Inferred gas hydrate on the Barents Sea shelf — a model for its formation and a volume estimate

 On the southwestern Barents Sea shelf, sediments containing gas hydrates that overlie free gas have been inferred from multichannel seismic data. The volume of suspected gas hydrate is tentatively estimated to about 1.9×10⁸ m³. The gas hydrate zone probably formed from thermogenic gas leaking from a deeper source. The hydrate zone may have thickened during the Neogene by including gas originally trapped as free gas below the hydrate following a significant downward migration of the isotherms caused by erosion and/or subsidence. Within the present oceanographic conditions, gas hydrate is suspected to be stable or slowly decomposing. Received: 20 December 1996 / Revision received: 20 August 1997     

Stratigraphic development of the south Vøring margin (Mid-Norway) since early Cenozoic time and its influence on subsurface fluid flow

The Cenozoic seismic stratigraphy and geological development of the south Vøring margin are analyzed to understand their relation to fluid flow and margin stability. The regional stratigraphy and palaeomorphology of the Møre and Vøring basins indicate gradual changes in depositional environment and tectonic compression between 55 Ma to 2.8 Ma during Brygge and Kai periods, and abrupt changes associated with glacial/interglacial cycles from last 2.8 Ma during Naust period. These changes resulted in deposition of various types of sediments and led to processes such as polygonal faulting and dewatering, inter-fingering of contouritic, stratified and glacigenic sediments, and margin progradation.A gas hydrate related bottom simulating reflector (BSR) occurs at Nyegga and within the central Vøring Basin while pockmarks are observed at Nyegga only. Diagentic reflectors due to Opal A - Opal CT conversion (DBSRs) occur along a wider area beyond the shelf edge. The DBSRs are located in oozes within the Kai and late Brygge Formations. The gas hydrate BSR occurrence is concentrated above Eocene depocenters in hemipelagic and contouritic sediments deposited during Late Plio-Pleistocene. The BSR overlies polygonal faults and DBSRs but are confined to the slope of anticlines indicating its formation being related to fluid pathways from methanogenic rocks through focused fluid flow. Microbial gas production in Kai, Brygge and deeper formations may have supplied the gas for gas hydrate formation. Fluid expulsion due to DBSR formation and polygonal faulting in oozes may have created overpressure development in permeable layers belonging to the overlying Naust Formation. Slide headwalls are also located close to the anticlines in the study area, implying that over pressured oozes and focussed fluid flow may have been important in creating weak surfaces in the overlying Naust sediments, promoting conditions for failures to occur.     

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.     

Analysis of shallow gas and fluid migration within the Plio-Pleistocene sedimentary succession of the SW Barents Sea continental margin using 3D seismic data

Three-dimensional (3D) seismic data acquired for hydrocarbon exploration reveal that gas accumulations are common within the 2–3 km thick Plio-Pleistocene stratigraphic column of the south-western Barents Sea continental margin. The 3D seismic data have relatively low-frequency content (<40 Hz) but, due to dense spatial sampling, long source-receiver offsets, 3D migration and advanced interpretation techniques, they provide surprisingly detailed images of inferred gas accumulations and the sedimentary environments in which they occur. The presence of gas is inferred from seismic reflection segments with anomalously high amplitude and reversed phase, compared with the seafloor reflection, so-called bright spots. Fluid migration is inferred from vertical zones of acoustic masking and acoustic pipes. The 3D seismic volume allows a spatial analysis of amplitude anomalies inferred to reflect the presence of gas and fluids. At several locations, seismic attribute maps reveal detailed images of flat spots, inferred to represent gas–water interfaces. The data indicate a focused fluid migration system, where sub-vertical faults and zones of highly fractured sediments are conduits for the migration of gas-bearing fluids in Plio-Pleistocene sediments. Gas is interpreted to appear in high-porosity fan-shaped sediment lobes, channel and delta deposits, glacigenic debris flows and sediment blocks, probably sealed by low-permeability, clayey till and/or (glacio)marine sediments. Gas and fluid flow are here attributed mainly to rapid Plio-Pleistocene sedimentation that loaded large amounts of sedimentary material over lower-density, fine-grained Eocene oozes. This probably caused pore-fluid dewatering of the high-fluid content oozes through a network of polygonal faults. The study area is suggested to have experienced cycles of fluid expulsion and hydrocarbon migration associated with glacial–interglacial cycles.     

南海北部天然气水合物研究进展

天然气水合物是一种新型的储量巨大的绿色能源,是目前世界各国研究界的研究热点之一。我国以及美国、日本、印度、韩国等国家都采集到了天然气水合物的实物样品。虽然我国对天然气水合物的研究起步较晚,但近年来的研究已经取得了飞速的进步,而且也于2007年5月在南海北部陆坡的神狐海域成功采集到天然气水合物的实物样品,这是在南海海域首次获取天然气水合物实物样品,证实了南海北部蕴藏着丰富的天然气水合物资源,标志着我国天然气水合物调查研究水平又上了一个新的台阶。目前,南海北部陆坡已经作为我国天然气水合物未来开发的战略选区之一。在总结我国天然气水合物以往十几年研究工作的基础上,综述了我国天然气水合物近年来在南海北部的地质、地球物理、地球化学3个方面的研究进展,提出了未来天然气水合物勘探和研究的方向和建议。     

Seismic expression of gas and gas hydrates across the western Black Sea

This study is a synthesis of gas-related features in recent sediments across the western Black Sea basin. The investigation is based on an extensive seismic dataset, and integrates published information from previous local studies. Our data reveal widespread occurrences of seismic facies indicating free gas in sediments and gas escape in the water column. The presence of gas hydrates is inferred from bottom-simulating reflections (BSRs). The distribution of the gas facies shows (1) major gas accumulations close to the seafloor in the coastal area and along the shelfbreak, (2) ubiquitous gas migration from the deeper subsurface on the shelf and (3) gas hydrate occurrences on the lower slope (below 750 m water depth). The coastal and shelfbreak shallow gas areas correspond to the highstand and lowstand depocentres, respectively. Gas in these areas most likely results from in situ degradation of biogenic methane, probably with a contribution of deep gas in the shelfbreak accumulation. On the western shelf, vertical gas migration appears to originate from a source of Eocene age or older and, in some cases, it is clearly related to known deep oil and gas fields. Gas release at the seafloor is abundant at water depths shallower than 725 m, which corresponds to the minimum theoretical depth for methane hydrate stability, but occurs only exceptionally at water depths where hydrates can form. As such, gas entering the hydrate stability field appears to form hydrates, acting as a buffer for gas migration towards the seafloor and subsequent escape.     

天然气水合物的聚集和形成及南海地质条件分析

天然气水合物目前已经成为世界范围的一个研究热点，而我国的天然气水合物研究起步则相对较晚，通过阅读国内外有关文献，总结了天然气水合物在海底的分布特征，聚集和形成机制，产状及其形成机理，甲烷羽的形成过程，天然气水合物在沉积物中的聚集位置通常有两种情况：一是较浅的沉积物（海底以下几米）中，受控于泥底辟，泥火山，断层等；二是较深的沉积物（海底以下几十米，甚至更深）中，受控于流体，当断层延伸至海底时，通常在水合物聚集处的上部发现甲烷羽，天然气以溶解气，游离气或分子扩散的形式运移，在温，压适宜的沉积物中，即水合物稳定带内聚集并形成水合物，水合物的形成过程是：最初形成晶体，呈分散状分布于沉积物中，之后逐渐聚集，生长成结核状，层状，最后形成块状，在细粒的浅层沉积物中，通常以较慢的速度生长，形成分散状的水合物；而在粗粒沉积物中，水合物通常呈填隙状，并且这种产状可能位于较深层位中，我国南海在温度，压力，构造条件，天然气来源等方面都能满足天然气水合物的形成条件，并且在南海也发现了一些水合物存在的标志，如似海底反射层（BSR）以及孔隙水中氯离子浓度的降低。因此，天然气水合物在我国南海海域可能有很好的前景。     

南海晚新生代构造运动与天然气水合物资源

南海在新生代经历过两次海底扩张产生了南海洋盆.南海北部和南部原来都是被动大陆边缘,但北部在晚新生代由于菲律宾海板块与欧亚板块在台湾地区发生了碰撞,使陆缘遭受到北西向挤压,在陆缘上产生了北西向左旋走滑活动,我们命名此次构造活动为东沙运动;南部陆缘在早中新世末由于南移的南沙地块与婆罗洲地块发生了碰撞,加上此时北移的菲律宾海板块在明都洛岛地区与欧亚板块发生碰撞,以及南部的东南苏拉威西地块与西北苏拉威西地块发生碰撞,在南海南部产生了挤压构造,我们命名此次构造运动为南沙运动.这两次新生代的构造运动改变了南北陆缘的性质,北部陆缘有人因此称之为准被动陆缘,而南部陆缘的南部则变成了挤压边缘.南海南北陆缘在晚新生代受到的挤压活动,对油气成藏和天然气水合物的形成有重要的推动作用,因为挤压活动有利于流体的流动,进而在适当的地方形成油气藏和天然气水合物.     

Evolution of the western Barents Sea

Information from multichannel seismic reflection data complemented by seismic refraction, gravity and magnetics forms the basis for a regional structural and evolutionary model of the western Barents Sea during post-Caledonian times. The western Barents Sea contains a thick succession, locally > 10 km, of Upper Paleozoic to Cenozoic sedimentary rocks covering a basement of probably Caledonian origin. The area is divided into three regional geological provinces: (1) an east-west trending basinal province between 74°N and the coast of Norway; (2) an elevated platform area to the north towards Svalbard; and (3) the western continental margin. Several structural elements of different origin and age have been mapped within each of these provinces. The main stratigraphic sequence boundaries have been tentatively dated from available well information, correlation with the geology of adjacent areas, and correlation with the interregional unconformities caused by relative changes of sea level. The main structural elements were developed during three major post-Caledonian tectonic phases: the Svalbardian phase in Late Devonian to Early Carboniferous times, the Mid and Late Kimmerian phase in Mid Jurassic to Early Cretaceous times and Cenozoic tectonism related to the progressive northward opening of the Norwegian-Greenland Sea. The sediments are predicted to be of mainly clastic origin except for a thick sequence of Middle Carboniferous — Lower Permian carbonates and evaporites. Salt diapirs have developed in several sub-basins, especially in the Nordkapp Basin where they form continuous salt walls that have pierced through > 7 km of sediments.     

Potential map of gas hydrate formation in Caspian Sea based on physicochemical stability evaluation of methane hydrate

Preliminary studies of Caspian Sea have shown the possibility of gas hydrate accumulations, because of suitable physicochemical conditions, existence of clayey deposits, and high concentrations of organic matter. Studies have indicated that gas hydrates are mainly composed of methane. Therefore, based on physicochemical equations for methane hydrate stability in different pressure, temperature, and salinity, this study was designed to calculate the potential of gas hydrate formation in the Caspian Sea basin. For this, data of more than 600 locations were analyzed and in each location, upper and lower limits of methane hydrate formation zone were calculated. Then, the zoning maps of upper and lower limits were prepared which can be useful for exploring the gas hydrate as an energy source or predicting gas hydrate hazards. According to the calculations and maps, methane hydrate formation in Caspian Sea, theoretically, can take place from near the seabed to 4000 and 2500 m beneath the sea surface when low and high geothermal gradient are supposed, respectively. By comparing the results with gas hydrate zones revealed in geophysical profiles, it has been shown that, in Caspian Sea, gas hydrates probably accumulate near the lower limit when a high geothermal gradient is assumed.     

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.     

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.     

Pockmark-like depressions near the Goliat hydrocarbon field,Barents Sea: Morphology and genesis

Pockmarks are observed worldwide along the continental margins and are inferred to be indicators of fluid expulsion. In the present study, we have analysed multibeam bathymetry and 2D/3D seismic data from the south-western Barents Sea, in relation to gas hydrate stability field and sediment type, to examine pockmark genesis. Seismic attributes of the sediments at and beneath the seafloor have been analysed to study the factors related to pockmark formation. The seabed depths in the study area are just outside the methane hydrate stability field, but the presence of higher order hydrocarbon gases such as ethane and/or propane in the expelled fluids may cause localised gas hydrate formation. The selective occurrence of pockmarks in regions of specific seabed sediment types indicates that their formation is more closely related to the type of seabed sediment than the source path of fluid venting such as faults. The presence of high acoustic backscatter amplitudes at the centre of the pockmarks indicates harder/coarser sediments, likely linked to removal of soft material. The pockmarks show high seismic reflection amplitudes along their fringes indicating deposition of carbonates precipitated from upwelling fluids. High seismic amplitude gas anomalies underlying the region away from the pockmarks indicate active fluid flow from hydrocarbon source rocks beneath, which is blocked by overlying less permeable formations. In areas of consolidated sediments, the upward flow is limited to open fault locations, while soft sediment areas allow diffused flow of fluids and hence formation of pockmarks over a wider region, through removal of fine-grained material.     

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.     

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 imaging of migration pathways by advanced attribute analysis,Alaminos Canyon 21, Gulf of Mexico

Potential accumulations of gas hydrates in Alaminos Canyon Block 21 (AC21) in the Gulf of Mexico are thought to occur in a shallow sand-rich interval, stratigraphically separated from sources of free gas below the base of the gas hydrate stability zone (BGHSZ), by an intervening thick layer of clay- and silt-rich sediments. Availability of sufficient gas charge from depth, in addition to local biogenic sourcing is considered key to the formation of gas hydrates in the GHSZ. Implicitly, a detailed understanding of geometries associated with fault and fracture networks in relation to potential gas migration pathways can provide additional confidence that seismic amplitude anomalies are related to gas hydrate accumulations. Delineation of fault and fracture systems from high resolution seismic data in and below the gas hydrates stability zone (GHSZ) was performed using an automated algorithm—Ant Tracking. The capturing of small-scale detail has particular significance at AC21, revealing a pervasive network of typically small-extent discontinuities, indicative of fracturing, throughout this intervening clay- and silt-rich layer of mass-transport deposits (MTDs). Ant Tracking features appear to correlate, to some extent, with potential gas hydrate accumulations, supporting the concept that fracturing possibly provides migration pathways albeit via a tortuous, complex path. This study demonstrates that the Ant Tracking attribute, in conjunction with detailed seismic interpretation and analysis, can provide valuable evidence of potential gas migration pathways.     

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.     

Gas hydrate within the Winona Basin, offshore western Canada

In western Canada gas hydrates have been thought to exist primarily in the Cascadia accretionary prism off southern Vancouver Island, British Columbia (BC). We present evidence for the existence of gas hydrate in folds and ridges of the Winona Basin up to 40 km seaward from the foot of the continental slope off northern Vancouver Island. The occurrence of a bottom-simulating reflector (BSR) observed in a number of vintage seismic reflection profiles is strongly correlated to faulted, and folded sedimentary ridges and buried folds. The observed tectonic structures of the Winona Basin are within the rapidly evolving Juan de Fuca - Cascadia - Queen Charlotte triple junction off BC. Re-processing of multi-channel data imaged mildly to strongly deformed sediments; the BSR is confined to sediments with stronger deformation. Changes in the amplitude character of sediment-reflections above and below the depth of the base of gas hydrate stability zone were also used as an indicator for the presence of gas hydrate. Additionally, regional amplitude and frequency reduction below some strong BSR occurrences may indicate free gas accumulations. Gas hydrate formation in the Winona Basin appears strongly constrained to folds and ridges and thus correlated to deeper-routed fluid-advection regimes. Methane production from in situ microbial activities as a source of gas to form gas hydrates, as proposed to be a major contributor for gas hydrates within the accretionary prism to the south, appears to be insufficient to produce the widespread gas hydrate occurrences in the Winona Basin. Potential reasons for the lack of sufficient in situ gas production may be that sedimentation rates are 5-100 times higher than those in the accretionary prism so that available organic carbon moves too quickly through the gas hydrate stability field. The confinement of BSRs to ridges and folds within the Winona Basin results in an areal extent of gas hydrate occurrences that is a factor of five less than what is expected from regional gas hydrate stability field mapping using water-depth (pressure) as the only controlling factor only.