Gas Chimneys Research Articles

Geological occurrence and accumulation mechanism of natural gas hydrates in the eastern Qiongdongnan Basin of the South China Sea: Insights from site GMGS5-W9-2018

Seismic and logging while drilling (LWD) data, gas hydrate samples, and hydrate gas geochemical testing results acquired during the GMGS5 expedition in 2018 have been used to explore the geological occurrence and accumulation mechanism of gas hydrates in the eastern Qiongdongnan Basin (QDNB) of the northern South China Sea (SCS). A large gas chimney with diameter of 4 km associated with the Songnan Low Uplift and the Central Channel and characterized by acoustic blanking on the seismic profile was identified at site GMGS5-W9-2018 in the deep water QDNB. Seismic indications for hydrocarbon migration and gas hydrate accumulation including low frequency features, enhanced reflections, pull-ups/downs and the bottom simulating reflector (BSR) were recognized within the chimney area. Gas hydrates were confirmed to be deposited at the depth of 7–158 mbsf based on the coring and sampling results and pilot hole LWD anomalies, which showed a high resistivity, low density, high gamma rays, and elevated acoustic velocity. Leakage type gas hydrates with multiple geomorphology occurrence, including massive, layers, nodules, fracture filling and disseminated, were recovered at the top of the gas chimney, suggesting favorable conditions for the accumulation of gas hydrates in the eastern QDNB. Although methane was the dominant hydrocarbon gas in all hydrate-bound gases, heavier hydrocarbons (C2+) were also prevalent. C2–C5 hydrocarbons made up to ~21% of the hydrocarbon gases, indicating their thermogenic origin and close relationship with deep reservoirs. Due to the presence of the C2+ hydrocarbons, structure I and structure II gas hydrates may coexist at site GMGS5-W9-2018. The base of structure II gas hydrate stability zone (BSIIGHSZ) was ~42–44 m deeper than the base of structure I gas hydrate stability zone (BSIGHSZ). The hydrocarbon migration and hydrate accumulation were closely related to the gas chimney in conjunction with the Songnan Low Uplift and the Central Channel, which transported the deep thermogenic gas to the GHSZ, forming hydrates in the fractured clay-dominated fine-grained sediments. The fractures occurred in the strata were likely driven by pneumatic forces. Overpressure in the sediments derived from gas charging and accumulation through the gas chimney may have caused the fractures in the sediments that dissipated the hydrocarbons into the GHSZ. We propose a model of gas hydrate accumulation that may aid in the future exploration of gas hydrates in the QDNB of the SCS.

One North Sea fairway analysis: revealing opportunities through data integration across scales

Abstract While the North Sea has many well-established prolific plays, the downwards trend of annual discovered resources has cast doubt on the continued ability of the stratigraphy to provide world-class future discoveries. However, despite this downwards trend, significant discoveries can still be made, such as Johan Sverdrup (Norwegian North Sea in 2010). Furthermore, new sub-plays within this petroleum system and new unproven plays with alternative source rocks are still being tested, although often with associated higher perceived pre-drill subsurface risk (Pg) and/or lower volume potential. There is room for new optimism, however, as new approaches and technologies are deployed, which could lead to future success. The analysis of regionally extensive integrated datasets allows for the interpretation of ‘One North Sea’ fairway maps that honour detailed observations from the prospect scale through to the play scale and beyond. By integrating source-rock maturity, hydrocarbon expulsion and migration, well shows, seismically defined gas chimneys, and direct hydrocarbon indications onto regional fairway maps, hydrocarbons can be traced from the source kitchens to the traps. By utilizing a zoom-in/zoom-out methodology, models can be developed, tested and applied across the basin, allowing new ideas to emerge, de-risk established prospects and reveal under-appreciated hydrocarbon volumetric upsides.

Late Cretaceous–Cenozoic sequence stratigraphic and paleogeographic controls on petroleum system elements of the Nicaraguan platform, western Caribbean Sea

Late Cretaceous–to–present-day mixed carbonate–clastic deposition along the Nicaraguan platform, western Caribbean Sea, has evolved from a tectonically controlled, rifted upper Eocene shallow–to–deep-marine carbonate–siliciclastic shelf to an upper Miocene–to–present-day tectonically stable shallow-marine carbonate platform and passive margin. By integrating subsurface data of 287 two-dimensional seismic lines and 27 wells, we interpret the Cenozoic stratigraphic sequence as 3 cycles of transgression and regression beginning with an upper Eocene rhodolitic–algal carbonate shelf that interfingered with marginal siliciclastic sediments derived from exposed areas of Central America bordering the margin to the west. During the middle Eocene, a carbonate platform was established with both rimmed reefs and isolated patch reefs. A late Eocene forced regression produced widespread erosion and subaerial exposure across much of the platform and was recorded by a regional unconformity. The Oligocene–upper Miocene sedimentary record includes a southeastward prograding delta of the proto-Coco river, which drained the emergent area of what is now northern Nicaragua. The late Miocene–to–present-day period marks a period of strong subsidence with the development of small pinnacle reefs. We describe favorable petroleum system elements of the Nicaraguan platform that include (1) Eocene fossiliferous limestone source rocks documented as thermally mature in vintage exploration wells and seen as active gas chimneys emanating from inferred carbonate reservoirs; (2) upper–to–middle Eocene reservoirs in patch and pinnacle reefs, middle Eocene calcareous slumps, and Oligocene fluvial-deltaic facies documented in wells; and (3) regional seal intervals that consist of both regional unconformities and Eocene–Oligocene intraformational shale.

Characteristics and dynamics of gas hydrate systems in the northwestern South China Sea - Results of the fifth gas hydrate drilling expedition

Abstract In 2018, Guangzhou Marine Geological Survey conducted the fifth gas hydrate drilling expedition (GMGS5) in the northwestern continental slope of South China Sea. We use pore water geochemistry, pressure core CT scanning, degassing quantification, seismic data and in situ temperature test at four drill sites (W01, W07, W08 and W09) to investigate the gas hydrate characteristics and discuss the dynamics of the gas hydrate system. Based on the CT scanning and pressure core degassing, we demonstrate the coexistence of pore-filling and vein-like gas hydrates. Gas hydrate saturation estimated based on the chloride concentration (0–92%) is higher than that based on the degassing quantification (0–52.9%). The saturation discrepancy is mainly resulted from the sampling discrimination. Based on the average gas hydrate saturation estimated using chloride concentration and degassing, the methane gas volume contained in the gas hydrate of one cubic meter sediment at drill sites W07, W08 and W09 are calculated to be 13.7 m3, 26.82 m3 and 22.24 m3 and 3.03 m3, 14.30 m3 and 16.25 m3. The geothermal gradients estimated based on the in situ temperature measurements at sites W07, W08 and W09 (102–111 °C/km) are more than 50% higher than that at the background site W01 (65 °C/km). It is inferred that the fractures and faults in the gas chimney at sites W07, W08 and W09 are the major contributors for the efficient fluid pathways and high geothermal gradients. The pore water sulfate profiles and inferred sulfate-reduction methane-oxidation interface (SMI) depths indicate that methane flux is lowest at site W01 and highest at site W08. The SMI depths and top of gas hydrate occurrence is linearly correlated which provides an experienced function for understanding the local gas hydrate distribution. Kick-type sulfate profiles at sites W07 and W09 indicate an ongoing increase of methane flux and corresponding shoaling SMI depths. In addition, chloride enrichment in the shallow sediment at site W08 indicates a recent fast gas hydrate formation process. The kick-type sulfate profiles, positive chloride anomalies and authigenic carbonates at multiple depths suggest that the system experienced dynamic changes in fluid flux and multi-stage gas hydrate evolution on time scale from months to thousands of years. Understanding the controlling mechanism and periodicity of the composite pressure system is important for evaluating the gas hydrate resource and carbon cycling.

Experimental investigation on biomass fired dryer for drying of agricultural products

This study focuses on the development and performance evaluation of a biomass-fired dryer. The system was designed after considering the needs of small-scale rural entrepreneurs. The designed system consisted of the combustion chamber, drying chamber, the passage of flue gases, flue gas chimney, etc. The thermal performance of the developed system was tested in terms of moisture removal rate, drying rate, thermal efficiency, and heat transfer efficiency. In order to check performance, the dryer was loaded with 4 kg of Coriander leaves having an initial moisture content of 89.40% (w.b.) were dried to the final moisture content of 9.54% (w.b.) in 7 h of operation, 6 kg of babul wood (Acacia nilotica) was consumed during the drying process. The average drying chamber temperature was 51.84°C. The thermal and heat transfer efficiency of the dryer was 28.22 and 56%, respectively. The economic parameters reveal that the developed system is technically viable and economically feasible as the payback period is very low. The net present worth, benefit-cost ratio and payback period were 3398 US$, 2.68 and 0.38 years, respectively.

Gas chimney and hydrocarbon detection using combined BBO and artificial neural network with hybrid seismic attributes

The exact interpretation of structure of the Earth is essential to avoid drilling of dry holes or locating hydrocarbon reservoirs in faulty regions. The economic approaches are required to locate the reservoirs without damaging the tectonic plates of the Earth. The identification of gas chimneys provides an impressive approach to indirectly interpret the hydrocarbon reservoirs as the chimneys form the migration pathway for gas from reservoirs, and it looks like a gas cloud in the seismic data. The proposed method using biogeography-based optimization (BBO) with supervised neural networks enables the effective interpretation of chimneys and hydrocarbons. Seismic attributes are measured on the pre-processed seismic data, where the attributes play an important role in providing qualitative information on structure of the Earth. Continuity, instantaneous and amplitude attributes are measured to provide the details on the areas of discontinuity, bright-spot regions with high amplitude and low frequency. The calculated attributes are picked from selected locations in the pre-processed seismic data, and the hybrid combination of attributes enables effective, economic chimney identification and provides indications to locate the reservoirs. The redundant and highly correlated features are eliminated using BBO which in turn improves the classification accuracy. BBO algorithm retains the best solution in each generation, and the fitness of the algorithm is verified with supervised neural networks. The proposed algorithm is applied on F3 block dataset, and BBO selects 20 predominant features among 75 features. The optimal solution after 50 generations enables the classification of chimneys through multilayer feed-forward supervised neural network. The results indicate that the chimney classification accuracy is improved by 90%, and the mean square error is minimized with BBO as a feature selection algorithm compared to other evolutionary algorithms.

Acoustic and biological characteristics of seafloor depressions in the North Yellow Sea Basin of China: Active fluid seepage in shallow water seafloor

High resolution multibeam bathymetry revealed the occurrence of numerous craterlike depressions, so-called pockmarks, in the sea floor of the North Yellow Sea Basin (NYSB). To investigate whether these pockmarks are related to any type of fluid flow or gas seepage from subsurface stratum, acoustic survey and box samplings were conducted in a selected area containing several typical pockmarks. This allowed for an integrated analysis of the morphology, distribution, and subsurface strata as well as a direct comparison of seafloor characteristics among different areas inside and outside of a pockmark. Additionally, surface sediments were analyzed for particle structure and grain size. Based on the combination of multibeam bathymetry and backscatter intensity data, 282 seafloor depressions were accurately identified. Moreover, the formation of pockmark can be attributed to existence of gas chimneys and other shallow gases revealed by shallow seismic profile records in the subsurface strata. Sediments inside and outside the pockmark showed remarkable discrepancies in terms of grain size, and benthic fauna. Additionally, strong internal reflection area was observed inside the depressions where benthic habitats are abundant and sediments contain strawberry pyrite and carbonate cementation. Side scan sonar also recorded sediment plumes in the water column, which indicates that the pockmarks are still active. Several “pearl-string-like” accumulations of pockmark were found in the study area, which well agree with the distribution of buried channels and lakes. These results thus provide new insights in interpreting shallow water fluid seepage, with implications for our current knowledge of overall sedimentation process in offshore continental shelves.

Seismic analysis of the gas hydrate system at Pointer Ridge offshore SW Taiwan

Pointer Ridge is a gas hydrate prospect on the South China Sea continental slope offshore SW Taiwan. It is characterized by densely distributed bottom simulating reflections (BSRs), active gas seepage, and potential sandy gas hydrate reservoirs. To understand how the fluids have migrated toward the seafloor, and the role of geological processes in the gas hydrate system, we have collected and analyzed high-quality 2D and 3D reflection seismic data. We first mapped the spatial distribution of the BSRs, and interpreted a major normal fault, Pointer Ridge Fault (PR Fault). The NE-SW trending fault dips to the east, and separates the erosional regime to the west from the depositional regime to the east. One active vent site was identified directly above the PR Fault, while another is located on a topographic high to the west of the fault. On the hanging block of the fault we found at least one major unconformity. The seismic data indicate refilled channels with coarser-grained sediments in the hanging wall of the normal fault. Seismic attribute analysis shows subsurface fluid conduits and potential gas hydrate reservoirs. We propose two types of gas chimneys, which are separated by the fault. Gas plumes derived from hydroacoustic data are mostly from the footwall block of the fault. We infer that fluid flow is more active in the erosional environment compared to the depositional one, and that this is the result of reduced overburden. The methane-bearing fluids migrate upward along the PR Fault and chimneys and form hydrates above the base of the gas hydrate stability zone. Based on seismic interpretation and seismic attribute analysis, we postulate that the channel infill constitutes the most promising hydrate reservoirs in this geological setting. In the surveyed area of Pointer Ridge these channels occur mainly below the gas hydrate stability zone.

Analysis of the Oldest Carbonate Gas Reservoir in China—New Geological Significance of the Dengying Gas Reservoir in the Weiyuan Structure, Sichuan Basin

The Weiyuan Structure is the largest surface structure in the Sichuan Basin. However, the abundance of the Dengying Formation gas reservoir in the Weiyuan Structure is low. The height of the gas column is 244 m, but the integrated abundance is only 26.4%. After nearly 40 years of exploration, the Gaoshi1 Well and Moxi8 Well yielded gas flows that marked an important exploration success after the discovery of the Sinian Dengying Formation gas reservoir in the Weiyuan Structure, Sichuan Basin, Lower-Paleozoic in 1964. Combined with research examples of oil and gas migration and gas chimneys around the world, the authors used comprehensive geological-geophysical-geochemical research methods to provide a reasonable explanation of the low abundance of the gas reservoir in the Weiyuan Structure based on the surface and subsurface data. The latest research results show that (1) currently, the Weiyuan Structure is the apex of the Dengying Formation in the Mid-Sichuan Basin. The Guang’an, Longnusi, Gaoshiti-Moxi, and Weiyuan structures are a series of traps in the Dengying Formation with gradual uplifting spill and closure points during the regional uplift of the Himalayan period. The natural gas of the Dengying Formation accumulated in different ways over a wide range and long distance in the Sichuan Basin. (2) At approximately 40 Ma, the Weiyuan area started to uplift and form the present structure, and it is the only outcropped area with the Triassic Jialingjiang Formation and Leikoupo Formation in the surface of the Sichuan Basin (except the steep structural belt in East Sichuan). Caused by the uplift and denudation, the core of the Weiyuan Structure has formed an escaping “skylight” for natural gas. The evidence of a gas chimney includes (1) the component percentage of non-hydrocarbon gas, which decreased from the bottom to the top, (2) the pressure coefficient is normal because the gas reservoir from the Upper Sinian to the Lower Permian commonly have a normal pressure coefficient (an average of 1.0), and (3) the isotope geochemistry of the argon mostly represents abiogenic characteristics of a deep source, and the 40Ar/36Ar ratio is as high as 2 855–5 222 in the Upper Permian. All of these characteristics provide sufficient evidence for a gas chimney effect. The characteristics of low abundance in the Weiyuan Structure can be a reference example for studying the late reconstruction of deep oil and gas reservoirs in the superimposed basins of western China.

Complex gas hydrate system in a gas chimney, South China Sea

Abstract Gas hydrates and deep-sea cold seeps are of interest to industry and academia for several reasons, including their implications for offshore geohazards and unconventional resources, and for global climate change and carbon cycling. Guangzhou Marine Geological Survey (GMGS) implemented its fifth gas hydrate drilling expedition in the QiongDongNan Sea Area (QDN-SA) of the northern South China Sea (SCS) from June to September 2018. Multidisciplinary investigations, including Logging While Drilling (LWD), pressure and non-pressure coring, gas composition analyses, in situ temperature and permeability measurements, pore water geochemical analyses, and a regional seismic survey, reveal a complex distribution of gas hydrate within a vertical chimney structure. LWD profiles record high resistivity anomalies and high P-wave velocity variations at 9–174 mbsf, suggesting the coexistence of free gas and gas hydrate in the sediment column throughout the investigated gas chimney. Hydrate-bound and void gas is an admixture of methane (70.1%–98.3%) and heavier hydrocarbons (C2H6: 0.16%–17.59%, C3H8: 0.05%–9.88%, i-C4H10:

Bright Spot Validation from 3-D Seismic Data: A Study of Naso-Field, Niger Delta Province, Nigeria

The validation of identified bright-spots A, B and C on 3D seismic data as Direct Hydrocarbon Indicator (DHI) in Naso field, Niger Delta was achieved through the integration of four surface attributes. These attributes include root mean square, average reflection strength, total energy, and total amplitude which are all amplitude dependent, where region of high value in every attribute indicated possible hydrocarbon prospects. Seismic data and check-shot data were used, which provided positive input for velocity extraction and structural mapping interpretation. A series of NW-SE trending normal growth faults A, B & C were interpreted defining the reservoir structural framework while fault D, E, F & G are post-depositional minor faults which also supported the trapping mechanism. Bright-spot A was mapped between 2,400-2550 milliseconds, while bright-spot B was mapped between 2,600-2,720 milliseconds and bright-spot C between 2,750-3,050 milliseconds. Two hydrocarbon prospects were identified. Prospect-1 was estimated to cover about 907.17, 1323.59 and 1075.44 acres, with subsurface total vertical depth of 9514.3ft, 10499.6ft, 10963ft in BS_A, B & C respectively. Prospect-2 covered about 653.49, 1080.33 and 493.77 acres with subsurface total vertical depth of 8650ft, 9525ft, and 10120ft, respectively. Hence, prospect-1 is considerably more prolific in hydrocarbon yield than prospect-2. The total area of the hydrocarbon prospects in-place has been estimated to about 5,533.79 acres which is likely to yield a prolific reserve for commercial purpose. The gas Chimney and push down highlight the subsurface leakage of gas from a poorly sealed cap rock and show a deep petroleum migration process. The study has been able to validate bright spot during exploration studies in effect to evaluate hydrocarbon reservoir, prospect identification and reservoir prediction.

Geological Controls on Fluid Flow and Gas Hydrate Pingo Development on the Barents Sea Margin

AbstractIn 2014, the discovery of seafloor mounds leaking methane gas into the water column in the northwestern Barents Sea became the first to document the existence of nonpermafrost‐related gas hydrate pingos (GHPs) on the Eurasian Arctic shelf. The discovered site is given attention because the gas hydrates occur close to the upper limit of the gas hydrate stability, thus may be vulnerable to climatic forcing. In addition, this site lies on the regional Hornsund Fault Zone marking a transition between the oceanic and continental crust. The Hornsund Fault Zone is known to coincide with an extensive seafloor gas seepage area; however, until now lack of seismic data prevented connecting deep structural elements to shallow seepages. Here we use high‐resolution P‐Cable 3‐D seismic data to study the subsurface architecture of GHPs and underlying glacial and preglacial deposits. The data show gas hydrates, authigenic carbonates, and free gas within the GHPs on top of gas chimneys piercing a thin section of low‐permeability glacial sediments. The chimneys connect to faults within the underlying tilted and folded fluid and gas‐hydrate‐bearing sedimentary rocks. Correlation of our data with regional 2‐D seismic surveys shows a spatial connection between the shallow subsurface fluid flow system and the deep‐seated regional fault zone. We suggest that fault‐controlled Paleocene hydrocarbon reservoirs inject methane into the low‐permeability glacial deposits and near‐seabed sediments, forming the GHPs. This conceptual model explains the existence of climate‐sensitive gas hydrate inventories and extensive seabed methane release observed along the Svalbard‐Barents Sea margin.

A semi-supervised learning framework for gas chimney detection based on sparse autoencoder and TSVM

A semi-supervised learning framework for gas chimney detection based on sparse autoencoder and TSVM

Repeatability of high-resolution 3D seismic data

High-resolution 4D (HR4D) seismic data have the potential for improving the current state-of-the-art in detecting shallow ([Formula: see text] below seafloor) subsurface changes on a very fine scale (approximately 3–6 m). Time-lapse seismic investigations commonly use conventional broadband seismic data, considered low to moderate resolution in our context. We have developed the first comprehensive time -lapse analysis of high-resolution seismic data by assessing the repeatability of P-cable 3D seismic data (approximately 30–350 Hz) with short offsets and a high density of receivers. P-cable 3D seismic data sets have for decades been used to investigate shallow fluid flow and gas-hydrate systems. We analyze P-cable high-resolution 4D (HR4D) seismic data from three different geologic settings in the Arctic Circle. The first two are test sites with no evidence of shallow subsurface fluid flow, and the third is an active seepage site. Using these sites, we evaluate the reliability of the P-cable 3D seismic technology as a time-lapse tool and establish a 4D acquisition and processing workflow. Weather, waves, tide, and acquisition-parameters such as residual shot noise are factors affecting seismic repeatability. We achieve reasonable quantitative repeatability measures in stratified marine sediments at two test locations. However, repeatability is limited in areas that have poor penetration of seismic energy through the seafloor, such as glacial moraines or rough surface topography. The 4D anomalies in the active seepage site are spatially restricted to areas of focused fluid flow and might likely indicate changes in fluid flow. This approach can thus be applied to detect migration of fluids in active leakage structures, such as gas chimneys.

Formation of linear planform chimneys controlled by preferential hydrocarbon leakage and anisotropic stresses in faulted fine-grained sediments, offshore Angola

Abstract. 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.

Evaluation of sealing potential near gas chimneys–the Gippsland Basin, Australia

Gas chimneys are geological features that are associated with seal bypass systems. Knowledge of the capacity of the seal is important for the evaluation of the petroleum systems and Carbon Capture and Storage (CCS) studies. The seal is breached when the capillary pressure threshold is exceeded by the underlying buoyancy pressure. The chaotic seismic character that is generally associated with the gas chimneys has been studied previously to evaluate the origin and geometry of the chimneys. Seismic attributes have also been used to determine the character of the gas chimneys and the potential association of hydrocarbon fields. However, it is important to evaluate the sealing potential of the cap rock in the vicinity of the gas chimney. This paper demonstrates that the chimney can be used as a calibration point and, with the use of seismic attributes, variations of sealing capacity can be evaluated. Out of many existing seismic attributes, understanding the shale structure is crucial to narrow the search criteria for the right class of attributes. Using the most useful stream of seismic attributes, the average sealing potential of the Lakes Entrance Formation is estimated. The sealing capacity map across the field is then shown to help evaluate the prospectivity of the area of study and may upgrade subsequent play fairway maps.

Distribution and characteristics of mud diapirs, gas chimneys, and bottom simulating reflectors associated with hydrocarbon migration and gas hydrate accumulation in the Qiongdongnan Basin, northern slope of the South China Sea

AbstractA comprehensive analysis on high‐resolution seismic data and the geological characteristics of the Qiongdongnan Basin (QDNB) in the northern South China Sea reveals multiple seismic evidence indicative of hydrocarbon migration and gas hydrate accumulation, including mud diapirs, gas chimneys, bottom simulating reflectors (BSRs), acoustic blanking, enhanced reflections, V‐AMP structures, elevated stacking velocity of sediments, and seafloor expressions. Mud diapirs and gas chimneys, which are characterized by acoustic turbidity, fuzziness, and blanking reflections on seismic profiles, with a diversity of structures and scales, are widespread and concentrated in the centres of sags and the transitional zones between sags and uplifts. Five BSR regions, with a total area of approximately 11,000 km2, have been delineated in the QDNB. They have high or medium‐high continuity amplitudes with overlying acoustic blanking on seismic profiles, indicating the occurrence of gas hydrate. Distinct fluid seepage phenomena are common below the BSRs, demonstrating that abundant free gas has migrated and accumulated beneath the gas hydrate. The distribution of BSRs and the developed zones of mud diapirs and gas chimneys presents a spatial superposition feature, and the BSRs are usually located on top of or within the structural highs of the mud diapirs and gas chimneys, indicating a close relationship between them. Mud diapirs, gas chimneys, and associated faults are efficient pathways for migrating hydrocarbons, through which the mature to highly mature thermogenic gas of the Palaeogene strata and the biogenic gas of the Neogene sediments migrate to the gas hydrate stability zones and form gas hydrate. Gas hydrate samples were recovered from the gas chimney area during the remote operated vehicle (ROV) investigation in deepwater QDNB in 2015. Based on the seismic interpretation and geochemical results, we propose two models of gas hydrate accumulation in the deepwater area of the QDNB: (a) near source biogenic gas hydrate with self‐generation and self‐accumulation, and (b) distal source thermogenic gas hydrate with lower generation and upper accumulation. We believe that the areas with a fine spatial coupling configuration between the mud diapirs, gas chimneys, and BSRs are favourable exploration targets for gas hydrate in the QDNB.

Discrete-element modelling of hanging wall deformation along the D-1 fault system, Danish North Sea, with implications for gas chimney interpretations

This study uses 3D seismic mapping, NNCC attribute analysis and 2D SDEM modelling of soft-sediment deformation to analyse hanging wall deformation related to faulting across a listric fault detaching along the top of a major salt roller. The computational model experiments used the overall fault and salt-structure evolution as boundary conditions to predict the distribution of faults within the hanging wall. The modelling successfully reproduced geometries similar to the syn- and antithetic faults that are visible on seismics, but it also showed the presence of well-defined vertical deformation zones, both in the hanging wall and the foot-wall of the listric fault. In the model, salt movement controls the formation of the deformation zones. The similarity of NNCC-inferred vertical chimneys and the modelled deformation zones suggests that the subtle vertical chimneys identified in the 3D seismic data might actually be related to vertical to sub-vertical deformation zones caused by the presence and movement of the salt structure.

Influence of faults and slumping on hydrocarbon migration inferred from 3D seismic attributes: Sanriku-Oki forearc basin, northeast Japan

Abstract Seismic imaging of the Sanriku-Oki forearc basin in the Japan Trench plate convergent margin provides insights into the geological controls on hydrocarbon systems in sediment deposited on the continental slope. We perform a series of seismic attribute analyses on 3D seismic reflection data to better define the influence of geological structures (e.g., faults, slumps and gas chimneys) on hydrocarbon sources, migration pathways and reservoirs. In order to identify coal-bearing strata from the 3D seismic volume, we calculate acoustic impedance distribution by integrating seismic and logging data via waveform inversion. Two Late Oligocene to Early Miocene major coal-bearing strata are characterized as low acoustic impedance strata. Gas-bearing strata characterized by anomalously high reflection strength are widely distributed beneath a bottom simulating reflector (BSR). A greater accumulation of gas occurs in the northeast of the study area, where successive slump deposits and gas chimneys are observed. We propose that a BSR with an underlying gas accumulation in the Sanriku-Oki forearc basin developed as a consequence of the expulsion of gas from Cretaceous to Oligocene coal beds and their subsequent upward migration through gas chimneys and faults. Furthermore, the migration and accumulation of gas and gas hydrate were strongly controlled by a series of porous slump deposits. As chimneys and gas pockets occur above the edges of the slumps, gas migration processes controlled by the slumps could be important in the forearc basin. Our study based on seismic attribute analyses demonstrates that features produced by tectonic movements at plate subduction margins (e.g., faults) and associated instability processes (e.g., slumps) represent important controls on the migration and accumulation of hydrocarbons in forearc basins.

Gas detection by seismic attributes in the Qiongdongnan Basin, northern South China Sea

Reflection characteristics of gas chimneys in seismic data in the Qiongdongnan Basin, northern South China Sea, indicate a large amount of gas, which migrates along the faults and fractures from deep to shallow layer, where strong attenuation of seismic waves under the seafloor occurs, leading to the decrease of Q factor. As it reaches to the seafloor, it also causes the decrease in density and P wave velocity of the seafloor, and then winds up in the seawater forming bubble plumes. In this paper, the recognition on gas and its migration, and therefore its distribution in the Qiongdongnan Basin, can be gained through multiple seismic attributes, such as Q-factor estimates at the shallow layer under the seafloor, elastic parameter inversion of the seafloor, and bubble plume imaging in the seawater near the seafloor, by using the quasi-3D seismic data in this area. It has great significance in study gas hydrate deposits in the Qiongdongnan Basin.