Base Of Gas Hydrate Stability Zone Research Articles

Submarine fluid flow system feeding methane emission in the northern South China Sea

AbstractSubmarine fluid flow system can transport methane into ocean. However, its evolution is not fully understood, particularly methane migration through the gas hydrate stability zone (GHSZ) in deep‐water settings. Here, we used 3D seismic and well‐logging data to show the currently active fluid flow system in the northern South China Sea. It was interpreted to have two parts and they together feed intermittent methane emission. Three gas clouds have been seismically imaged beneath the base of gas hydrate stability zone (BGHSZ) and a set of new faults can be identified within them. Twenty‐eight seismic pipes were found to penetrate three vertically stacked mass transport deposits (MTDs) above the gas clouds. Log‐seismic correlation shows that the seismic reflections in the pipe represent MTD sediment, bulk carbonate and gas hydrate‐ or free gas‐bearing sediments. We interpreted faults and pipes as the main migration conduits below and above the BGHSZ respectively. The MTD within the GHSZ could seal the underlying free gas transported by faults and thus overpressure built up at the base prior to the occurrences of the pipes and the fracturing through the overlying sedimentary succession. Subsequently, focused fluid flow entered the GHSZ, with the methane probably bypassing the GHSZ before pore clogging of gas hydrates occurred. Additionally, mapping of high‐amplitude reflections surrounding the upper portion of gas clouds reveals the relict free gas associated with three paleo‐GHSZ bases. Episodic emplacements of new MTDs repeatedly caused the upward shifts of the BGHSZ and the resultant gas hydrate dissociation, contributing to methane emission. We proposed that the occurrences of MTDs may facilitate methane emission by intermittently trapping methane and inducing gas hydrate dissociation in deep‐water settings.

Dynamic evolution of gas hydrate systems affected by magmatism and Quaternary mass-transport deposits in the Qiongdongnan Basin, South China Sea, and implications for the global carbon cycle

Methane release from hydrate systems has long been considered as one of the important mechanisms for global carbon cycle perturbations and ocean acidification. Here, a Quaternary active gas hydrate system in Qiongdongnan Basin, South China Sea (SCS), has been analyzed by using seismic, logging while drilling (LWD), and core data. The results show that post-rift magmatism led to the reactivation of a magma diapir, and the faults and fractures related to its emplacement created efficient pathways for deep gas migration. The increase in overburden temperature and circulation of deep thermal fluids, especially in the center of the diapir area, thinned the gas hydrate stability zone (GHSZ). The accumulation of natural gas at the base of GHSZ and the thinning of GHSZ decreased the ability of hydrate bearing sediments to seal free gas and lead to hydraulic fracturing. The free gas entering the GHSZ was buried by mass transport deposits (MTDs) and then formed hydrates due to the overlying compaction and so further sealed the free gas under the GHSZ. This study indicates that two contrasting mechanisms are acting: (1) tectonic activity and magmatism favor the release of deep natural gas which may reach the ocean, (2) MTDs and the overlying compacted sections act as seals, inhibiting the release of such gas to the ocean. This study shows how the seal capacity of MTDs may reduce methane release from deep-seated reservoirs into the ocean. Given their widespread distribution along continental margins, we suggest that the impact of MTDs on vertical fluid flow, and potential consequences for ocean chemistry and the global carbon cycle, might be underestimated.

Gas Hydrate Accumulation Related to Pockmarks and Faults in the Zhongjiannan Basin, South China Sea

The amplitude and coherence attributes of three-dimensional (3D) seismic data are used to confirm gas hydrate occurrence and to delineate its distribution in the Zhongjiannan basin, South China Sea. High amplitude anomalies (HAAs) are distributed above or below the regional base of gas hydrate stability zone (BGHSZ), which intersect with the bottom simulating reflectors (BSRs) or are interrupted by different types of pockmarks. The maximum amplitude attribute extracted along T1 (5.5 Ma) horizon is controlled by the widely distributed faults. The layer-bound polygonal faults (PFs) show networks of small normal faults, and the dominant orientations of PFs are similar or orthogonal to the regional tectonic faults, which provide the fluid migration pathways for gas and fluids to form HAAs. BSR shows the strong amplitude and continuous reflection where the faults or PFs can reach the BGHSZ without the influence of the pockmarks. Most of the pockmarks are related to the reactivation of faults and magmation, and some pockmarks are caused by the dissociation of gas hydrate. Around the matured pockmark, the BSR is discontinuous, and HAAs locally appear within the pockmarks. The inverted acoustic impedance profile shows obviously high values of HAAs except in pockmark zones. Partial HAAs occur above BGHSZ, and the continuity is interrupted by the pockmarks with only high values around the pockmarks. We propose that BSR, HAAs, pockmarks, and different types of faults are closely related to the occurrence and distribution of gas hydrates in the study area. This work allows us to understand the relationship between gas hydrate occurrence and accumulation with pockmarks, faults, and magmatic activities.

The many double BSRs across the northern Hikurangi margin and their implications for subduction processes

The bottom simulating reflection (BSR) is widely observed along continental margins and is believed to mark the base of gas hydrate stability zone (BGHSZ). In some regions, double or multiple overlapping BSRs are observed, yet their formation mechanisms and geologic implications are not well understood. Here we present 3D seismic images from the 2018 NZ3D experiment that covers a 14×60km2 survey area on New Zealand's northern Hikurangi subduction margin. We observe double BSRs in five locations. Beneath the Tuaheni Basin in the mid-slope, a secondary BSR (BSR2) lies ∼100-360 m deeper than the primary BSR (BSR1) and its 3D geometry mimics the unconformity at the base of the basin. At three thrust ridges located 18-38 km from the deformation front, BSR2 lies ∼55-130 m below and is sub-parallel to BSR1. At another thrust ridge ∼14 km from the deformation front, BSR2 forms above the BSR1, and the two BSRs converge towards the peak of the ridge. Through 3D modeling of BGHSZ and analysis of the geometry and reflection characteristics of the double BSRs, we identify three potential mechanisms for their formation (1) rapid sedimentation, (2) tectonic uplift, (3) overpressure/heat advection caused by fluid migration. Our study demonstrates that formation of double BSRs is closely linked to subduction processes along the northern Hikurangi margin, and double BSRs may be used as indicators for areas with recent sedimentation, tectonic and/or fluid activities.

Thermogenic gas controls high saturation gas hydrate distribution in the Pearl River Mouth Basin: Evidence from numerical modeling and seismic anomalies

We integrate three-dimensional (3D) seismic data, well log and two-dimensional (2D) modeling to understand the accumulation and saturation of gas hydrates and their relationships with biogenic and thermogenic gases in the Pearl River Mouth Basin, South China Sea (Sites SH2, SH5, W17, and W19). The model based on physical, geochemical and geological parameters from shallow gas hydrate sites, deep oil sites and seismic interpretation, and it is tied to well-log and seismic quantitative interpretation, to predict the main controlling factors for gas hydrate distribution. The modeling results show that the biogenic methane generation is distributed within the strata shallower than 1500 mbsf (meters below the seafloor), whereas thermogenic gas generation ranges from 2300 mbsf to 6000 mbsf. The erosion surfaces of migrating canyons represent important pathways for upward migration of biogenic gas resulting in average hydrate saturation <20% below the ridges of canyons. The total organic carbon content is an important factor related to biogenically-sourced gas hydrate distribution. Higher gas saturation (>40%) at the base of gas hydrate stability zone (BGHSZ) is related to the focused migration of thermogenic gas along faults and throughout permeable carrier beds. Although gas chimneys are widely identified from the coherence attribute extracted from 3D seismic data, their impact on the obviously higher (locally up to 60%) gas hydrate saturations in the fine-grained sediments appears site-dependent and associated with fault activity. By comparing well log-derived gas hydrate saturations, our modeling shows that about 80% of the methane-forming hydrate is thermogenically-sourced at Sites W19 and W17. The integrated modeling effort presented in this work, based on provides a better understanding of the different contributions from different gas sources for gas hydrate accumulation along the northern slope of the South China Sea.

Distribution and genesis of submarine landslides in the northeastern South China Sea

Submarine landslide is a widespread marine geohazard and an important part of the global ‘source to sink’ system. In this article, multi‐channel seismic and topographic data are utilized to investigate the distribution, calculate the geomorphologic parameters, and discuss the genesis of the submarine landslides in the northeastern South China Sea. In the study area, which can be divided into two tectonic provinces: passive continental margin province and accretionary wedge province, 101 landslides are identified, with a total size of 2,358.22 km2. Statistics show that most landslides are of small sizes (&lt;15 km2) and developed along small slope gradient regions. There is a high correlation between the slope gradient and the ratio of headscarp height to runout distance. The size and runout distance of landslide also correlate well. However, there is a low correlation coefficient including slope gradient versus runout distance and slope gradient versus size. The distribution of submarine landslides is consistent with that of bottom simulating reflections (BSRs) and seabed activities, which may imply a possible inner correlation among them. The calculated base of gas hydrate stability zone (BGHSZ) pinches out the seafloor at about 555–556 m below the sea level, where most of the landslide headscarps are located. And the BGHSZ depths are often shallower than the BSRs bottom depths in the upper slope area. Combined with previous studies, we consider that the influence factors of submarine landslides in the study area are earthquakes, canyons, seabed fluid activities, and gas hydrate dissociation. Gas hydrate dissociation is a global influence factor in the study area. In the passive continental margin province, the submarine landslides are mainly canyon‐associated landslides. While in the accretionary wedge province, they are mainly controlled by seabed fluid activities and earthquakes.

Anomalously Deep BSR Related to a Transient State of the Gas Hydrate System in the Western Black Sea

AbstractA comprehensive characterization of gas hydrate system offshore the western Black Sea was performed through an integrated analysis of geophysical data. We detected the bottom‐simulating reflector (BSR), which marks, in this area, the base of gas hydrate stability. The observed BSR depth does not fit the theoretical steady state base of gas hydrate stability zone (BGHSZ). We show that the disparity between the BSR and predicted BGHSZ is the result of a transient state of the hydrate system due to the ongoing reequilibrium since the Last Glacial Maximum. When gas hydrates are brought outside the stability zone due to changes in temperature and sea level, their dissociation generates an increase in interstitial pore pressure. This process is favorable to the recrystallization of gas hydrates and delays the upward migration of the hydrate stability zone explaining the anomalously deep BSR. The BSR depth, which is commonly used to derive geothermal gradient values by assuming steady state conditions, is used here to derive the maximum excess pore pressure at the BGHSZ. Derived excess pore pressure values of 1–2 MPa are probably the result of the low permeability of hydrate‐bearing sediments. Higher pore pressure values derived at the location of a fault system could cause hydrofracturing enabling the free gas to cross the gas hydrate stability zone and emerge at the seafloor, forming the flares observed in close vicinity to where the shallow gas hydrates were sampled.

Geophysical evidence for gas hydrate accumulation related to methane seepage in the Taixinan Basin, South China Sea

Seafloor features, seismic amplitude anomaly, geophysical attributes, and chemosynthetic community are different in the methane seepage areas of Taixinan Basin, South China Sea. We integrate the new acquired seafloor images with seismic data to present the differences of three cold seeps which are associated with fluid flows. These three cold seeps include two inactive sites (CS02-8 and CS02-9) and one active site (Site F). At site CS02-8 area, the seafloor carbonate and buried carbonate layer are identified from core samples at Site GMGS2-08. BSR is interrupted by the upward migration of fluids from the base of gas hydrate stability zone (BGHSZ) and gas hydrate-bearing sediments show pull-up seismic reflections below the buried carbonate layer. At site CS02-9 area, near seafloor carbonate was found at hole GMGS2-09B where a 62 m-high pinnacle and gas hydrate were also found from the core samples. The LWD data at hole 09A indicate that free gas is present below the depth of 64 m without gas hydrate. The average saturations of free gas estimated from P-wave velocity are about 3% with a patchy distribution and 0.3% with homogenous distribution respectively, which is accord with the enhanced reflection below BGHSZ. At Site F area, the exposed gas hydrate, carbonate and flourishing chemosynthetic communities are detected using ROV with a high amplitude seismic reflection at seafloor. The BSR is interrupted by a chimney structure from the seismic data which indicates that the fluids are migrating vertically and rise to the seafloor as gas bubbles. The geophysical evidence at these three seep sites show that pore-filling gas hydrates with saturations ranging from 10% to 40% are widely distributed above the BSR. However, massive or vein gas hydrates only occur around cold seeps.

Quantitative interpretations and assessments of a fractured gas hydrate reservoir using three-dimensional seismic and LWD data in Kutei basin, East Kalimantan, offshore Indonesia

Concentrated fractured gas hydrate reservoirs in fine-grained sediments have been considered to have limited potential for commercially-viable production of natural gas. They may also pose significant geohazard where they overly conventional oil and gas production targets. The genesis of such accumulations is not well known and the characterization of the reservoirs is difficult. Thus, integrate characterization using 3D seismic interpretation, well logs, and geotechnical analyses are vital. The present study is the first detailed description of a fractured gas hydrate reservoir in the Kutei basin, offshore Indonesia.In this study, vertical fluid flow features (such as mud volcanoes and chimney structures) and other indicators of gas hydrate occurrence (including Bottom Simulating Reflectors (BSRs) and high-resistivity zones) are interpreted over a region of 500 km2. Seismic amplitude anomalies provide evidence of gas migration that has reached the seafloor. An interpreted double BSR below a mud volcano is inferred to indicate thermogenic gas present in the shallow sections. Geological and geotechnical analyses show overpressure associated with gas migration across the base of gas hydrate stability zone (BGHSZ) results in the formation of hydraulic and pneumatic fracturing. Seismic and well logs data indicate that the vertical migration of ascending free gas results in large-size macro-fractures whereas free gas accumulation below less permeable hydrated sediments allows for the generation of local small micro-fractures. Consequently, the fractures play roles as conduits and reservoirs for gas hydrate accumulations.Hydrate saturation was estimated from seismic and well log data by using seismic inversion and Archie equation, respectively. Gas hydrate is inferred to uniformly occur above the base of hydrate stability zone and its average thickness is estimated to be approximately 30 m. The gas hydrate is estimated to occupy generally 5–10% by volume in micro-fracture reservoirs, with as high as 30–35% near the top of macro-fracture reservoirs.

Gas hydrate destabilization and methane release events in the Krishna–Godavari Basin, Bay of Bengal

Methane release events have been linked to global warming, alteration of the carbon cycle and influence on biota. However, unequivocal evidence of paleomethane release events are limited. We report several negative carbon stable isotope excursions in planktic and benthic foraminifera in a core (MD161-8) from the Krishna-Godavari (K–G) Basin, Bay of Bengal. The most negative δ13C spikes are recorded during the marine isotope stages MIS-4 and at the transition of MIS-5 to 4. Occurrence of highly 13C depleted (average δ13C = −48 ± 2.4‰ VPDB) authigenic high magnesian calcite are also reported within this time window from the core MD161-8. In the present work an unequivocal explanation for the observed 13C depletion in the marine planktic and benthic foraminifera is difficult to achieve solely from the optical/electron microscopy or C–O stable isotope ratio analyses due to possible influence of diagenetic alteration. We attribute the observed episodic methane expulsion events, as inferred from the negative δ13C excursions and earlier reports on the occurrence chemosynthetic bivalves and Mo concentration anomaly to the destabilization of the base of gas hydrate stability zone (BGHSZ). Sea level drop and shale tectonics induced focused fluid flow are the two possible causes of hydrate destabilization discussed here. Shale tectonics were possibly responsible for creating fault systems which acted as the conduit for gas flow through the sediment column and subsequent seepage. Shale and salt tectonics in the passive continental margins being a globally observed phenomenon, its role as an important driving force for enhanced methane emission needs detailed investigation to understand the climatic perturbations through geologic time. Additional evidence of methane emission from site MD161-15 further supports the link between shale tectonics and methane emission.

Heat flow and gas hydrate saturation estimates from Andaman Sea, India

Gas hydrate was recovered in the Andaman Sea along the eastern coast of the Andaman Islands during the India National Gas Hydrate Program (NGHP) Expedition-01 at Site NGHP-01-17. Coring confirmed gas hydrate occurs predominantly in discrete volcanic ash layers. Pore water chemistry, electrical resistivity and P-wave velocity logs are used to estimate gas hydrate saturations at Site NGHP-01-17. Gas hydrate saturation estimated from chloride concentrations shows values up to ∼85% of the pore space for distinct ash layers from ∼270 m below seafloor to the base of gas hydrate stability zone (BGHSZ). Gas hydrate saturations estimated from the electrical resistivity and acoustic velocity logs using standard empirical relations and modeling approaches are comparable to each other, but saturations are only ∼20% of the pore space on average. This much lower gas hydrate saturation estimate from the log data is a result of overall reduced resolution of the logging tools relative to the typically 20–30 cm thick hydrate-bearing ash layers. Available 2D multi-channel seismic data were also analyzed and a bottom-simulating reflector (BSR) was imaged along several seismic profiles. The depth of the BSR is more than 600 m along the seismic line crossing Site NGHP-01-17, which makes this one of the deepest BSRs observed worldwide. To understand the unusual depth of the BSR, we mapped its depth and estimated heat flow from the BSR depth using a simple conductive model. BSR-derived heat flow values range from ∼12 to ∼41.5 mW/m2 from the study area and follow the bathymetry trend of dominant North–South ridges and can be explained with the east-ward trending increase in heat-flow toward the current seafloor spreading center. We also modeled the BGHSZ to analyze the linkage between gas hydrate occurrences in the Andaman Sea and its relation to the tectonic activity. Our analysis suggests an extensively variable BGHSZ in the Andaman Sea controlled mainly by overall low geothermal gradients. Consistent local minor variations were observed with lower heat flow values over prominent topographic highs and higher values in valleys/troughs due to focusing and defocusing effects of the topography.

Mass-transport deposits and gas hydrate occurrences in the Ulleung Basin, East Sea – Part 2: Gas hydrate content and fracture-induced anisotropy

Mass-transport-deposits (MTDs) and hemipelagic mud interbedded with sandy turbidites are the main sedimentary facies in the Ulleung Basin, East Sea, offshore Korea. The MTDs show similar seismic reflection characteristics to gas-hydrate-bearing sediments such as regional seismic blanking (absence of internal reflectivity) and a polarity reversed base-reflection identical to the bottom-simulating reflector (BSR). Drilling in 2007 in the Ulleung Basin recovered sediments within the MTDs that exhibit elevated electrical resistivity and P-wave velocity, similar to gas hydrate-bearing sediments. In contrast, hemipelagic mud intercalated with sandy turbidites has much higher porosity and correspondingly lower electrical resistivity and P-wave velocity.At drill-site UBGH1-4 the bottom half of one prominent MTD unit shows two bands of parallel fractures on the resistivity log-images indicating a common dip-azimuth direction of about ∼230° (strike of ∼140°). This strike-direction is perpendicular to the seismically defined flow-path of the MTD to the north-east. At Site UBGH1-14, the log-data suggest two zones with preferred fracture orientations (top: ∼250°, bottom: ∼130°), indicating flow-directions to the north-east for the top zone, and north-west for the bottom zone. The fracture patterns may indicate post-depositional sedimentation that gave rise to a preferred fracturing possibly linked to dewatering pathways. Alternatively, fractures may be related to the formation of pressure-ridges common within MTD units.For the interval of observed MTD units, the resistivity and P-wave velocity log-data yield gas hydrate concentrations up to ∼10% at Site UBGH1-4 and ∼25% at Site UBGH1-14 calculated using traditional isotropic theories such as Archie's law or effective medium modeling. However, accounting for anisotropic effects in the calculation to honor observed fracture patterns, the gas hydrate concentration is overall reduced to less than 5%. In contrast, gas hydrate was recovered at Site UBGH1-4 near the base of gas hydrate stability zone (GHSZ). Log-data predict gas hydrate concentrations of 10–15% over an interval of 25 m above the base of GHSZ. The sediments of this interval are comprised of the hemipelagic mud and interbedded thin sandy turbidites, which did contain pore-filling gas hydrate as identified from pore-water freshening and core infra-red imaging. Seismically, this unit reveals a coherent parallel bedding character but has overall faint reflection amplitude. This gas-hydrate-bearing interval can be best mapped using a combination of regular seismic amplitude and seismic attributes such as Shale indicator, Parallel-bedding indicator, and Thin-bed indicator.

Geothermal modeling for the base of gas hydrate stability zone and saturation of gas hydrate in the Krishna-Godavari basin, eastern Indian margin

The passive eastern Indian margin is rich in gas hydrates, as inferred from the wide-spread occurrences of bottom-simulating reflectors (BSRs) and recovery of gas hydrate samples from various sites in the Krishna Godavari (KG) and Mahanadi (MN) basins drilled by the Expedition 01 of the Indian National Gas Hydrate Program (NGHP). The BSRs are often interpreted to mark the thermally controlled base of gas hydrate stability zone (BGHSZ). Most of the BSRs exhibit moderate to typically higher amplitudes than those from other seismic reflectors. We estimate the average geothermal gradient of ∼40°C/km and heat flow varying from 23 to 62 mW/m2 in the study area utilizing the BSR’s observed on seismic sections. Further we provide the BGHSZ where the BSR is not continuous or disturbed by local tectonics or hidden by sedimentation patterns parallel to the seafloor with a view to understand the nature of BSR.

Gas-hydrate stability thickness map along the Indian continental margin

The gas-hydrate stability thickness (GHST) map along the Indian continental margin is prepared from available bathymetry, sea-bottom temperature and geothermal gradient data. The bottom-simulating reflector (BSR) often marks the base of gas-hydrate stability zone. The prior information about the stability thickness in a particular area will help in identifying BSR on seismic data. The map is also useful to the exploration scientists to set a depth window within which proxies for gas-hydrate can be looked into. A GHST map was initially prepared in 1998 based on the-then available data. A lot of new data has been generated by various organizations under the Indian National Gas Hydrate Programs for the advancement of exploration and exploitation activities. By incorporating the new data from the published and available documents, we have modified the GHST map along the Indian margin. Besides filling the data gap, the new map shows the gas-hydrate stability zone in the Andaman offshore. In addition, we show maps of sea-bottom temperature, sediment thickness, geothermal gradient and heat flow to provide a bird’s eye view of these parameters along the continental margin of India.

Gas-Hydrate System of the Shenhu Area, Northern South China Sea

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 20485, ’Gas Hydrate System of Shenhu Area, Northern South China Sea: Wire-line Logging, Geo chem ical Results and Preliminary Resources Estimates,’ by Nengyou Wu, Shengxiong Yang, Haiqi Zhang, Jinqiang Liang, Hongbin Wang, and Jing'an Lu, China Geological Survey, originally prepared for the 2010 Offshore Technology Conference, Houston, 3-6 May. The paper has not been peer reviewed. The Shenhu area, one of the most favorable locations for gas hydrates in the north slope of the South China Sea, is in the Pearl River mouth basin. The geological setting and temperature and pressure conditions are favorable for the formation of gas hydrates. Drilling recovered high concentrations of hydrate (maximum 26 to 48%, with gas composition of greater than 99% methane) in a disseminated form in foram-rich clay sediments. Introduction Gas hydrate is an ice-like solid substance formed by the combination of low-molecular-weight gases (e.g., methane, ethane, and carbon dioxide) with water. Gas hydrate mainly occurs naturally in sediments beneath the permafrost and in the sediments of the continental slope in water depths greater than 300 m. The reason that marine gas-hydrate systems are important economically, socially, and the environmentally is not only that gas hydrate in some places may be sufficiently concentrated to be an economically viable fossil-fuel resource, but also because gas hydrate can cause geohazards through large-scale slope destabilization and can release methane, a potential greenhouse gas, to affect the global climate. Gas hydrate and its associated sediments also have become an important focus for biogeochemical study of the deep biosphere. The stability of gas hydrate fundamentally depends on four factors: temperature, pressure, gas composition, and pore-water salinity. The nucleation and growth of gas hydrate also depend on sediment grain size, shape, and mineral composition. These factors, which control gas-hydrate formation and stability, are affected by a series of physical and chemical processes in the marine sediments and result in a variation of gas-hydrate dynamics on different time scales. Therefore, gas hydrate is not distributed continuously all over the world in the vertical and horizontal scales. Because the geological setting and the factors controlling gas-hydrate formation are different in various locations, scientists have established different geological models on the basis of the formation mechanisms, gas sources, and dynamics. Gas-hydrate deposits can be classified into the two end-member regimes of the high-flux gas-hydrate deposit and the low-flux gas-hydrate deposit, on the basis of the mechanisms that control gas transport into the gas-hydrate-stability zone (GHSZ), although both regimes operate simultaneously in many regions. One investigator grouped the hydrates in the marine environment into these same two categories, calling them diffusion gas hydrates and vent gas hydrates. The diffusion gas hydrates occur widely in areas where bot-tom simulating reflectors (BSRs) are recorded in seismic profiles, and they reflect a thermodynamic equilibrium system of hydrates and water with dissolved methane within a GHSZ. The hydrates are buried within the sediment, significantly deeper than the sediment/water interface, and are characterized by low concentrations. The vent gas hydrates occur in an area where gas vents out of the seafloor. This is a thermodynamic-disequilibrium system of hydrate, water, and free gas, occurring in a zone that extends from the base of GHSZ to the seafloor, and is characterized by high concentration. However, these conceptual models should be verified by actual evidence from sites around the world.

Seismic and heat flow constraints from the gas hydrate system in the Krishna–Godavari Basin, India

High resolution multi-channel seismic reflection data acquired in the offshore domain of the Krishna–Godavari (KG) Basin, provide seismic indicators of gas hydrate including bottom-simulating reflectors (BSR), and high reflectivity zones below the BSR as an indication of free gas. The recent recovery of gas hydrate by drilling across various sites in the KG Basin, together with the seismic indicators, suggests overall widespread favourable gas hydrate stability conditions. The BSRs provide indirect estimates of geothermal gradients and heat flow. A simple conductive model is used to calculate heat flow from the depth of the BSR. BSR-derived heat flow values range from about 30 to 102 mW/m 2 across the KG Basin. We also modeled the base of gas hydrate stability zone (BGHSZ) to analyse the linkage between gas hydrate occurrences in the basin and its relation to the fault systems in the basin. The influence of bottom water temperature, pore water salinity and geothermal gradient variation on gas hydrate stability zone (GHSZ) thickness is analysed in relation to both geological formations and tectonics. Our analysis suggests a highly variable GHSZ in the KG Basin controlled by local variations in the parameters of stability condition and faulting. The BSR-derived heat flow increases towards deeper water depths across the basin. Consistent local variations were also observed with low heat flow values over prominent topographic highs and high heat flow values over the flanks of the topographic highs. This variation may be due to focusing and defocusing effects of the topography. However, local BSR-depths patterns may also be due to dynamic effects, including the displacement of isotherms by thrust-faulting. Additionally, recent mass-wasting events and normal faulting resulted in the BGHSZ being too shallow relative to the predicted model (which assumed equilibrium conditions).

Seismic character of the BSR in the Ulleung Basin plain, East Sea (Japan Sea)

A bottom simulating reflector (BSR) was identified in multichannel seismic reflection profiles collected in the Ulleung Basin plain, East Sea (Japan Sea). The BSR, occurring in densely stratified sediments, is observed very close to the boundary between two sequences interpreted as low-energy turbidites/hemipelagic sediments and mass-transport deposits. The essential characteristics of the BSR include its cross-cutting relationship to strata at a small angle, strong amplitude, and reverse polarity with respect to the seafloor reflection. Stochastic inversion of seismic data indicates that the BSR reflection coefficients reach maximally the range from 0.23 to 0.26 and are 1.5∼1.7 times that of the seafloor reflection. A decrease in interval velocities, disrupted seismic signals, and noticeable drop of the frequency content are recognized below the BSR. These features strongly suggest that the BSR represents the base of gas hydrate stability zone (BHSZ) in the Ulleung Basin plain and a free gas zone is present below the BSR with high amplitude. Inversion also discriminates the lower amplitude BSR that is not easily identified from interfering reflection signals, suggesting that gas hydrates exist more extensively in the Ulleung Basin than recognized in seismic profiles.

Evaluation of normative EEG spectral parameters in normal children. A five-year follow-up study : K. Sadowsky, E. Mullner, S.J. Pöppl, W.S. Tirsch and H.M. Weinmann (Munich, W. Germany)

Concentrated fractured gas hydrate reservoirs in fine-grained sediments have been considered to have limited potential for commercially-viable production of natural gas. They may also pose significant geohazard where they overly conventional oil and gas production targets. The genesis of such accumulations is not well known and the characterization of the reservoirs is difficult. Thus, integrate characterization using 3D seismic interpretation, well logs, and geotechnical analyses are vital. The present study is the first detailed description of a fractured gas hydrate reservoir in the Kutei basin, offshore Indonesia.In this study, vertical fluid flow features (such as mud volcanoes and chimney structures) and other indicators of gas hydrate occurrence (including Bottom Simulating Reflectors (BSRs) and high-resistivity zones) are interpreted over a region of 500 km2. Seismic amplitude anomalies provide evidence of gas migration that has reached the seafloor. An interpreted double BSR below a mud volcano is inferred to indicate thermogenic gas present in the shallow sections. Geological and geotechnical analyses show overpressure associated with gas migration across the base of gas hydrate stability zone (BGHSZ) results in the formation of hydraulic and pneumatic fracturing. Seismic and well logs data indicate that the vertical migration of ascending free gas results in large-size macro-fractures whereas free gas accumulation below less permeable hydrated sediments allows for the generation of local small micro-fractures. Consequently, the fractures play roles as conduits and reservoirs for gas hydrate accumulations.Hydrate saturation was estimated from seismic and well log data by using seismic inversion and Archie equation, respectively. Gas hydrate is inferred to uniformly occur above the base of hydrate stability zone and its average thickness is estimated to be approximately 30 m. The gas hydrate is estimated to occupy generally 5–10% by volume in micro-fracture reservoirs, with as high as 30–35% near the top of macro-fracture reservoirs.