches of cemented sediment that can provide hardgrounds attractive to a variety of benthonic organisms. Detailed examination of a sandstone sample from a pockmark in Norwegian Block 25/7 in the North Sea has revealed the presence of Mg calcite and aragonite cements, some of the latter forming botryoids. All cement types are characterized by extremely light carbon isotopic compositions, with a mean 13C value of -56.1 PDB, which shows that the cements contain carbonate produced by oxidation of biogenic methane. Oxidation occurred in both the oxic and anoxic diagenetic zones. Trace amounts of interstitial methane (mean 13C = -40.8%) and higher hydrocarbon gases (up to C5) with a C1/Cn ratio of 0.855-0.874 in the pockmark sediments indicate that some thermogenic methane may be mixed with the biogenic gas.
1. Introduction : This paper presents results of a deepwater pockmark detailed investigation from a geophysical and geomorphologic point of view. AUV high-resolution results are compared to seabed and sub-seabed imagery from 3D exploration data, and provide new insights with regard to pockmark generation and fluid migration.
We describe 8 fossil-seep sites from the western margin of North America that range in age from Jurassic to Pliocene (~150 million to 5 million years old). They all occur in a broadly similar geologic context: marine sedimentary rock sequences once deposited in ancient, subduction-related plate tectonic settings, now uplifted and exposed in northern California, Oregon, and Washington. These localities have been identified through recognition of a distinctive co-occurrence of chemosymbiotic fossil bivalves (Thyasira, Lucinoma, Solemya, and Modiolus) and probable worm tube structures, with anomalous nodular masses of limestone in otherwise deep-water sandstone and mudstone deposits. These ancient seep occurrences also resmble the modern chemosymbiotic faunas and carbonate deposits that surround the methane seeps on the seafloor off the Oregon coast, in deep-water marine sedimentary environments above today's active subduction zone.
Some pockmarks on the Norwegian continental shelf contain patches of cemented sediment that can provide hardgrounds attractive to a variety of benthonic organisms. Detailed examination of a sandstone sample from a pockmark in Norwegian block 25/7 in the North Sea has revealed the presence of Mg calcite and aragonite cements, some of the latter forming botryoids. All cement types are characterized by extremely light carbon isotopic compositions. -from Authors
Numerous slabs and indurated crusts of limestones and carbonate cemented sandstone were found on the floor of pockmarks in the North Sea. Sea-floor pockmarks are local depressions found in unconsolidated fine-grained sediments. Their origin has been ascribed to percolation of gas or liquids through the sea floor. A group of 5 pockmarks believed to be associated with shallow gas, located in Norwegian Block 25/7, were studied by the use of a remotely operated vehicle (ROV). Petrographic studies of the cements in one of these slabs (the '25/7-slab') show a variety of aragonite and Mg-calcite crystals. The delta 13C-value of the aragonite cement in the 25/7-slab is -58.9per mille (PDB) which indicates a methanogenic origin. It is concluded that both the sea-floor pockmarks and the carbonate crusts are probably caused by the seepage of hydrocarbon gases through the sea-floor. -from Authors
As offshore petroleum exploration and development move into deeper water, industry must contend increasingly with gas hydrate, a solid compound that binds water and a low-molecular-weight gas. However, hydrate occurs naturally wherever there are high pressures, low temperatures, and sufficient concentrations of gas and water. This paper discusses the seismic detection of gas hydrate in marine sediments.
Seabed fluid flow involves the flow of gases and liquids through the seabed. Such fluids have been found to leak through the seabed into the marine environment in seas and oceans around the world - from the coasts to deep ocean trenches. This geological phenomenon has widespread implications for the sub-seabed, seabed, and marine environments. Seabed fluid flow affects seabed morphology, mineralization, and benthic ecology. Natural fluid emissions also have a significant impact on the composition of the oceans and atmosphere; and gas hydrates and hydrothermal minerals are potential future resources. This book describes seabed fluid flow features and processes, and demonstrates their importance to human activities and natural environments. It is targeted at research scientists and professionals with interests in the marine environment. Colour versions of many of the illustrations, and additional material - most notably feature location maps - can be found at www.cambridge.org/9780521819503.
Hydrocarbon-based communities occur in connection with gas-related seabed features, pockmarks and 'mottled' seabed. Sediments beneath the larger patches of 'mottled' seabed are gas-charged and it is assumed that slow seepage (micro-seepage) of pore-water and hydrocarbons occurs from these. The distribution of the benthos, nekton, and plankton associated with 'mottled' seabed in the Tommeliten area and specific pockmarks in the Holene and Gullfaks areas was studied by video-tape recordings. Besides the faunal elements, biogenic sedimentary structures were recorded.
Analysis of well log data from Cenozoic basin fill of the Deer Lodge Valley, southwestern Montana, provides evidence for identifying paleosols and paleosol stacks in the subsurface. The paleosol stacks are continental sequence boundary markers and appear as several relatively thin, high-velocity/high-density zones within basin fill. Zone thickness ranges from 1 to 1.5 m; zones are stacked to thicknesses of up to 15 m. Density varies within the zones by as much as 0.6 g/cm³, and differs by as much as 0.9 g/cm³ from material immediately above these zones. Velocity differs by as much as 10 ft/ms from the overlying material and causes bright reflections on seismic sections. Synthetic seismograms are used to tie well log and seismic data. Basing our interpretation upon well log data and well cuttings analyses, we determined the high-velocity/high-density zones to be limestone. The pedogenic origin of the limestone is shown by (1) well cutting chips from the high-velocity/high-density zones that exhibit pedogenic features associated with calcic paleosols, (2) paleosol horizonation interpreted from well log analysis, (3) the absence of minerals normally associated with lacustrine deposits, and (4) comparison with surface paleosol exposures.
Seepage of shallow gas, mainly methane, recently has been documented visually in the central section of the North Sea at water depths of about 70-80 m. The surface expressions caused by these seeps are not dramatic. They mainly consist of small funnel-shaped craters (20 cm across) in the cover sand. However, in deeper parts of the North Sea, where the cover sediments consist of silt and clay, the surface features are more dramatic. Here the craters (pockmarks) caused by shallow gas eruptions or seepages are up to 20 m deep and 100 m wide. In some areas, the pockmarks are paved with a crust of calcium carbonate cement, which is believed to have formed as a result of gas efflux through the seabed. Furthermore, the presence of these calcium carbonate reefs seems to have attracted a wide variety of marine life. The presence of shallow gas has caused an enrichment of marine life that seems to be significant. Beside the semicircular and composite pockmarks, gas-induced erosion also has caused elongate depressions. These occur where the top sediment bedding has caused the gas to migrate along certain axes. The mode and speed of formation of the gas-induced erosion features are of major concern to oil exploration and development in the northern North Sea. Research into these aspects recently has been stimulated by the discovery of one of the world's largest offshore gas reservoirs, which coincidentally lies within a pockmarked area.
2D and 3D seismic data from the mid-Norwegian margin show that polygonal
fault systems are wide-spread within the fine-grained, Miocene sediments
of the Kai For- mation that overlie the Mesozoic/ Early Cenozoic rift
basins. De-watering and devel- opment of polygonal faults commenced
shortly after burial and is an ongoing process since Miocene times. This
is evident from the polygonal fault system's stratigraphic setting, the
statistical properties of fault throw, and the stratigraphic setting of
fluid flow features that are related to de-watering of the polygonal
fault systems.
Two occurrences of active gas seepages are described from the North Sea. The southernmost one, situated above a salt diapir in Norwegian block , has been studied and sampled by use of a remotely operated vehicle (ROV). This seepage consists of about 120 single seeps located within a diameter of ∼ 100 m. It is estimated to produce ∼ 24 m3 of methane gas per day (at ambient pressure, 75 m water depth). Isotope values of the methane gas and higher hydrocarbon gases in the surrounding seafloor sediments, show that their origin is from a deep seated, thermogenic source. No typical gas-induced erosion features are found on the seafloor at this location, probably due to the lack of very fine grained material.The second occurrence is located in U.K. block (Geoteam, 1984), where the seepage is associated with a very large pockmark depression, measuring 17 m in depth and 700×450 m in width. This depression represents an eroded fine grained sediment volume of ∼ 7.105 cubic metres. No detailed inspection or sampling of the gas has been performed here. However seismic reflection anomalies are seen on airgun seismic records at various levels down to a depth of at least 1100 m below seafloor. The seeping gas, possibly mixed with liquids, at this location is therefore also expected to be of a thermogenic origin.
A small area of 3.1 km2 in the Norwegian Trench has been selected for a detailed analysis of possible relationships between the Recent geology and the formation of pockmarks. Two sets of side-scan sonar and deep-towed boomer (DTS) data were used for the analysis.The most recent deposit is Unit 1, consisting of finely layered soft, silty clays. Unit 1 has been divided into three seismic subunits (1A, 1B and 1C), based upon the boomer records. The underlying Unit 2 is seismically homogeneous, consisting of stiff, silty, sandy clay with fragments of chalk. Most of the irregularities in the surface of Unit 2 are found to be furrows traversing the study area in a N—S general direction.The seabed is heavily pockmarked (mean density: 30/km2) and some of the pockmarks are asymmetric with steep ESE walls. The maximum pockmark depth within the study area is 5.8 m, and the maximum wall angle is about 12°.In addition to describing the Recent geology and surface pockmarks the following features are described and their relationship with pockmark formation discussed: (1) “sloping reflectors”, found below some of the pockmarks; (2) “columnar disturbances”, seen under many of the buried pockmarks; and (3) “masked reflectors”, intermittent strong reflectors found within Subunit 1A, which are probably caused by a raised gas concentration in porous layers.Pockmarks and associated features are assumed to be caused by gas escaping through the seabed. A possible gas migration mechanism through the interface between Units 1 and 2 and through Unit 1 is also discussed.
Discusses triggering of pockmark eruptions which may be by earthquakes, by seabed pressure perturbations caused by tidal or gravity waves or, in deep water, by a combination of tidal waves and low atmospheric pressure and storm waves. -from Author
Hundreds of pockmarks and mounds, which seismic reflection sections show to be underlain by chimney-like structures, exist in southeast part of the Vøring plateau, Norwegian continental margin. These chimneys may be representative of a class of feature of global importance for the escape of methane from beneath continental margins and for the provision of a habitat for the communities of chemosynthetic biota. Thinning of the time intervals between reflectors in the flanks of chimneys, observed on several high-resolution seismic sections, could be caused by the presence of higher velocity material such as hydrate or authigenic carbonate, which is abundant at the seabed in pockmarks in this area. Evidence for the presence of hydrate was obtained from cores at five locations visited by the Professor Logachev during TTR Cruise 16, Leg 3 in 2006. Two of these pockmarks, each about 300-m wide with active seeps within them, were the sites of high-resolution seismic experiments employing arrays of 4-component OBS (Ocean-Bottom Seismic recorders) with approximately 100-m separation to investigate the 3D variation in their structure and properties. Shot lines at 50-m spacing, run with mini-GI guns fired at 8-m intervals, provided dense seismic coverage of the sub-seabed structure. These were supplemented by MAK deep-tow 5-kHz profiles to provide very high-resolution detail of features within the top 1-40 m sub-seabed. Travel-time tomography has been used to detail the variation in Vp and Vs within and around the chimneys. Locally high-amplitude reflectors of negative polarity in the flanks of chimneys and scattering and attenuation within the interiors of the chimneys may be caused by the presence of free gas within the hydrate stability field. A large zone of free gas beneath the hydrate stability field, apparently feeding several pockmarks, is indicated by attenuation and velocity pull-down of reflectors.
Pockmarks on the slope of the Lower Congo Basin are distributed along a meandering band on seafloor coincident with a shallow buried palaeochannel imaged from the 3D-seismic database. Each pockmark originates systematically at the channel-levee interface and the seafloor expression of the palaeochannel's sinuosity is mimicked by the sinuous trend of pockmarks. 3D-seismic on the slope, calibrated by biostratigraphic data from cores of the Leg ODP 175, indicate a seaward decrease of the sedimentation rate. We suggest that this condition induces a differential loading of the hemipelagic cover over the palaeochannel and propose a model for episodic dewatering of fluids trapped in the buried turbiditic channel. The consequence is a fluid flow caused by a longitudinal pressure gradient along the buried channel. A hydromechanical model proposed for the formation of shallow pockmarks indicates that the sedimentation rate cannot generate the overpressure required for pockmark formation on the seafloor. Therefore, it is suggested that hydrocarbon migration from deeper overpressured reservoirs is added to the pore fluid pressure in the shallow subsurface sediments. Horizontal drainage by the turbiditic palaeochannel and vertical migration along many vertical conduits (seismic chimneys) probably initiated at shallow subbottom depth. It is concluded that these shallow processes have important implications for fluid migration from deeply buried hydrocarbon reservoirs.
V-shaped seismic reflectors have been seen on seismic for several decades, but were interpreted as
“erosional features” until 3-D seismic became available. The advent of 3-D revealed that a number of
these, especially in the North Sea, were actually closed features, i.e. funnels rather than valleys. As the
quality of survey increased, a distinction could be made between erosional funnels, which initially
developed as craters on the seafloor, and conical intrusions. The distinction is based on the comparison
between the inner and outer part of the funnel: in the former case, the infill bears no resemblance to the
surrounding sediments, while conical intrusions just push up a keystone of sediment, which can be
perfectly correlated with the outside of the anomaly.
A third category was recently observed on seismic surveys from a passive continental margin: funnelshaped
anomalies are filled with sediments that can be correlated with what is deposited around the
anomaly, but these sediments have collapsed into the funnel after being deposited, implying that some
material removal took place below sedimentary cover. The model proposed involved the emplacement
of pingo-like massive hydrate structures followed by their dissociation, leaving only a weld at the end of
the process. The volumes involved are quite high, on the order of a few km3. Multiple episodes of
funnel-shaped anomalies are stacked, suggesting a rhythmic succession of hydrate growth and
dissociation over several My.
It is therefore crucial to correctly correlate the inner and outer sides of conical seismic anomalies in
order to be able to discriminate between the three possible mechanisms mentioned above. All three are
related with fluid expulsion, but they have quite different meanings in terms of fluid nature and process.
Advancing pockmarks composed of multiphase, migrated infill-sets are observed above gas-bearing channels and deep-seated extensional faults in a high-resolution 3D seismic survey of the Lower Congo Basin, offshore Angola. They occur within a Neogene-Quaternary-aged sedimentary sequence deformed by polygonal faults. “Advancing” pockmarks are so termed based on the presence of migrating stratal packages which represent episodes of infill; i.e., where apices are laterally offset by hundred(s) of meters. Migration occurs systematically in the same direction over the evolution of the pockmark array. “Normal” vertical-stacked-up pockmarks on the contrary show vertically stacked successive episodes of individual pockmark formation.
The objective of this research is to establish what factors control the development of advancing pockmark arrays and what implications they have for leakage of hydrocarbons. Advancing and “normal” pockmarks occur in different settings. “Normal” pockmarks develop on horizontal slopes whilst “advancing” pockmarks are located on inclined slopes or in channels. Migration occurs in the downslope direction, implying that bottom currents play a role in the migration process. These pockmarks developed above a turbidite channel which is overlain by a gas hydrate highlighted in seismic data by a bottom-simulating reflector (BSR). The BSR is restricted to channel areas. High seismic amplitudes within the pockmarks below the BSR suggest that they are filled with free gas.
The following emplacement mechanism is interpreted for advancing pockmarks arrays:
1) Initial slope topography of the system is created by expulsions of gas focused through the underlying turbidite channel;
2) Fine-grained sands are draped over the pre-existing pockmark crater;
3) The vortex resulted from a higher influx rate of bottom currents, in combination with seepages, resuspended the unconsolidated sediments on the downstream pockmark sidewall. A local starving area was thus created. This area is covered directly by another fine-grained layer which in turn is deformed in the downstream side. The same process is repeated, starving areas are stacked up and vertically connected to each other. A permeable pathway for fluid flow is developed along the downstream pockmark sidewall.
4) Fluid leakage through the permeable pathway determines the location of future pockmarks which are preferentially developed on the downslope flank. The result is a downslope migrating pockmark fill.
In conclusion the two main factors that control the formation of advancing pockmarks are:
1) fluid flux,
2) the interaction of local bottom currents with sedimentation and pre-existing pockmark topography.
Hundreds of pockmarks and mounds, which seismic reflection sections show to be underlain by chimney-like structures, exist in southeast part of the Vøring plateau, Norwegian continental margin. These chimneys may be representative of a class of feature of global importance for the escape of methane from beneath continental margins and for the provision of a habitat for the communities of chemosynthetic biota. Thinning of the time intervals between reflectors in the flanks of chimneys, observed on several high-resolution seismic sections, could be caused by the presence of higher velocity material such as hydrate or authigenic carbonate, which is abundant at the seabed in pockmarks in this area. Evidence for the presence of hydrate was obtained from cores at five locations visited by the Professor Logachev during TTR Cruise 16, Leg 3 in 2006. Two of these pockmarks, each about 300-m wide with active seeps within them, were the sites of high-resolution seismic experiments employing arrays of 4-component OBS (Ocean-Bottom Seismic recorders) with approximately 100-m separation to investigate the 3D variation in their structure and properties. Shot lines at 50-m spacing, run with mini-GI guns fired at 8-m intervals, provided dense seismic coverage of the sub-seabed structure. These were supplemented by MAK deep-tow 5-kHz profiles to provide very high-resolution detail of features within the top 1-40 m sub-seabed. Travel-time tomography has been used to detail the variation in Vp and Vs within and around the chimneys. Locally high-amplitude reflectors of negative polarity in the flanks of chimneys and scattering and attenuation within the interiors of the chimneys may be caused by the presence of free gas within the hydrate stability field. A large zone of free gas beneath the hydrate stability field, apparently feeding several pockmarks, is indicated by attenuation and velocity pull-down of reflectors.
Early Cretaceous rifting of the South Atlantic basin resulted in the development of three unconformity-bound tectono-stratigraphic megasequences that are recognizable in petroliferous basins along the present-day margins of both Brazil and west Africa. These megasequences have been termed nonmarine/synrift, transitional marine, and marine. These megasequences provide a framework for understanding the character and distribution of hydrocarbon reservoirs within the South Atlantic petroleum systems. Hydrocarbons occur in nonmarine and marine stratal packages in both carbonates and siliciclastics. Reservoirs in the marine megasequences contain an estimated 70% of the region's known oil reserves, most of which have been discovered in the last two decades in deep-water fields. Each reservoir system requires a comprehensive evaluation of depositional systems and diagenetic modification. Deep-water siliciclastic reservoirs are controlled by sediment provenance and transport mechanisms to the deep-water setting and less so by diag