Mapping gas hydrate and fluid flow indicators and modeling gas hydrate stability zone (GHSZ) in the Ulleung Basin,East (Japan) Sea: Potential linkage between the occurrence of mass failures and gas hydrate dissociation

The Ulleung Basin, East (Japan) Sea, is well-known for the occurrence of submarine slope failures along its entire margins and associated mass-transport deposits (MTDs). Previous studies postulated that gas hydrates which broadly exist in the basin could be related with the failure process. In this study, we identified various features of slope failures on the margins, such as landslide scars, slide/slump bodies, glide planes and MTDs, from a regional multi-channel seismic dataset. Seismic indicators of gas hydrates and associated gas/fluid flow, such as the bottom-simulating reflector (BSR), seismic chimneys, pockmarks, and reflection anomalies, were re-compiled. The gas hydrate occurrence zone (GHOZ) within the slope sediments was defined from the BSR distribution. The BSR is more pronounced along the southwestern slope. Its minimal depth is about 100 m below seafloor (mbsf) at about 300 m below sea-level (mbsl). Gas/fluid flow and seepage structures were present on the seismic data as columnar acoustic-blanking zones varying in width and height from tens to hundreds of meters. They were classified into: (a) buried seismic chimneys (BSC), (b) chimneys with a mound (SCM), and (c) chimneys with a depression/pockmark (SCD) on the seafloor. Reflection anomalies, i.e., enhanced reflections below the BSR and hyperbolic reflections which could indicate the presence of gas, together with pockmarks which are not associated with seismic chimneys, and SCDs are predominant in the western-southwestern margin, while the BSR, BSCs and SCMs are widely distributed in the southern and southwestern margins. Calculation of the present-day gas-hydrate stability zone (GHSZ) shows that the base of the GHSZ (BGHSZ) pinches out at water depths ranging between 180 and 260 mbsl. The occurrence of the uppermost landslide scars which is below about 190 mbsl is close to the range of the GHSZ pinch-out. The depths of the BSR are typically greater than the depths of the BGHSZ on the basin margins which may imply that the GHOZ is not stable. Close correlation between the spatial distribution of landslides, seismic features of free gas, gas/fluid flow and expulsion and the GHSZ may suggest that excess pore-pressure caused by gas hydrate dissociation could have had a role in slope failures.     

南海北部揭阳凹陷天然气水合物的地震异常特征分析

大量研究表明南海北部珠江口盆地是天然气水合物发育区,但是该盆地东部揭阳凹陷水合物研究较少。本文利用揭阳凹陷新采集三维地震资料,对该三维地震资料进行成像道集优化和叠前时间偏移处理,得到针对水合物的新处理地震数据体,并通过高精度网格层析反演得到层速度数据体。利用该数据开展叠后约束稀疏脉冲反演,获得含天然气水合物地层波阻抗异常,综合分析反演与地震属性识别水合物。从新处理地震资料看,该区域似海底反射(bottom simulation reflection,BSR)反射呈连续、不连续与地层斜交等特征,BSR发育在一个继承性小型水道上,且下部断裂和气烟囱发育。通过分析BSR特征及BSR上下地层的速度、波阻抗、振幅、频率、相干等属性异常,结合水合物成藏条件,发现了南海北部新的天然气水合物有利富集区,为该区域水合物勘探提供基础。     

Qualitative assessment of gas hydrate and gas concentrations from the AVO characteristics of the BSR in the Ulleung Basin, East Sea (Japan Sea)

The presence of gas hydrate in the Ulleung Basin, East Sea (Japan Sea), inferred by various seismic indicators, including the widespread bottom-simulating reflector (BSR), has been confirmed by coring and drilling. We applied the standard AVO technique to the BSRs in turbidite/hemipelagic sediments crosscutting the dipping beds and those in debris-flow deposits to qualitatively assess the gas hydrate and gas concentrations. These BSRs are not likely to be affected by thin-bed tuning which can significantly alter the AVO response of the BSR. The BSRs crosscutting the dipping beds in turbidite/hemipelagic sediments are of low-seismic amplitude and characterized by a small positive gradient, indicating a decrease in Poisson’s ratio in the gas-hydrate stability zone (GHSZ), which, in turn, suggests the presence of gas hydrate. The BSRs in debris-flow deposits are characterized by a negative gradient, indicating decreased Poisson’s ratio below the GHSZ, which is likely due to a few percent or greater gas saturations. The increase in the steepness of the AVO gradient and the magnitude of the intercept of the BSRs in debris-flow deposits with increasing seismic amplitude of the BSRs is probably due to an increase in gas saturations, as predicted by AVO model studies based on rock physics. The reflection strength of the BSRs in debris-flow deposits, therefore, can be a qualitative measure of gas saturations below the GHSZ.     

Qualitative assessment of gas hydrate and gas concentrations from the AVO characteristics of the BSR in the Ulleung Basin,East Sea (Japan Sea)

The presence of gas hydrate in the Ulleung Basin, East Sea (Japan Sea), inferred by various seismic indicators, including the widespread bottom-simulating reflector (BSR), has been confirmed by coring and drilling. We applied the standard AVO technique to the BSRs in turbidite/hemipelagic sediments crosscutting the dipping beds and those in debris-flow deposits to qualitatively assess the gas hydrate and gas concentrations. These BSRs are not likely to be affected by thin-bed tuning which can significantly alter the AVO response of the BSR. The BSRs crosscutting the dipping beds in turbidite/hemipelagic sediments are of low-seismic amplitude and characterized by a small positive gradient, indicating a decrease in Poisson’s ratio in the gas-hydrate stability zone (GHSZ), which, in turn, suggests the presence of gas hydrate. The BSRs in debris-flow deposits are characterized by a negative gradient, indicating decreased Poisson’s ratio below the GHSZ, which is likely due to a few percent or greater gas saturations. The increase in the steepness of the AVO gradient and the magnitude of the intercept of the BSRs in debris-flow deposits with increasing seismic amplitude of the BSRs is probably due to an increase in gas saturations, as predicted by AVO model studies based on rock physics. The reflection strength of the BSRs in debris-flow deposits, therefore, can be a qualitative measure of gas saturations below the GHSZ.     

南海北部陆坡某海域浅层气的声学特征及其对水合物勘探的指示意义

海底天然气渗漏是海洋环境中广泛分布的自然现象,在世界各大洋中都有发现。海底渗漏的气体赋存于浅部地层,可以改变近表层沉积物的物理性质,使其在声学剖面上得以反映。通过对南海北部陆坡某海域研究区浅地层剖面和地震数据分析,在浅地层剖面上发现了声空白、声混浊、增强反射层、速度下拉等特征,在地震剖面上则识别出气烟囱或泥底辟、亮点、速度下拉、增强反射层等特征。以似海底反射层（BSR）作为地震剖面上明显的含气层标志,划分了2套含气系统。通过浅地层剖面与地震剖面联合解释认为,BSR之下气烟囱／泥底辟的发育导致了亮点、速度下拉、增强反射层等声学特征的发生,BSR之上水合物层的存在则可能起到封堵天然气而使其发生侧向运移的作用,在气体封堵相对薄弱的位置,天然气向上运移形成声空白、声混浊、增强反射层、速度下拉等特征。以声空白代表的天然气聚集带可能成为块状水合物的发育场所,可能成为较有潜力的勘探目标。     

Seismic Insights into Bottom Simulating Reflection (BSR) in the Krishna-Godavari Basin,Eastern Margin of India

The multichannel seismic data along one long-offset survey line from Krishna-Godavari (K-G) basin in the eastern margin of India were analyzed to define the seismic character of the gas hydrate/free gas bearing sediments. The discontinuous nature of bottom simulating reflection (BSR) was carefully examined. The presence of active faults and possible upward fluid circulation explain the discontinuous nature and low amplitude of the BSR. The study reveals free gas below gas hydrates, which is also indicated by enhancement of seismic amplitudes with offsets from BSR. These findings were characterized by computing seismic attributes such as the reflection strength and instantaneous frequency along the line. Geothermal gradients were computed for 18°C and 20°C temperature at the depth of BSR to understand the geothermal anomaly that can explain the dispersed nature of BSR. The estimated geothermal gradient shows an increase from 32°C/km in the slope region to 41°C/km in the deeper part, where free gas is present. The ray-based travel time inversion of identifiable reflected phases was also carried out along the line. The result of velocity tomography delineates the high-velocity (1.85–2.0 km/s) gas hydrate bearing sediments and low-velocity (1.45–1.5 km/s) free gas bearing sediments across the BSR.     

Geological controls on focused fluid flow through the gas hydrate stability zone on the southern Hikurangi Margin of New Zealand, evidenced from multi-channel seismic data

Highly concentrated gas hydrate deposits are likely to be associated with geological features that promote increased fluid flux through the gas hydrate stability zone (GHSZ). We conduct conventional seismic processing techniques and full-waveform inversion methods on a multi-channel seismic line that was acquired over a 125 km transect of the southern Hikurangi Margin off the eastern coast of New Zealand’s North Island. Initial processing, employed with an emphasis on preservation of true amplitude information, was used to identify three sites where structures and stratal fabrics likely encourage focused fluid flow into and through the GHSZ. At two of the sites, Western Porangahau Trough and Eastern Porangahau Ridge, sub-vertical blanking zones occur in regions of intensely deformed sedimentary layering. It is interpreted that increased fluid flow occurs in these regions and that fluids may dissipate upwards and away from the deformed zone along layers that trend towards the seafloor. At Eastern Porangahau Ridge we also observe a coherent bottom simulating reflection (BSR) that increases markedly in intensity with proximity to the centre of the anticlinal ridge. 1D full-waveform inversions conducted at eight points along the BSR reveal much more pronounced low-velocity zones near the centre of the ridge, indicating a local increase in the flux of gas-charged fluids into the anticline. At another anticline, Western Porangahau Ridge, a dipping high-amplitude feature extends from the BSR upwards towards the seafloor within the regional GHSZ. 1D full-waveform inversions at this site reveal that the dipping feature is characterised by a high-velocity zone overlying a low-velocity zone, which we interpret as gas hydrates overlying free gas. These results support a previous interpretation that this high-amplitude feature represents a local “up-warping” of the base of hydrate stability in response to advective heat flow from upward migrating fluids. These three sites provide examples of geological frameworks that encourage prolific localised fluid flow into the hydrate system where it is likely that gas-charged fluids are converting to highly concentrated hydrate deposits.     

Geological controls on focused fluid flow through the gas hydrate stability zone on the southern Hikurangi Margin of New Zealand,evidenced from multi-channel seismic data

Highly concentrated gas hydrate deposits are likely to be associated with geological features that promote increased fluid flux through the gas hydrate stability zone (GHSZ). We conduct conventional seismic processing techniques and full-waveform inversion methods on a multi-channel seismic line that was acquired over a 125 km transect of the southern Hikurangi Margin off the eastern coast of New Zealand’s North Island. Initial processing, employed with an emphasis on preservation of true amplitude information, was used to identify three sites where structures and stratal fabrics likely encourage focused fluid flow into and through the GHSZ. At two of the sites, Western Porangahau Trough and Eastern Porangahau Ridge, sub-vertical blanking zones occur in regions of intensely deformed sedimentary layering. It is interpreted that increased fluid flow occurs in these regions and that fluids may dissipate upwards and away from the deformed zone along layers that trend towards the seafloor. At Eastern Porangahau Ridge we also observe a coherent bottom simulating reflection (BSR) that increases markedly in intensity with proximity to the centre of the anticlinal ridge. 1D full-waveform inversions conducted at eight points along the BSR reveal much more pronounced low-velocity zones near the centre of the ridge, indicating a local increase in the flux of gas-charged fluids into the anticline. At another anticline, Western Porangahau Ridge, a dipping high-amplitude feature extends from the BSR upwards towards the seafloor within the regional GHSZ. 1D full-waveform inversions at this site reveal that the dipping feature is characterised by a high-velocity zone overlying a low-velocity zone, which we interpret as gas hydrates overlying free gas. These results support a previous interpretation that this high-amplitude feature represents a local “up-warping” of the base of hydrate stability in response to advective heat flow from upward migrating fluids. These three sites provide examples of geological frameworks that encourage prolific localised fluid flow into the hydrate system where it is likely that gas-charged fluids are converting to highly concentrated hydrate deposits.     

Seismic character of gas hydrates on the Southeastern U.S. continental margin

Gas hydrates are stable at relatively low temperature and high pressure conditions; thus large amounts of hydrates can exist in sediments within the upper several hundred meters below the sea floor. The existence of gas hydrates has been recognized and mapped mostly on the basis of high amplitude Bottom Simulating Reflections (BSRs) which indicate only that an acoustic contrast exists at the lower boundary of the region of gas hydrate stability. Other factors such as amplitude blanking and change in reflection characteristics in sediments where a BSR would be expected, which have not been investigated in detail, are also associated with hydrated sediments and potentially disclose more information about the nature of hydratecemented sediments and the amount of hydrate present.Our research effort has focused on a detailed analysis of multichannel seismic profiles in terms of reflection character, inferred distribution of free gas underneath the BSR, estimation of elastic parameters, and spatial variation of blanking. This study indicates that continuous-looking BSRs in seismic profiles are highly segmented in detail and that the free gas underneath the hydrated sediment probably occurs as patches of gas-filled sediment having variable thickness. We also present an elastic model for various types of sediments based on seismic inversion results. The BSR from sediments of high ratio of shear to compressional velocity, estimated as about 0.52, encased in sediments whose ratios are less than 0.35 is consistent with the interpretation of gasfilled sediments underneath hydrated sediments. This model contrasts with recent results in which the BSR is explained by increased concentrations of hydrate near the base of the hydrate stability field and no underlying free gas is required.     

Seismic attribute study for gas hydrates in the Andaman Offshore India

Seismic data from the Andaman offshore region has been examined to investigate for the presence of gas hydrates. The seismic data displays reflection characteristics such as blanking, enhanced reflection patterns, shadows in instantaneous frequency, and increase in amplitude with the offset, which are indicative of gas hydrates and underlying free gas. A prominent bottom-simulating reflection, BSR, coupled with reverse polarity is observed around 650–700 ms. Seismic attributes such as the reflection strength and instantaneous frequency are computed along this reflector in order to probe for the presence of gas hydrates or free gas in this region. The reflection plot shows a strong reflector paralleling the seafloor. In addition, attenuation of the high frequency signal is noticed, indicating the presence of free gas below the BSR.     

南海北部陆坡东沙海域海底丘状体气体与水合物分布

海底丘状体在天然气水合物发育区是一种常见的微地貌,对丘状体的研究有助于理解海底流体渗漏模式以及水合物的赋存规律。本文研究南海北部陆坡东沙海域天然气水合物发育区海底丘状体的特征及其与水合物的关系。研究所用的数据包括准三维多道地震数据、多波束数据以及浅地层剖面数据。在多波束海底地形图上,丘状体表现为局部的正地形,直径大约为300 m,高出周围海底约50 m。浅地层剖面上存在明显的声空白以及同相轴下拉现象,指示了海底丘状体气体的分布以及流体运移的路径。丘状体周围明显的BSR表明局部区域可能发育有水合物,水合物钻探结果也证实了这一推测。三维多道地震剖面上,丘状体正下方存在空白反射区域,这与泥火山的地震反射特征类似。但空白反射区域内存在强振幅能量,而且丘状体正下方存在连续的反射层,这表明该丘状体并非泥火山成因。综合钻探结果以及三维地震成像结果,认为水合物形成过程引起的沉积物膨胀以及海底碳酸盐岩的沉淀是形成该丘状体的主要原因。     

Multi-frequency seismic study of gas hydrate-bearing sediments in Lake Baikal,Siberia

In this paper we present and discuss the frequency-dependent behaviour of the acoustic characteristics of methane hydrate-bearing sediments in Lake Baikal, Siberia. Five different types of seismic sources (airgun-array, two types of single airguns, watergun and sparker) are used, encompassing a frequency bandwidth from 10 up to 1000 Hz. On low-frequency airgun-array data, the base of the hydrate stability zone (HSZ) is observed as a high-amplitude bottom-simulating reflection (BSR) with reversed polarity. The amplitude and continuity of the BSR decrease or even disappear on medium- to high-frequency data, a feature explained in terms of vertical and horizontal resolution. The increasing reflection amplitude of the BSR with increasing offset, the calculated reflection coefficient of the BSR and the occurrence of enhanced reflections below the BSR suggest the presence of free gas below the HSZ. The observation of some enhanced reflections extending above the BSR may be interpreted as an indication for free gas co-existing with hydrates within the HSZ. Amplitude blanking above the BSR is highly variable while the BSR itself appears to act as a low-pass frequency filter for medium- to high-frequency data.New single-channel airgun profiles provide the first seismic information across the Baikal Drilling Project (BDP-97) deep drilling site, at which hydrate-bearing sediments were retrieved at about 200 m above the base of the local HSZ. At the drilling site there are no seismic characteristics indicative of the presence of hydrates. Combination of the drilling and seismic information has allowed us to make a rough estimation of the volume of hydrates and carbon stored in the sediments of Lake Baikal, which lead us to conclude that the Lake Baikal gas hydrate reservoirs do not form a prospective energy resource.     

Evidence for gas hydrate occurrences in the Canadian Arctic Beaufort Sea within permafrost-associated shelf and deep-water marine environments

The presence of a wedge of offshore permafrost on the shelf of the Canadian Beaufort Sea has been previously recognized and the consequence of a prolonged occurrence of such permafrost is the possibility of an underlying gas hydrate regime. We present the first evidence for wide-spread occurrences of gas hydrates across the shelf in water depths of 60–100 m using 3D and 2D multichannel seismic (MCS) data. A reflection with a polarity opposite to the seafloor was identified ∼1000 m below the seafloor that mimics some of the bottom-simulating reflections (BSRs) in marine gas hydrate regimes. However, the reflection is not truly bottom-simulating, as its depth is controlled by offshore permafrost. The depth of the reflection decreases with increasing water depth, as predicted from thermal modeling of the late Wisconsin transgression. The reflection crosscuts strata and defines a zone of enhanced reflectivity beneath it, which originates from free gas accumulated at the phase boundary over time as permafrost and associated gas hydrate stability zones thin in response to the transgression. The wide-spread gas hydrate occurrence beneath permafrost has implications on the region including drilling hazards associated with the presence of free gas, possible overpressure, lateral migration of fluids and expulsion at the seafloor. In contrast to the permafrost-associated gas hydrates, a deep-water marine BSR was also identified on MCS profiles. The MCS data show a polarity-reversed seismic reflection associated with a low-velocity zone beneath it. The seismic data coverage in the southern Beaufort Sea shows that the deep-water marine BSR is not uniformly present across the entire region. The regional discrepancy of the BSR occurrence between the US Alaska portion and the Mackenzie Delta region may be a result of high sedimentation rates expected for the central Mackenzie delta and high abundance of mass-transport deposits that prohibit gas to accumulate within and beneath the gas hydrate stability zone.     

Characteristics of Gas Hydrate and Free Gas Offshore Southwestern Taiwan from a Combined MCS/OBS Data Analysis

In this study, we present the results of the combined analyses of ocean bottom seismometer and multi-channel seismic reflection data collection offshore southwestern Taiwan, with respect to the presence of gas hydrates and free gas within the accretionary wedge sediments. Estimates of the compressional velocities along EW9509-33 seismic reflection profile are obtained by a series of pre-stack depth migrations in a layer stripping streamlined Deregowski loop. Strong BSR is imaged over most of the reflection profile while low velocity zones are imaged below BSR at several locations. Amplitude versus angle analysis that are performed within the pre-stack depth migration processes reveal strong negative P-impedance near the bottom of the hydrate stability zone, commonly underlain by sharp positive P impedance layers associated with negative pseudo-Poisson attribute areas, indicating the presence of free gas below the BSR. Ray tracing of the acoustic arrivals with a model derived from the migration velocities generally fits the vertical and hydrophone records of the four ocean-bottom seismographs (OBS). In order to estimate the Poisson’s ratios in the shallow sediments at the vicinity of the OBSs, we analyze the mode-converted arrivals in the wide-angle horizontal component. P-S mode converted reflections are dominant, while upward P-S transmissions are observed at large offsets. We observe significant compressional velocity and Poisson’s ratio pull-down in the sediment below the BSR likely to bear free gas. When compared to Poisson’s ratio predicted by mechanical models, the values proposed for the OBSs yield rough estimates of gas hydrate saturation in the range of 0–10% in the layers above the BSR and of free gas saturation in the range of 0–2% just below the BSR.     

Variability of the bottom-simulating reflector (BSR) and its association with tectonic structures in the Chilean margin between Arauco Gulf (37°S) and Valdivia (40°S)

Multichannel seismic reflection data recorded between Arauco Gulf (37°S) and Valdivia (40°S), on the Chilean continental margin, were processed and modeled to obtain seismic images and sub-surface models, in order to characterize the variability of the bottom-simulating reflector (BSR), which is a geophysical marker for the presence of gas hydrates. The BSR is discontinuous and interrupted by submarine valleys, canyons, as well as by faults or fractures. The BSR occurrence is more common south of Mocha Island due to moderate slopes and greater organic matter contribution by rivers in that area. Tectonic uplift and structural instability change the stability gas hydrate zone and consequently the BSR position, creating in some cases missing or double BSRs. Our modeling supports the presence of gas hydrate above the BSR and free gas below it. Higher BSR amplitudes support higher hydrate or free gas concentrations. In the study area, gas hydrate concentration is low (an average of 3.5%) suggesting disseminated gas hydrate distribution within the sediments. Also higher BSR amplitudes are associated with thrust faults in the accretionary prism, which serve as conduits for gas flow from deeper levels. This extra gas supply produces a wider thickness of gas hydrates or free gas.     

Migration of methane at the diapiric structure of the western continental margin of India — insights from seismic data

Investigations are being carried out to locate the indirect evidences of gas occurrences in the Indian Continental Margins by identifying features like pockmarks, shale diapirs, venting as well as bottom simulating reflectors (BSR). We investigate one such case of a shale diapirs. Occurrence of free gas and gas hydrates is reported using a complex trace analysis to study the variation of reflection strength and instantaneous frequency along individual reflections. A frequency analysis over the flank of the shale diapir indicates attenuation of high frequencies, possibly representing the migration of fluids (probably methane) along the fault pathways. Similarly a plot of reflection strength along an individual reflector shows the migration pathways and significant blanking.     

Gas hydrate formation in compressional,extensional and un-faulted structural settings – Examples from New Zealand's Hikurangi margin

We investigate gas hydrate formation processes in compressional, extensional and un-faulted settings on New Zealand's Hikurangi margin using seismic reflection data. The compressional setting is characterized by a prominent subduction wedge thrust fault that terminates beneath the base of gas hydrate stability, as determined from a bottom-simulating reflection (BSR). The thrust is surrounded by steeply dipping strata that cross the BSR at a high angle. Above the BSR, these strata are associated with a high velocity anomaly that is likely indicative of relatively concentrated, and broadly distributed, gas hydrates. The un-faulted setting—sedimentary infill of a slope basin on the landward side of a prominent thrust ridge—is characterized by a strong BSR, a thick underlying free gas zone, and short positive polarity reflection segments that extend upward from the BSR. We interpret the short reflection segments as the manifestation of gas hydrates within relatively coarse-grained sediments. The extensional setting is a localized, shallow response to flexural bending of strata within an anticline. Gas has accumulated beneath the BSR in the apex of folding. A high-velocity zone directly above the BSR is probably mostly lithologically-derived, and only partly related to gas hydrates. Although each setting shows evidence for focused gas migration into the gas hydrate stability zone, we interpret that the compressional tectonic setting is most likely to contain concentrated gas hydrates over a broad region. Indeed, it is the only setting associated with a deep-reaching fault, meaning it is the most likely of the three settings to have thermogenic gas contributing to hydrate formation. Our results highlight the importance of anisotropic permeability in layered sediments and the role this plays in directing sub-surface fluid flow, and ultimately in the distribution of gas hydrate. Each of the three settings we describe would warrant further investigation in any future consideration of gas hydrates as an energy resource on the Hikurangi margin.     

Canyon-infilling and gas hydrate occurrences in the frontal fold of the offshore accretionary wedge off southern Taiwan

We utilized reflection seismic and bathymetric data to infer the canyon-infilling, fold uplift, and gas hydrate occurrences beneath the frontal fold at the toe of the accretionary wedge, offshore SW Taiwan. The lateral migrating paleo-Penghu canyons has cut across the frontal fold with six distinct canyon/channel incisions marked by channel infills. The longitudinal bathymetric profile along the modern canyon course shows a knickpoint of ~300 m relief at this frontal fold, indicating that the rate of fold uplift is greater than that of canyon incision. The age for the initial thrusting of this fontal fold is around 240 kyr ago, as estimated by using the maximum thickness of growth strata of this fold divided by the sedimentation rate obtained from a nearby giant piston core. Bottom simulating reflector (BSR) on seismic sections indicates the base of gas hydrate stability zone. Beneath the frontal fold, there is a widespread occurrence of BSRs, suggesting the highly probable existence of substantial quantities of gas hydrates. A seismic flat spot and a few push-down reflectors below BSR are found lying beneath the anticlinal axis with bathymetric four-way dip closure. The flat spot, cutting across a series of dipping reflections beneath BSR, may indicate the contact between free gas and its underlying formation water. The push-down reflectors beneath BSRs are interpreted to result from abundant free gas hosted beneath the gas hydrate stability zone. The multiple paleo-canyon infills seen along and beneath the frontal fold and above BSRs may provide thick porous sands to host gas hydrates in the frontal fold.     

Multiple seismic attribute analyses for determination of bottom simulating reflector of gas hydrate seismic data in the Ulleung Basin of Korea

The bottom simulating reflector (BSR), the boundary between the gas hydrate and the free gas zone, is considered to be the most common evidence in seismic data analysis for gas hydrate exploration. Multiple seismic attribute analyses of reflectivity and acoustic impedance from the post-stack deconvolution and complex analysis of instantaneous attribute properties including the amplitude envelope, instantaneous frequency, phase, and first derivative of the amplitude of seismic data have been used to effectively confirm the existence of a BSR as the base of gas hydrate stability zone. In this paper, we consider individual seismic attribute analysis and integrate the results of those attributes to locate the position of the BSR. The outputs from conventional seismic data processing of the gas hydrate data set in the Ulleung Basin were used as inputs for multiple analyses. Applying multiple attribute analyses to the individual seismic traces showed that the identical anomalies found in two-way travel time (TWT) between 3.1 and 3.2 s from the results of complex analyses and l ₁ norm deconvolution indicated the location of the BSR.     

Gas hydrate stability zone modelling in areas of salt tectonics and pockmarks of the Barents Sea suggests an active hydrocarbon venting system

The Barents Sea seabed exhibits an area of major glacial erosion exposing parts of the old hydrocarbon basins. In this region, we modelled the gas hydrate stability field in a 3D perspective, including the effects of higher order hydrocarbon gases. We used 3D seismic data to analyse the linkage between fluid-flow expressions and hydrate occurrences above old sedimentary basin systems and vertical faults. Pockmarks showed a relation to fault systems where some of them are directly connected to hydrocarbon bearing sedimentary formations. The influence of bottom water temperature, pore water salinity and geothermal gradient variation on gas hydrate stability zone (GHSZ) thickness is critically analysed in relation to both geological formations and salt tectonics. Our analysis suggests a highly variable GHSZ in the Barents Sea region controlled by local variations in the parameters of stability conditions. Recovery of gas-hydrate sample from the region and presence of gas-enhanced reflections below estimated BSR depths may indicate a prevalent gas-hydrate stable condition.