Gas-charged Sediments Research Articles

Characteristics of seismic strata in the southeast Zhoushan Archipelago (East China Sea) with emphasis on shallow gas

Based on high-resolution sub-bottom seismic profiles collected on the coast of the Zhoushan Archipelago in the East China Sea, the distribution and characteristics of shallow gas have been analyzed. The strata in the area were divided into three geological units: Holocene fine-grained neritic facies muddy strata, Late-Pleistocene coarse-grained fluvial or lacustrine facies sandy strata, and bedrock. The presence of shallow gas was evidenced by various acoustic indicators, including acoustic blanking, enhanced reflections, gas chimneys, bright spots, and small mounds. Shallow gas accumulations are observed through the Lower and Upper Holocene strata, presumably related to the degradation of organic matter buried in the Holocene deposits and sealed by soft muddy deposits. The presence of ultra-shallow gas-charged sediment sealed by fine-grained muddy sediment in coastal areas with frequent human disturbance suggests a high potential for sediment instability. Moreover, frequent upward gas migration signatures were found in the study area. These gas migration features mainly occur in nearshore areas where tidal disturbance and human activities are both active. We therefore deduced that the deduction of sediment above probably decreased the pressure, thereby resulting in increased upward migration. The presence of active shallow gas within thin, soft muddy strata, along with the strong tidal turbulence and human activities, indicates that the seafloor of the study area is vulnerable to sediment erosion, and shallow gas is easily released, posing a potential danger to the stability of marine engineering foundations. Since the Changjiang sediment supply has been decreasing over the past decades, it will inevitably lead to corresponding morphological adjustments in the study area, such as seafloor erosion. To what extent coastal erosion will release the buried shallow gas and lead to unpredictable consequences is worthy of our sustained attention.

Seismic characterization of a fluid escape structure in the North Sea: the Scanner Pockmark complex area

SUMMARYSubsurface fluid escape structures are geological features which are commonly observed in sedimentary basins worldwide. Their identification and description have implications for various subsurface fluid flow applications, such as assuring integrity of overburden rocks to geological CO2 storage sites. In this study, we applied 3-D first-arrival traveltime tomography to a densely sampled wide-azimuth and wide-angle ocean bottom seismometer (OBS) data set collected over the Scanner Pockmark complex, a site of active gas venting in the North Sea. Seismic reflection data show a chimney structure underlying the Scanner Pockmark. The objective of this study was to characterize this chimney as a representative fluid escape structure in the North Sea. An area of 6$\times $6 km2 down to a depth of 2 km below sea level was investigated using a regularized tomography algorithm. In total, 182 069 manually picked traveltimes from 24 OBS were used. Our final velocity model contains compressional wave velocity perturbations ranging from −125 to +110 ms−1 relative to its average 1-D model and compares favourably with a coincident seismic reflection data set. The tomographic velocity model reveals that the chimney as observed in seismic reflection data is part of a larger complex fluid escape structure, and discriminates the genuine chimney from seismic artefacts. We find that part of the seeping gas migrates from a deep source, accumulates beneath the Crenulate Reflector unconformity at ∼250 m below seafloor (mbsf) before reaching the porous sediments of the Ling Bank and Coal Pit formation at <100 mbsf. In addition, the model shows that the venting gas at Scanner Pockmark is also being fed laterally through a narrow NW–SE shallow channel. Quantitative velocity analysis suggests a patchy gas saturation within the gas-charged sediments of the Ling Bank and the Coal Pit formations. Confined to the well-resolved regions, we estimate a base case average gas saturation of ∼9 per cent and in-situ gas volume of ∼1.64 $\times {10^6}\ {{\rm{m}}^3}$ across the Ling Bank and Coal Pit Fm. that can sustain the observed methane flux rate at the Scanner Pockmark for about 10 to 17 yr.

Reservoir Potential Assessment of the Tortonian–Calabrian Sediments from Offshore Makran (Southwest Pakistan) through a Multidisciplinary Approach

AbstractThe Tortonian–Calabrian strata of Offshore Makran (Pakistan) is investigated for the purpose of reservoir assessment. The stratigraphy and petrophysics indicate that the Neogene sediments have tight reservoir potential with porosities in the range of 3%–9% and 40%–50% water saturation. The mineralogical cross plots indicate a mixed lithology with an abundance of dolomite and calcite, together with minor quartz content and shale. The seismic interpretation demonstrates medium–high amplitude reflection patterns, mud diapirs coupled with onlapping strata and the occurrence of bottom simulating reflectors (BSRs). The BSRs are characterized by diminished amplitude, low continuity and exhibiting acoustic blanking zones. The high formation pressure results indicate overpressure zones, suggesting the occurrence of overpressured shales in the Jal Pari‐1A. The presence of mud diapirs and gas chimneys are the result of tectonic forces acting at the junction of the Arabian, Indian and Eurasian plates, whereas, BSRs prove the existence of gas charged sediments supporting the formation of mud diapirism in the region. It is concluded that the high rates of sedimentation during the Neogene are likely to have contributed to the development of the high formation pressure. Revised mud weights, casing policies, pore pressure transients and geophysical inversion studies will help alleviate drilling risks in future exploration strategies.

New Seismoacoustic Data on Shallow Gas in Holocene Marine Shelf Sediments, Offshore from the Cilento Promontory (Southern Tyrrhenian Sea, Italy)

High-resolution seismoacoustic data represent a useful tool for the investigations of gas-charged sediments occurring beneath the seabed through the identification of the diagnostic intrasedimentary features associated with them. Acoustic blanking revealed shallow gas pockets in the seismostratigraphic units of the inner shelf off the Northern Cilento promontory. Six main seismostratigraphic units were recognized based on the geological interpretation of the seismic profiles. Large shallow gas pockets, reaching a lateral extension of 1 km, are concentrated at the depocenter of Late Pleistocene–Holocene marine sediments that are limited northwards by the Solofrone River mouth and southwards by the Licosa Cape promontory. A morphobathymetric interpretation, reported in a GIS environment, was constructed in order to show the main morphological lineaments and to link them with the acoustic anomalies interpreted through the Sub-bottom chirp profiles. A newly constructed workflow was assessed to perform data elaboration with Seismic Unix software by comparing and improving the seismic data of the previously processed profiles that used Seisprho software. The identification of these anomalies and the corresponding units from the offshore Cilento promontory represent a useful basis for an assessment of marine geohazards and could help to plan for the mitigation of geohazards in the Cilento region.

Petroleum systems in a buried stratovolcano: Maturation, migration and leakage

Buried volcanic systems are being increasingly studied due to their potential for hydrocarbon and geothermal exploration and subsurface storage of fluids such as CO2. Whilst widespread secondary porosity and fracturing increase reservoir potential in volcanic rocks and associated epiclastic deposits, these processes often also compromise seal integrity, leading to hydrocarbon leakage. Volcanism was widespread offshore northwestern New Zealand in the Miocene and petroleum has been discovered in the buried Kora stratovolcano. We analyse the petroleum system in this volcano using basin modelling to better understand controlling factors on the petroleum potential of buried volcanos. We reproduce charge of this accumulation from a marine source rock, the Waipawa Formation. In addition, there is significant potential for gas generation from Late Cretaceous coal measures of the Rakopi Formation. Modelling results are consistent with an effective fluid plumbing system and suggest that good quality connected reservoirs are present. In addition, Cretaceous faults that were reactivated in the Miocene act as migration pathways. However, models also suggest that significant amounts of petroleum have leaked during volcanic activity and related early maturation of source rocks. Models further predict that overburden and seal rock quality remain insufficient to support large petroleum accumulations. Low seal quality is consistent with the observation of high amplitude anomalies in reflection seismic data above the Kora volcano that are likely related to gas charged sediments and gas leakage. The occurrence of leakage is also consistent with extensive fracturing and oil shows in marine deposits overlying the volcano. These results highlight that the potential of buried volcanic systems to store petroleum and other fluids depends to a large extent on their structural evolution and burial history. On a global scale, hydrocarbon and greenhouse gas leakage of volcanos emplaced in petroliferous basins could have been significant in earth history with the potential to influence global climate.

Influence of anisotropy in mechanical properties of muddy aquatic sediment on CH4 bubble growth direction and migration pattern

Gas-charged sediments of shallow water bodies are significant sources of atmospheric methane (CH4), an important greenhouse gas. They are a source of a permanent concern for their contribution to destabilization of coastal and aquatic infrastructure. Past accounts of gas bubbles developed in shallow muddy aquatic sediments and in their surrogates have reported a controversial occurrence of vertical as well as horizontal bubbles morphologies. Our study suggests an explanation of the apparent conundrum about preferred orientations of bubbles in muddy sediments. It is conducted by employing mechanical reaction–transport numerical model, which couples CH4 diffusion-led expansion of gas bubble and elastic-fracturing mechanical sediment response to its growth. Muddy sediment is assumed to exhibit a transverse anisotropy in fracture toughness (describing an easiness of breaking the inter particle bonds), attributed to partial or full alignment of plate-like clay particles. Bubbles growing in isotropic sediment develop a vertically oriented morphology and start their ascent once reaching their mature sizes. Under an increasing measure of anisotropy, the bubbles grow preferentially horizontally. These bubbles can coalesce with neighboring ones and form interconnected permeable horizontal gas networks, as observed in some lab experiments. For the first time, our results suggest that anisotropy-led lateral bubble growth can also play a crucial role in accumulating gas reserve from long distances around large and small scale outlets in aquatic sediments. Moreover, in contrast to the vertical bubbles, horizontal bubbles tend to be stationary, thus being responsible for high gas storage capacity of anisotropic sediments.

Sediment deformation atop the Lomonosov Ridge, central Arctic Ocean: Evidence for gas-charged sediment mobilization?

We have used a hovercraft platform drifting with the sea ice to acquire the first digitally recorded seismic reflection data transects across the Canada/Greenland (89°N-85°N) section of the Lomonosov Ridge, central Arctic Ocean. The flat-lying, laterally uniform Cenozoic sediment package on top of the ridge at 87°N, 60° W shows at least four sites with local seismic amplitude anomalies. The common feature is a column (<600 m wide) of partly discontinuous or chaotic bright reflection events at the center of a <1.5 km wide dome (amplitude <25 m) terminating at the seabed in a 8–12 m deep depression. The amplitude anomalies are interpreted as gas-charged fluid escape pipes marked by a pockmark at the seabed. Gas and fluids introduced from below have mobilized the overlying high porosity, low density Eocene bio-siliceous ooze causing the doming. The gas and fluids appear to originate from the top of rotated fault blocks and sub-basalt sediments of Mesozoic or older age deposited when the Lomonosov Ridge was part of the pre-Late Cretaceous continental margin north of Franz Josef Land.

Shallow seismic characteristics and distribution of gas in lacustrine sediments at Lake Erçek, Eastern Anatolia, Turkey, from high-resolution seismic data

The high-resolution analysis of single-channel, seismic reflection data from Lake Erçek (Eastern Anatolia) revealed a wide range of shallow gas anomalies consisting of enhanced reflections, seismic chimneys, acoustic blanking/acoustic turbidity, strong reflectors, and pockmarks, including both surface and buried pockmarks. The enhanced reflections are represented by the higher amplitude reflection patterns resulting from high acoustic impedance variations. They are mostly clustered in the NW-corner of the lake. Seismic chimneys are represented by vertical and thinned columnar disturbances of amplitude blanking and mostly occurred in deep basinal and faulted sections in the West and East of the lake. Some seismic chimneys, occurring together with pockmarks, represent vertical vent activations. Acoustic gas masking was represented by chaotic and diffuse seismic reflection patterns, including acoustic blanking and acoustic turbidity. As diffuse acoustic turbidity indicates gas-charged sediments, columnar disturbances showing acoustic blanking indicate degassing of the sediments. These features extend from SE to NW, coinciding with the deep basin morphology of the lake. A very local strong reflector was identified in the W-section of the lake, simulating the lake floor. This reflector is due to extended enhanced reflections, suggesting shallow free gas. Pockmarks observed in the lake are structurally classified into the two distinct types; surface (active) pockmarks found in the SE-part of the lake and buried (passive) pockmarks found in the NW. The former enlarge through deeper gas reservoir feedback, as the layering is impermeable, while the latter have resulted from a cessation of the reservoir feedback mechanism and/or permeable layering. In the lake, shallow gas distribution is controlled by faults, that provide the faulting-driven depositional control and earthquakes, that provide the seismicity-driven overpressure control. The shallow gas is then vertically–horizontally distributed and shaped by asymmetric depositional–stratigraphic factors. This study of Lake Erçek presents complementary information about a possible tectono-thermal origin of observed shallow gas.

Seismic Anisotropy Within an Active Fluid Flow Structure: Scanner Pockmark, North Sea

Understanding sub-seabed fluid flow mechanisms is important for determining their significance for ocean chemistry and to define fluid pathways above sub-seafloor CO2storage reservoirs. Many active seabed fluid flow structures are associated with seismic chimneys or pipes, but the processes linking structures at depth with the seabed are poorly understood. We use seismic anisotropy techniques applied to ocean bottom seismometer (OBS) data, together with seismic reflection profiles and core data, to determine the nature of fluid pathways in the top tens of meters of marine sediments beneath the Scanner pockmark in the North Sea. The Scanner pockmark is 22 m deep, 900 m × 450 m wide and is actively venting methane. It lies above a chimney imaged on seismic reflection data down to ∼1 km depth. We investigate azimuthal anisotropy within the Scanner pockmark and at a nearby reference site in relatively undisturbed sediments, using the PS converted (C-) waves from a GI gun source, recorded by the OBS network. Shear-wave splitting is observed on an OBS located within the pockmark, and on another OBS nearby, whereas no such splitting is observed on 23 other instruments, positioned both around the pockmark, and at an undisturbed reference site. The OBSs that show anisotropy have radial and transverse components imaging a shallow phase (55–65 ms TWT after the seabed) consistent with PS conversion at 4–5 m depth. Azimuth stacks of the transverse component show amplitude nulls at 70° and 160°N, marking the symmetry axes of anisotropy and indicating potential fracture orientations. Hydraulic connection with underlying, over pressured gas charged sediment has caused gas conduits to open, either perpendicular to the regional minimum horizontal stress at 150–160 N or aligned with a local stress gradient at 50–60 N. This study reports the first observation of very shallow anisotropy associated with active methane venting.

The interplay of stratal and vertical migration pathways in shallow hydrocarbon plumbing systems

AbstractHydrocarbon 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.

Assessment of gas hydrate using prestack seismic inversion in the Mahanadi Basin, offshore eastern India

The Indian National Gas Hydrate Program Expedition-01 in 2006 has discovered gas hydrate in the Mahanadi offshore basin along the eastern Indian margin. However, well-log analysis, pressure core measurements, and infrared anomalies reveal that gas hydrates are distributed as disseminated within the fine-grained sediment, unlike massive gas hydrate deposits in the Krishna-Godavari Basin. The 2D multichannel seismic section, which crosses holes NGHP-01-9A and 19B located at approximately 24 km apart, indicates a continuous bottom-simulating reflector (BSR) along it. We aim to investigate the prospect of gas hydrate accumulation in this area by integrating well-log analysis and seismic methods with rock-physics modeling. First, we estimate gas hydrate saturation at these two holes from the observed impedance using the three-phase Biot-type equation. Then, we establish a linear relationship between the gas hydrate saturation and the impedance contrast with respect to the water-saturated sediment. Using this established relation and impedance obtained from prestack inversion of seismic data, we produce a 2D gas hydrate-distribution image over the entire seismic section. The gas hydrate saturation estimated from resistivity and sonic data at well locations varies within 0%–15%, which agrees well with the available pressure core measurements at hole 19. However, the 2D map of gas hydrate distribution obtained from our method indicates that the maximum gas hydrate saturation is approximately 40% just above the BSR between the common-depth points of 1450 and 2850. The presence of gas-charged sediments below the BSR is one of the reasons for the strong BSR observed in the seismic section, which is depicted as low impedance in the inverted impedance section. Closed sedimentary structures above the BSR are probably obstructing the movements of free-gas upslope, for which we do not see the presence of gas hydrate throughout the seismic section above the BSR.

An improved specimen preparation method for marine shallow gas-bearing sand sediments and its validations

Instabilities of shallow gas-charged seabed are potential geological hazards in ocean engineering. In practice, the conventional field sampling techniques failed to obtain undisturbed gas-bearing sediments from the seabed for laboratory mechanical testing because of sensitive gas exsolution and escape from sediments. However, preparation of representative remoulded gas-charged specimens is a challenging issue, because it is rather difficult to quantitatively control the gas content and obtain uniform distribution of gas bubbles within the specimen. Given the above problems, this work proposes a reliable approach to reconstitute the high-saturation specimen of gas-charged sediments in the laboratory by an improved multifunction integrated triaxial apparatus (MITA). This apparatus is developed based on an advanced stress path triaxial system by introducing a temperature-controlled system and a wave-monitoring system. The temperature-controlled system is used to accurately mimic the in situ environments of sediments in the seabed. The wave-monitoring system is utilized to identify exsolution point of free gas and examine the disturbance of gas to specimens during gas exsolution. The detailed procedure of gassy specimen preparation is introduced. Then, the quality of prepared specimens using our improved apparatus is validated by the high-resolution micro-X-ray computed tomography (μCT) scanning test, from which bubble occurrence and size distribution within the gassy sand specimen can be obtained; and preliminary mechanical tests on gassy sand specimens with various initial saturation degrees are performed. The proposed specimen preparation procedure succeeds in proving the postulated occurrence state of gas bubbles in coarse-grained sediments and accurately controlling the gas content.

Observation of Diffusive Methane Emission from Holocene Mud Deposits on the Continental Shelf Offshore Southeastern Korea

Chun, J.-H.; Kim, Y.; Choi, J.-Y.; Kim, Y.J.; Lee, H.-J., and Kim, Y.H., 2020. Observation of diffusive methane emission from Holocene mud deposits on the continental shelf offshore southeastern Korea. In: Jung, H.-S.; Lee, S.; Ryu, J.-H., and Cui, T. (eds.), Advances in Geospatial Research of Coastal Environments. Journal of Coastal Research, Special Issue No. 102, pp. 127-136. Coconut Creek (Florida), ISSN 0749-0208.This study investigated gas-charged sediments found in Holocene mud deposits on the inner continental shelf offshore southeastern Korea. Headspace gases obtained from seven piston cores of Holocene mud deposits were analyzed using gas chromotography, and methane dissolved in the water column were analyzed using in situ methane sensors and gas chromotography. The methane concentration data from both sediments and water columns provide the evidences of diffusive methane emission from the inner continental shelf. The methane concentration of headspace gases ranged from 22.2 to 123.9 mM below the upper front of acoustic blanking zone (ABZ). The maximum anomaly of dissolved methane in the water column was 38.3 nM at a site 5 over rough seafloor in the ABZ, which is surrounded by water masses of relatively lower methane concentration. Dissolved methane concentrations were significantly lower (< 10 nM) in the upper water column (< 20 m water depth). Concentrations of dissolved methane within the whole water column in the gas-free zone (GFZ) were < 5 nM. These results imply that diffusive methane emission occurred from a rough seafloor with erosional features on the inner continental shelf at water depth of about 54 m; this diffusive methane emission was not related to high methane concentrations in piston core sediments of the shallow upper front of the ABZ.

Assessment of gas hydrate reservoir from inverted seismic impedance and porosity in the northern Hikurangi margin, New Zealand

Hikurangi margin, New Zealand, the shallowest subduction zone on Earth is the unique place to study the slow slip creep-like deformation and seafloor failure, and their probable link to the presence of gas hydrate, cold seeps and fluid migration. International Ocean Discovery Program expedition 372 discovered gas hydrate within thin intervals (ranging from ~5 to 25 m) of the deformed sediments in the Tuaheni landslide complex area of the northern Hikurangi margin. Seismic profiles show typical BSRs (bottom simulating reflectors) cutting across the dipping strata throughout the sections. To characterize the reservoir, here, we invert both acoustic impedance (product of velocity and density) and porosity along two perpendicular seismic profiles crossing the well using a model-based post-stack seismic inversion technique. Results show the layer structure of alternate high-impedance with low-porosity and low-impedance with high-porosity sediments. Interestingly, each layer is consisting of thin and relatively low impedance intra-layers, which clearly indicates the possible pathways for fluid and gas migration, and one of the reasons for seafloor collapse in this region. Very low impedance observed below the BSR is due to the gas-charged sediments at the dipping strata, and is the probable source of free-gas migrating upward. Next, we apply rock physics theory to know the distribution of gas hydrate at Site U1517 as well as along two perpendicular seismic profiles. Rock physics modeling using sonic velocity at well location shows that gas hydrate is distributed mainly within the depth intervals of 5–50 m, 90–115 m and 130–155 m with an average saturation of about 10–15% and the maximum concentration of about 35% of the pore space at 100 m depth. Prediction of gas hydrate saturation from the resistivity method using neutron porosity and NMR (nuclear magnetic resonance) logs in Archie's empirical formula shows less saturation compared to that from the sonic log. Whereas, estimated gas hydrate saturation from the chloride concentrations ranges from 0 to 45% of the pore space. The spatial distributions of gas hydrate along seismic profiles are predicted by rock physics theory using the inverted impedance and porosity. Saturation of gas hydrate along the seismic profiles varies from 0 to 40% with an average of ~10% of the pore space. The correlation between inverted impedance and porosity indicates that the high impedance layers are due to low porosity consolidated sediment without significant gas hydrate concentration. Gas hydrate is distributed in relatively high porosity and low impedance layers along the seismic profiles. Our result shows that the amount of gas hydrate concentration is low in this region and sediments are highly deformed/disturbed in the presence of many fluid/gas migration pathways.

Sound Velocity in a Thin Shallowly Submerged Terrestrial-Marine Quaternary Succession (Northern Adriatic Sea)

Estimating sound velocity in seabed sediment of shallow near-shore areas submerged after the Last Glacial Maximum is often difficult due to the heterogeneous sedimentary composition resulting from sea-level changes affecting the sedimentary environments. The complex sedimentary architecture and heterogeneity greatly impact lateral and horizontal velocity variations. Existing sound velocity studies are mainly focused on the surficial parts of the seabed sediments, whereas the deeper and often more heterogeneous sections are usually neglected. We present an example of a submerged alluvial plain in the northern Adriatic where we were able to investigate the entire Quaternary sedimentary succession from the seafloor down to the sediment base on the bedrock. We used an extensive dataset of vintage borehole litho-sedimentological descriptions covering the entire thickness of the Quaternary sedimentary succession. We correlated the dataset with sub-bottom sonar profiles in order to determine the average sound velocities through various sediment types. The sound velocities of clay-dominated successions average around 1530 m/s, while the values of silt-dominated successions extend between 1550 and 1590 m/s. The maximum sound velocity of approximately 1730 m/s was determined at a location containing sandy sediment, while the minimum sound velocity of approximately 1250 m/s was calculated for gas-charged sediments. We show that, in shallow areas with thin Quaternary successions, the main factor influencing average sound velocity is the predominant sediment type (i.e. grain size), whereas the overburden influence is negligible. Where present in the sedimentary column, gas substantially reduces sound velocity. Our work provides a reference for sound velocities in submerged, thin (less than 20 m thick), terrestrial-marine Quaternary successions located in shallow (a few tens of meters deep) near-shore settings, which represent a large part of the present-day coastal environments.

Tectonic and structural controls on Neogene fluid release in the Patagonian Continental Margin

Abstract Analysis of high-resolution multi-beam bathymetry, 2D multi-channel seismic, and high-resolution seismic sub-bottom profiles revealed the presence of widespread fluid escape features in the middle slope of the Patagonian Continental Margin. On the sea-bottom, these features correspond to pockmarks and mud volcanoes, whereas in the sub-surface they are represented in the seismic records by several acoustic anomalies such as chimneys, acoustic blanking, enhanced reflectors, and reverse-phase enhanced reflectors among others. Some of these acoustic signatures can be traced to syn-rift deposits that fill a deep re-activated and inverted graben. Analysis of pockmarks elongation and pockmarks alignments show a good correlation with inverted normal faults, suggesting that faults and fractures have influenced the pockmark's shape and might have acted as pathways for upward fluid migration. Some of the acoustic anomalies associated with pockmarks are interpreted as evidence of gas. The gas observed in the seismic data seems to be thermogenic, and the seismic data suggests a deep origin associated with over-pressured syn-rift deposits. Two morphometrically different sets of pockmarks were identified: an older set hosted by Miocene aged rocks, and a younger set hosted by Quaternary deposits. Different potential triggers are discussed for the genesis of the Miocene pockmarks in relation to the seismostratigraphy, structural geology and regional tectonics. It is concluded that, tectonic activity associated with the Neogene inversion of the graben faults, due to Andean compression, is the most likely cause for the formation these pockmarks. The presence of gas charged sediment and young pockmarks also suggest that after the Middle Miocene tectonic climax, a more recent pulse of release of fluids occurred.

A hybrid discrete bubble-lattice Boltzmann–discrete element model for gas-charged sediments

This paper presents a hybrid discrete bubble-lattice Boltzmann–discrete element modelling framework for simulating gas-charged sediments, especially in the seabed. A discrete bubble model proposed in chemical engineering is adapted in the coupled discrete element/lattice Boltzmann method to model the migration of gas bubbles in saturated sediments involving interactions between gas bubbles and fluid/solid phases. Surface tension is introduced into the discrete bubble model in this work, so that it can handle the complex gas–fluid–solid interface. The lattice Boltzmann and discrete element methods are, respectively, employed to simulate fluid flows and mechanical behaviours of sediments. A velocity interpolation-based immerse boundary method is utilised to resolve the coupling between the fluid flow and the solid/gas phase. The proposed technique is preliminarily validated using simulations of bubble migration in fluids, which is followed by high-resolution investigations of the transport of a gas bubble in seabed sediments. It is demonstrated that this hybrid method can reproduce, to a certain degree, the characters of bubbles moving in seabed sediment tests.

Origin and Distribution of Shallow Gas-Charged Sediment on the Inner Continental Shelf of Central West Coast of India

Origin and Distribution of Shallow Gas-Charged Sediment on the Inner Continental Shelf of Central West Coast of India

High-resolution impedance estimation using refraction and reflection FWI constraints: the Fortuna region, offshore Equatorial Guinea

Seismic imaging and reservoir characterization in the Fortuna region, offshore Equatorial Guinea, is beset with various geophysical challenges related to the presence of extensive, but small-scale low-velocity gas pockets, which give rise to significant and cumulative image distortion at target level. This distortion had not been resolved in a vintage 2013 broadband pre-stack depth migration project, as the velocity model was not sufficiently well resolved, but was subsequently addressed successfully in a project conducted using high-resolution non-parametric tomography with improved broadband deghosted data. The primary objective of that subsequent project was to improve the understanding of the internal structure of the Viscata and Fortuna reservoirs, and this objective was met via clearer internal imaging of these reservoir intervals and the overlying gas-charged sediments. The follow-on work considered here deals with the use of full waveform inversion to further delineate small-scale velocity anomalies associated with the highly compartmentalized reservoir units, and also to use the full waveform inversion velocity model as a constraint during acoustic impedance inversion. We compare the results of impedance inversion using both a conventional approach (with the well-log velocities to build the background trend), with a new approach using a trend derived from high-resolution waveform inversion velocities.

Integration of 3D seismic attributes for preliminary shallow geohazard identification in deep water exploration area with no well data

Drilling in the deep water is very challenging as the safety of drilling rigs and other installations could be threatened by the presence of shallow geohazards. These hazards are a global problem, and while the industry has matured in handling these problems, significant losses owing to the improper assessment of the geohazards prior to drilling have been widely reported (Campbell, 1999). In the Gulf of Mexico (GoM) alone, it was estimated that the associated losses continue to be more than $1.7 million per well (Dutta et al., 2010). ISO 17776 defines a hazard as a ‘potential source of harm’, thus shallow geohazards in the context of offshore drilling activities can be defined as local and/or regional shallow geological features having potential to cause loss of life or damage to health, environment or assets. The geohazards characteristics vary from place to place depending on the regional geology, tectonic history and sedimentation pattern. The US Department of Interior classify the hazards into two categories: 1. Sea-floor geohazards which include fault scarps, gas vents, hydrate mounds, unstable slopes, slumping, active mud gullies, crown cracks, collapse depressions, furrows, sink-holes, mass sediments movements, surface channels, pinnacles and reefs. 2. Subsurface geohazards include faults, gas-charged sediments, abnormal pressure zones, gas hydrates, shallow water-flow sands and buried channel.