Probable gas hydrate/free gas model over western continental margin of India

Multi-channel seismic reflection data from the western continental margin of India (WCMI) have been analyzed to construct a plausible model for gas hydrate formation. A reflector at 2950 ms two way travel time (TWT) on one of the sections is interpreted to represent the base of the layer of the methane hydrate, identified by a bottom simulating reflector (BSR) that lies almost 500 ms beneath the sea floor. BSRs of similar origin are common world wide, where they are usually interpreted to mark the base of gas hydrate bearing clastic sediment, with or without underlying free gas. In this study we present a model with the contrasting physical properties that produce synthetic wavelets that match with the observed BSR amplitude and waveforms for varying source-receiver offsets of multi-channel seismic reflection data. The preliminary results presented here put important constraints on models that predict the distribution and formation of hydrate. Offset-dependent amplitude recovery also gives an appropriate response for hydrate characterization.     

In situ geotechnical characterization of sediments on the Nova Scotian Slope, eastern Canadian continental margin

A seabed 2-m-long cone penetrometer and coring system (Geotechnical Module) has been used at 17 stations in four transects on the Scotian Slope to characterise in situ shear strength and induced pore pressure on several different types of late Pleistocene and early Holocene failure. Study sites were selected using the SAR high-resolution deep-towed acoustic system equipped with a digital 160–190 kHz sidescan sonar and a 3.5 kHz subbottom profiler.

Several distinctive types of “geotechnical signature” were recognised from plots of cone resistance and induced pore pressure with depth in the sediment. Normally consolidated sediments show a progressive increase in cone resistance with depth (to about 75 kPa at 2 m subbottom). Holocene surficial muds show spectacular apparent overconsolidation, reaching a peak of 250 kPa at about 50 cm subbottom and then decreasing down to 1.5 m. This overconsolidation is associated with Zoophycos burrows. Late Pleistocene sediments exhumed by bedding plane slides show strong true overconsolidation consistent with the original depth of burial inferred from high-resolution seismic stratigraphy. Debris flows show only a slight shear stress gradient with depth (40–45 kPa over 0.5–1 m subbottom) with under-consolidation due to remoulding of sediment.     

Seismic stratigraphy of the central Scotian Rise: A record of continental margin glaciation

3.5-k Hz profiles from low channel levees on the Scotian Rise show transparent Holocene acoustic facies overlying stratified glacial facies, dated by Carbon-14 in cores. Corresponding acoustic facies in seismic records are correlated with glaciations by counting back from the present sea floor using sedimentation rates from Carbon-14 dating and biostratigraphy from wells as a guide. The regional thickness and character of the seismic units correlate with the duration and intensity of glacial periods inferred from the global isotopic record. Changes in glacial supply to the continental margin are interpreted using this chronology, which shows stage 12 as the first widespread erosional glacial event.     

Gas hydrate and free gas distribution from inversion of seismic data on the South Shetland margin (Antarctica)

Travel-time inversion is applied to seismic data to produce acoustic velocity images of the upper 800 m of the South Shetland margin (Antarctic Peninsula) in three different geological domains: (i) the continental shelf; (ii) the accretionary prism; (iii) the trench. The velocity in the continental shelf sediments is remarkably higher, up to 1000 m/s at 600–700 m below seafloor, than that of the other two geological domains, due to the sediment overcompaction and erosion induced by the wax and waning of a grounded ice sheet. Pre-stack depth migration was applied to the data in order to improve the seismic image and to test the quality of the velocity fields. Where the Bottom Simulating Reflector (BSR) is present, positive and negative velocity anomalies were found with respect to a reference empirical velocity profile. The 2D-velocity section was translated in gas hydrate and free gas distribution by using a theoretical approach. The analysis revealed that the BSR is mainly related to the presence of free gas below it. The free gas is distributed in the area with variable concentration and thickness, while the gas hydrate is quite uniformly distributed across the margin.     

Estimation of seismic velocities and gas hydrate concentrations: A case study from the Shenhu area,northern South China Sea

In the Shenhu area of the northern South China Sea (SCS), canyon systems and focused fluid flow systems increase the complexity of the gas hydrate distribution in the region. It also induces difficulties in predicting the hydrate reservoir characteristics and quantitatively evaluating reservoir parameters. In this study, several inversion methods have been executed to estimate the velocities of strata and gas hydrate concentrations along a profile in the Shenhu area. The seismic data were inverted to obtain the reflection coefficient of each stratum via a spectral inversion method. Stratigraphic horizons were then delineated by tracking the inverted reflectivities. Based on the results of spectral inversion, a low-frequency velocity field of the strata was constructed for acoustic impedance inversion. Using a new iterative algorithm for acoustic impedance inversion, reflection coefficients were converted into velocities, and the velocity variations of the strata along a 2D seismic line were then obtained. Subsequently, gas hydrate saturations at well SH2 were estimated via the shale-corrected resistivity method, the chloride ion concentration method and three different rock physics models. The results were then compared to determine the optimal rock physics model, and the modified Wood equation (MWE) was found to be appropriate for this area. Finally, the inverted velocities and MWE were used to predict the distribution and concentrations of gas hydrates along the seismic line. The estimated spatial distribution of gas hydrates is consistent with that from sonic logging and resistivity data at well SH2, and with the drilling results. Therefore, this method is applicable in areas with no well data, or with few wells, and provides an effective tool for predicting and evaluating gas hydrates using seismic data.     

Anomalous seismic reflections related to gas/gas hydrate occurrences along the western continental margin of India

Gas hydrates along continental margins are commonly inferred from the presence of bottom simulating reflectors (BSRs) on reflection seismic records. Shale and mud diapirs are often observed in the proximity of BSR-inferred gas hydrates. Analysis of data from documented gas-hydrate occurrences suggests that the areas where mud volcanoes exist on the seafloor are promising locations for sediments with high gas-hydrate concentration. Along the western continental margin of India (WCMI), we have identified several anomalous reflections on single-channel, analogue seismic records in the proximity of BSRs, from which the presence of gas-charged sediments and gas seepages was inferred. These features characterize both the shelf-slope region of the WCMI and the adjoining deep-sea areas. The seismic records also reveal mud/shale diapiric activity and pockmarks near the gas hydrates.     

Low-amplitude BSRs and gas hydrate concentration on the northern margin of the South China Sea

The passive northern continental margin of the South China Sea is rich in gas hydrates, as inferred from the occurrence of bottom-simulating reflectors (BSR) and from well logging data at Ocean Drilling Program (ODP) drill sites. Nonetheless, BSRs on new 2D multichannel seismic reflection data from the area around the Dongsha Islands (the Dongsha Rise) are not ubiquitous. They are confined to complex diapiric structures and active fault zones located between the Dongsha Rise and the surrounding depressions, implying that gas hydrate occurrence is likewise limited to these areas. Most of the BSRs have low amplitude and are therefore not clearly recognizable. Acoustic impedance provides information on rock properties and has been used to estimate gas hydrate concentration. Gas hydrate-bearing sediments have acoustic impedance that is higher than that of the surrounding sediments devoid of hydrates. Based on well logging data, the relationship between acoustic impedance and porosity can be obtained by a linear regression, and the degree of gas hydrate saturation can be determined using Archie’s equation. By applying these methods to multichannel seismic data and well logging data from the northern South China Sea, the gas hydrate concentration is found to be 3–25% of the pore space at ODP Site 1148 depending on sub-surface depth, and is estimated to be less than values of 5% estimated along seismic profile 0101. Our results suggest that saturation of gas hydrate in the northern South China Sea is higher than that estimated from well resistivity log data in the gas hydrate stability zone, but that free gas is scarce beneath this zone. It is probably the scarcity of free gas that is responsible for the low amplitudes of the BSRs.     

冲绳海槽天然气水合物BSR的地震研究

根据多道地震反射资料分析,在冲绳海槽南部和中部发现了拟海底反射层(BSR)现象。通过对海底异常反射层的振幅特征、速度异常和AVO属性分析,说明该BSR可能反映了天然气水合物的存在,并发现冲绳海槽断层与天然气水合物的形成有密切关系。     

Seismic velocities and quantification of gas-hydrates from AVA modeling in the western continental margin of India

The most commonly used marker for the investigation of gas-hydrates is the bottom simulating reflector (BSR), which is caused by gas-hydrate laden sediment underlain by either brine or gas-saturated sediment. A BSR has been identified by seismic experiment in the Kerala-Konkan Basin of the western continental margin of India. Here we perform AVA modeling of seismic reflection data from a BSR to investigate the seismic velocities for quantitative assessment of gas-hydrates and to understand the origin of the BSR. The result reveals a P-wave velocity of 2.245 km/s and an S-wave velocity of 0.895 km/s for the sediments above the BSR. This corresponds to a Poisson ratio of 0.406 and hydrates saturation of ∼30% in the study area. The comparison of estimated P-wave velocity (1.77 km/s) above the hydrates-bearing sediment to that (1.78 km/s) below the BSR implies that the origin of the BSR is mainly due to gas-hydrates, as the presence (even in small quantities) of free-gas reduces the P-wave velocity considerably.     

Source and accumulation of gas hydrate in the northern margin of the South China Sea

The northern South China Sea (NSCS) experienced continuous evolution from an active continental margin in the late Mesozoic to a stable passive continental margin in the Cenozoic. It is generally believed that the basins in the NSCS evolved as a result of Paleocene–Oligocene crustal extension and associated rifting processes. This type of sedimentary environment provides a highly favourable prerequisite for formation of large-scale oil- and gas–fields as well as gas hydrate accumulation. Based on numerous collected data, combined with the tectonic and sedimentary evolution, a preliminary summary is that primitive coal-derived gas and reworked deep gas provided an ample gas source for thermogenic gas hydrate, but the gas source in the superficial layers is derived from humic genesis. In recent years, the exploration and development of the NSCS oil, gas and gas hydrate region has provided a basis for further study. A number of 2D and 3D seismic profiles, the synthetic comparison among bottom simulating reflector (BSR) coverage characteristics, the oil-gas area, the gas maturity and the favourable hydrate-related active structural zones have provided opportunities to study more closely the accumulation and distribution of gas hydrate. The BSR has a high amplitude, with high amplitude reflections below it, which is associated with gas chimneys and pockmarks. The high amplitude reflections immediately beneath the BSR are interpreted to indicate the presence of free gas and gas hydrate. The geological and geochemical data reveal that the Cenozoic northern margin of the NSCS has developed coal-derived gas which forms an abundant supply of thermogenic gas hydrate. Deep-seated faults and active tectonic structures facilitate the gas migration and release. The thermogenic gas hydrate and biogenic gas are located at different depths, have a different gas source genesis and should be separately exploited. Based on the proven gas hydrate distribution zone, we have encircled and predicted the potential hydrate zones. Finally, we propose a simple model for the gas hydrate accumulation system in the NSCS Basin.     

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.     

Velocity structure and gas hydrate saturation estimation on active margin off SW Taiwan inferred from seismic tomography

This paper presents results of a seismic tomography experiment carried out on the accretionary margin off southwest Taiwan. In the experiment, a seismic air gun survey was recorded on an array of 30 ocean bottom seismometers (OBS) deployed in the study area. The locations of the OBSs were determined to high accuracy by an inversion based on the shot traveltimes. A three-dimensional tomographic inversion was then carried out to determine the velocity structure for the survey area. The inversion indicates a relatively high P wave velocity (Vp) beneath topographic ridges which represent a series of thrust-cored anticlines develop in the accretionary wedge. The bottom-simulating reflectors (BSR) closely follow the seafloor and lies at 325 ± 25 m within the well-constrained region. Mean velocities range from ~1.55 km/s at the seabed to ~1.95 km/s at the BSR. We model Vp using an equation based on a modification of Wood’s equation to estimate the gas hydrate saturation. The hydrate saturation varies from 5% at the top ~200 m below the seafloor to 25% of pore space close to the BSR in the survey area.     

Evidence of shallow- and deep-water gas hydrate destabilizations in North Atlantic polar continental margin sediments

 Destabilization of gas hydrates from the North Atlantic polar continental margins is geophysically detectable within hydrate stability zones (HSZ). High-frequency seismic surveys of structures and propagation velocities of compressional waves have changed the classic conception of a consistently stable hydrate zone. The results are important in two respects: (1) unstable shallow-water gas hydrates can substantially contribute to the transfer of methane into the atmosphere, and (2) deep-water gas hydrates also indicate destabilization, which results in slope instability with probably only a secondary role in the transfer of methane to the atmosphere and thus in the greenhouse effect. Received: 6 August 1997 / Revision received: 26 January 1998     

Occurrence and exploration of gas hydrate in the marginal seas and continental margin of the Asia and Oceania region

Supplies of conventional natural gas and oil are declining fast worldwide, and therefore new, unconventional forms of energy resources are needed to meet the ever-increasing demand. Amongst the many different unconventional natural resources are gas hydrates, a solid, ice-like crystalline compound of methane and water formed under specific low temperature and high pressure conditions. Gas hydrates are believed to exist in large quantities worldwide in oceanic regions of continental margins, as well as associated with permafrost regions in the Arctic. Some studies to estimate the global abundance of gas hydrate suggest that the total volume of natural gas locked up in form of gas hydrates may exceed all known conventional natural gas reserves, although large uncertainties exist in these assessments. Gas hydrates have been intensively studied in the last two decades also due to connections between climate forcing (natural and/or anthropogenic) and the potential large volumes of methane trapped in gas hydrate accumulations. The presence of gas hydrate within unconsolidated sediments of the upper few hundred meters below seafloor may also pose a geo-hazard to conventional oil and gas production. Additionally, climate variability and associated changes in pressure-temperature regimes and thus shifts in the gas hydrate stability zone may cause the occurrence of submarine slope failures.Several large-scale national gas hydrate programs exist especially in countries such as Japan, Korea, Taiwan, China, India, and New Zealand, where large demands of energy cannot be met by domestic supplies from natural resources. The past five years have seen several dedicated deep drilling expeditions and other scientific studies conducted throughout Asia and Oceania to understand gas hydrates off India, China, and Korea. This thematic set of publications is dedicated to summarize the most recent findings and results of geo-scientific studies of gas hydrates in the marginal seas and continental margin of the Asia, and Oceania region.     

Occurrence and exploration of gas hydrate in the marginal seas and continental margin of the Asia and Oceania region

Supplies of conventional natural gas and oil are declining fast worldwide, and therefore new, unconventional forms of energy resources are needed to meet the ever-increasing demand. Amongst the many different unconventional natural resources are gas hydrates, a solid, ice-like crystalline compound of methane and water formed under specific low temperature and high pressure conditions. Gas hydrates are believed to exist in large quantities worldwide in oceanic regions of continental margins, as well as associated with permafrost regions in the Arctic. Some studies to estimate the global abundance of gas hydrate suggest that the total volume of natural gas locked up in form of gas hydrates may exceed all known conventional natural gas reserves, although large uncertainties exist in these assessments. Gas hydrates have been intensively studied in the last two decades also due to connections between climate forcing (natural and/or anthropogenic) and the potential large volumes of methane trapped in gas hydrate accumulations. The presence of gas hydrate within unconsolidated sediments of the upper few hundred meters below seafloor may also pose a geo-hazard to conventional oil and gas production. Additionally, climate variability and associated changes in pressure-temperature regimes and thus shifts in the gas hydrate stability zone may cause the occurrence of submarine slope failures.Several large-scale national gas hydrate programs exist especially in countries such as Japan, Korea, Taiwan, China, India, and New Zealand, where large demands of energy cannot be met by domestic supplies from natural resources. The past five years have seen several dedicated deep drilling expeditions and other scientific studies conducted throughout Asia and Oceania to understand gas hydrates off India, China, and Korea. This thematic set of publications is dedicated to summarize the most recent findings and results of geo-scientific studies of gas hydrates in the marginal seas and continental margin of the Asia, and Oceania region.     

Rock physics modeling for assessing gas hydrate and free gas: a case study in the Cascadia accretionary prism

We investigate the estimation of gas hydrate and free gas concentration using various rock physics models in the Cascadia accretionary prism, which is one of the most intensively studied regions of natural gas hydrate occurrences. Surface seismic reflection data is the most useful and cost-effective in deriving seismic velocity, and hence estimating gas hydrate and free gas across a BSR with depth, if a proper background (without gas hydrate and free gas) velocity is chosen. We have used effective medium theory of Helgerud et al. (EMTH) and, a combination of self-consistent approximation and differential effective medium (SCA-DEM) theory coupled with smoothing approximation for crystalline aggregate. Using the SCA-DEM (non-load-bearing) and EMTH (load-bearing) modeling, we calculate the average saturations of gas hydrate as 17 and 19%, respectively within ~100 m thick sedimentary column using velocity, derived from the surface seismic data. The saturations of gas hydrate are estimated as 15 and 18% using the SCA-DEM, and 20 and 25% using EMTH from the logging-while-drilling and wire-line sonic velocities, respectively. Estimations of gas hydrate from Poisson’s ratio are in average 50% for EMTH and 10% for SCA-DEM theory. We obtain the maximum saturation of free gas as 1–2% by employing the SCA-DEM theory either to seismic or sonic velocities, whereas the free-gas saturation varies between 0.1 and 0.4% for EMTH model. The gas hydrate saturation estimated from the sonic velocity and the free gas saturation derived from both the seismic and sonic velocities using the SCA-DEM modeling match quite well with those determined from the pressure core data in the study region.     

A seismic study to investigate the prospect of gas hydrate in Mahanadi deep water basin,northeastern continental margin of India

The presence of gas hydrates, one of the new alternative energy resources for the future, along the Indian continental margins has been inferred mainly from bottom simulating reflectors (BSR) and the gas stability zone thickness mapping. Gas hydrate reserves in Krishna Godawari Basin have been established with the help of gas-hydrate related proxies inferred from multidisciplinary investigations. In the present study, an analysis of 3D seismic data of nearly 3,420 km² area of Mahanadi deep water basin was performed in search of seismic proxies related with the existence of natural gas hydrate in the region. Analysis depicts the presence of BSR-like features over a large areal extent of nearly 250 km² in the central western part of the basin, which exhibit all characteristics of a classical BSR associated with gas hydrate accumulation in a region. The observed BSR is present in a specific area restricted to a structural low at the Neogene level. The coherency inversion of pre-stack time migration (PSTM) gathers shows definite inversion of interval velocity across the BSR interface which indicates hydrate bearing sediments overlying the free gas bearing sediments. The amplitude versus offset analysis of PSTM gathers shows increase of amplitude with offset, a common trend as observed in BSR associated with gas hydrate accumulation. Results suggest the possibility of gas hydrate accumulation in the central part of the basin specifically in the area of structural low at the Neogene level. These results would serve as preliminary information for selecting prospective gas hydrate accumulation areas for further integrated or individual study from geophysical, geological, geochemical and microbiological perspectives for confirmation of gas hydrate reserves in the area. Further, on the basis of these results it is envisaged that biogenic gas might have been generated in the region which under suitable temperature and pressure conditions might have been transformed into the gas hydrates, and therefore, an integrated study comprising geophysical, geological, geochemical and microbiological data is suggested to establish the gas hydrate reserves in Mahanadi deep water basin.     

Velocity and AVO analysis for the investigation of gas hydrate along a profile in the western continental margin of India

The occurrence of gas hydrate has been inferred from the presence of Bottom-Simulating Reflectors (BSRs) along the western continental margin of India. In this paper, we assess the spatial and vertical distribution of gas hydrates by analyzing the interval velocities and Amplitude Versus Offset (AVO) responses obtained from multi-channel seismics (MCSs). The hydrate cements the grains of the host sediment, thereby increasing its velocity, whereas the free gas below the base of hydrate stability zone decreases the interval velocity. Conventionally, velocities are obtained from the semblance analysis on the Common Mid-Point (CMP) gathers. Here, we used wave-equation datuming to remove the effect of the water column before the velocity analysis. We show that the interval velocities obtained in this fashion are more stable than those computed from the conventional semblance analysis. The initial velocity model thus obtained is updated using the tomographic velocity analysis to account for lateral heterogeneity. The resultant interval velocity model shows large lateral velocity variations in the hydrate layer and some low velocity zones associated with free gas at the location of structural traps. The reflection from the base of the gas layer is also visible in the stacked seismic data. Vertical variation in hydrate distribution is assessed by analyzing the AVO response at selected locations. AVO analysis is carried out after applying true amplitude processing. The average amplitudes of BSRs are almost constant with offset, suggesting a fluid expulsion model for hydrate formation. In such a model, the hydrate concentrations are gradational with maxima occurring at the base of hydrate stability zone.     

南海神狐水合物钻探区不同形态流体地震反射特征与水合物产出的关系

天然气水合物的分布在很大程度上受到含气流体运移的影响。南海北部陆坡区,尤其是珠江口盆地的白云凹陷,普遍存在流体渗漏的现象,暗示了水合物赋存的良好前景。神狐海域水合物钻探区内的高分辨率地震资料显示,区域内发育大量流体运移通道,在地震剖面上表现为不同形态的地震反射模糊带,根据其形态特征,可以划分为花冠状和穹顶状两大类模糊反射带。模糊反射带的存在意味着研究区内具有良好的含气流体运移条件,能够为甲烷气体的垂向运移提供通道。神狐海域水合物的钻探结果表明,水合物的分布与模糊反射带的分布范围具有良好的空间匹配关系,其中,花冠状地震反射模糊带侧翼部与中尺度正断层相连,促进了含气流体的侧向运移,顶部与可能的微裂隙相通,气体可向上运移至水合物稳定带,形成了水合物藏;而穹顶状地震反射模糊带顶部则通过疑似流体通道与海底沟通,这种结构极易形成气体逃逸而无法形成水合物。因此,不同形态特征的模糊反射带可能对水合物的分布具有一定的指示意义。     

南海北部大陆边缘天然气水合物稳定带厚度的地热学研究

The exploration of unconventional and/or new energy resources has become the focus of energy research worldwide,given the shortage of fossil fuels.As a potential energy resource,gas hydrate exists only in the environment of high pressure and low temperature,mainly distributing in the sediments of the seafloor in the continental margins and the permafrost zones in land.The accurate determination of the thickness of gas hydrate stability zone is essential yet challenging in the assessment of the exploitation potential.The majority of previous studies obtain this thickness by detecting the bottom simulating reflectors(BSRs) layer on the seismic profiles.The phase equilibrium between gas hydrate stable state with its temperature and pressure provides an opportunity to derive the thickness with the geothermal method.Based on the latest geothermal dataset,we calculated the thickness of the gas hydrate stability zone(GHSZ) in the north continental margin of the South China Sea.Our results indicate that the thicknesses of gas hydrate stability zone vary greatly in different areas of the northern margin of the South China Sea.The thickness mainly concentrates on 200–300 m and distributes in the southwestern and eastern areas with belt-like shape.We further confirmed a certain relationship between the GHSZ thickness and factors such as heat flow and water depth.The thickness of gas hydrate stability zone is found to be large where the heat flow is relatively low.The GHSZ thickness increases with the increase of the water depth,but it tends to stay steady when the water depth deeper than 3 000 m.The findings would improve the assessment of gas hydrate resource potential in the South China Sea.