首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
Integrated geological, geochemical, and geophysical exploration since 2004 has identified massive accumulation of gas hydrate associated with active methane seeps on the Umitaka Spur, located in the Joetsu Basin on the eastern margin of Japan Sea. Umitaka Spur is an asymmetric anticline formed along an incipient subduction zone that extends throughout the western side of the Japanese island-arc system. Seismic surveys recognized chimney structures that seem strongly controlled by a complex anticlinal axial fault system, and exhibit high seismic amplitudes with apparent pull-up structures, probably due to massive and dense accumulation of gas hydrate. Bottom simulating reflectors are widely developed, in particular within gas chimneys and in the gently dipping eastern flank of the anticline, where debris can store gas hydrates that may represent a potential natural gas resource. The axial fault system, the shape of the anticline, and the carrier beds induce thermogenic gas migration to the top of the structure, and supply gas to the gas hydrate stability zone. Gas reaching the seafloor produces strong seepages and giant plumes in the sea water column.  相似文献   

2.
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.  相似文献   

3.
《Marine and Petroleum Geology》2012,29(10):1915-1931
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.  相似文献   

4.
A gas hydrate reservoir is hosted in marine sediments of an accretionary prism, located offshore the South Shetland Islands (Antarctic Peninsula), and affected by widespread deformations. To analyse gas hydrate distribution and fluid circulation inside sediments, available velocity models were used. Seismic velocities are translated in terms of hydrate porosity, which is the difference between the reference porosity (i.e., the porosity without gas hydrate) and the effective porosity (i.e., the porosity reduced by the gas hydrate presence). The pre-stack depth migration sections underlined the presence of several geological features, such as gentle and open folds, fractures and faults. In this paper, we observed a relationship between syncline–anticline structures and hydrate presence. In particular, a relationship is underlined between the hydrate porosity values and the distance from the hinge of the anticline: the hydrate porosity increases toward the limbs of anticline. The micro-fracturing model supports the idea that the syncline favours the hydrate formation, while the anticline favours the free gas accumulation below the bottom simulating reflector.  相似文献   

5.
Muri Basin in the Qilian Mountain is the only permafrost area in China where gas hydrate samples have been obtained through scientific drilling. Fracture-filling hydrate is the main type of gas hydrate found in the Qilian Mountain permafrost. Most of gas hydrate samples had been found in a thin-layer-like, flake and block group in a fracture of Jurassic mudstone and oil shale, although some pore-filling hydrate was found in porous sandstone. The mechanism for gas hydrate formation in the Qilian Mountain permafrost is as follows: gas generation from source rock was controlled by tectonic subsidence and uplift--gas migration and accumulation was controlled by fault and tight formation--gas hydrate formation and accumulation was controlled by permafrost. Some control factors for gas hydrate formation in the Qilian Mountain permafrost were analyzed and validated through numerical analysis and laboratory experiments. CSMGem was used to estimate the gas hydrate stability zone in the Qilian permafrost at a depth of 100–400 m. This method was used to analyze the gas composition of gas hydrate to determine the gas composition before gas hydrate formation. When the overlying formation of gas accumulation zone had a permeability of 0.05 × 10−15 m2 and water saturation of more than 0.8, gas from deep source rocks was sealed up to form the gas accumulation zone. Fracture-filling hydrate was formed in the overlap area of gas hydrate stability zone and gas accumulation zone. The experimental results showed that the lithology of reservoir played a key role in controlling the occurrence and distribution of gas hydrate in the Qilian Mountain permafrost.  相似文献   

6.
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.  相似文献   

7.
In this paper, we address the irregular behaviour and geometry of the gas hydrate stability zone (HSZ) inferred from reflection seismic data in relation to heat-flow measurements. The study area lies in the hanging wall of the Posolsky fault in the Southern Baikal Basin (SBB). Side-scan sonar imagery already revealed an undulating antithetic active fault structure and several isolated active vent structures. Remarkably, these fluid discharge structures occur only where the base of the hydrate stability zone (BHSZ), as inferred from seismic reflection profiles, is fluctuating and discontinuous, independent of lake floor morphology. The correlation between the interpreted BHSZ and heat-flow data across the Malenki seep is reasonable. On a seismic profile south of the fluid escape features, the BHSZ is expressed as an oscillatory, but continuous reflection, and shows poor correlation with heat-flow measurements. In nearly all cases, measured heat-flow exceeds inferred heat-flow. Additionally, the local inferred minima are anomalously low compared to the expected background values in the SBB. These observations suggest that the present-day hydrate accumulation and its (meta-)stability are more complicated than originally suspected. The limited area of these anomalies, their amplitudes and their occurrence in the immediate vicinity of faults and fluid escape features suggest that fluid convection cells disturb local gas hydrate stability conditions.  相似文献   

8.
Gas hydrate was discovered in the Krishna–Godavari (KG) Basin during the India National Gas Hydrate Program (NGHP) Expedition 1 at Site NGHP-01-10 within a fractured clay-dominated sedimentary system. Logging-while-drilling (LWD), coring, and wire-line logging confirmed gas hydrate dominantly in fractures at four borehole sites spanning a 500 m transect. Three-dimensional (3D) seismic data were subsequently used to image the fractured system and explain the occurrence of gas hydrate associated with the fractures. A system of two fault-sets was identified, part of a typical passive margin tectonic setting. The LWD-derived fracture network at Hole NGHP-01-10A is to some extent seen in the seismic data and was mapped using seismic coherency attributes. The fractured system around Site NGHP-01-10 extends over a triangular-shaped area of ∼2.5 km2 defined using seismic attributes of the seafloor reflection, as well as “seismic sweetness” at the base of the gas hydrate occurrence zone. The triangular shaped area is also showing a polygonal (nearly hexagonal) fault pattern, distinct from other more rectangular fault patterns observed in the study area. The occurrence of gas hydrate at Site NGHP-01-10 is the result of a specific combination of tectonic fault orientations and the abundance of free gas migration from a deeper gas source. The triangular-shaped area of enriched gas hydrate occurrence is bound by two faults acting as migration conduits. Additionally, the fault-associated sediment deformation provides a possible migration pathway for the free gas from the deeper gas source into the gas hydrate stability zone. It is proposed that there are additional locations in the KG Basin with possible gas hydrate accumulation of similar tectonic conditions, and one such location was identified from the 3D seismic data ˜6 km NW of Site NGHP-01-10.  相似文献   

9.
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.  相似文献   

10.
In order for methane to be economically produced from the seafloor, prediction and detection of massive hydrate deposits will be necessary. In many cases, hydrate samples recovered from seafloor sediments appear as veins or nodules, suggesting that there are strong geologic controls on where hydrate is likely to accumulate. Experiments have been conducted examining massive hydrate accumulation from methane gas bubbles within natural and synthetic sediments in a large volume pressure vessel through temperature and pressure data, as well as visual observations. Observations of hydrate growth suggest that accumulation of gas bubbles within void spaces and at sediment interfaces likely results in the formation of massive hydrate deposits. Methane hydrate was first observed as a thin film forming at the gas/water interface of methane bubbles trapped within sediment void spaces. As bubbles accumulated, massive hydrate growth occurred. These experiments suggest that in systems containing free methane gas, bubble pathways and accumulation points likely control the location and habit of massive hydrate deposits.  相似文献   

11.
This paper presents a computational model for mapping the regional 3D distribution in which seafloor gas hydrates would be stable, that is carried out in a Geographical Information System (GIS) environment. The construction of the model is comprised of three primary steps, namely: (1) the construction of surfaces for the various variables based on available 3D data (seafloor temperature, geothermal gradient and depth-pressure); (2) the calculation of the gas function equilibrium functions for the various hydrocarbon compositions reported from hydrate and sediment samples; and (3) the calculation of the thickness of the hydrate stability zone. The solution is based on a transcendental function, which is solved iteratively in a GIS environment.The model has been applied in the northernmost continental slope of the Gulf of Cadiz, an area where an abundant supply for hydrate formation, such as extensive hydrocarbon seeps, diapirs and fault structures, is combined with deep undercurrents and a complex seafloor morphology. In the Gulf of Cadiz, the model depicts the distribution of the base of the gas hydrate stability zone for both biogenic and thermogenic gas compositions, and explains the geometry and distribution of geological structures derived from gas venting in the Tasyo Field (Gulf of Cadiz) and the generation of BSR levels on the upper continental slope.  相似文献   

12.
To confirm the seabed fluid flow at the Haima cold seeps, an integrated study of multi-beam and seismic data reveals the morphology and fate of four bubble plumes and investigates the detailed subsurface structure of the active seepage area. The shapes of bubble plumes are not constant and influenced by the northeastward bottom currents, but the water depth where these bubble plumes disappear (630–650 m below the sea level) (mbsl) is very close to the upper limit of the gas hydrate stability zone in the water column (620 m below the sea level), as calculated from the CTD data within the study area, supporting the “hydrate skin” hypothesis. Gas chimneys directly below the bottom simulating reflectors, found at most sites, are speculated as essential pathways for both thermogenic gas and biogenic gas migrating from deep formations to the gas hydrate stability zone. The fracture network on the top of the basement uplift may be heavily gas-charged, which accounts for the chimney with several kilometers in diameter (beneath Plumes B and C). The much smaller gas chimney (beneath Plume D) may stem from gas saturated localized strong permeability zone. High-resolution seismic profiles reveal pipe-like structures, characterized by stacked localized amplitude anomalies, just beneath all the plumes, which act as the fluid conduits conveying gas from the gas hydrate-bearing sediments to the seafloor, feeding the gas plumes. The differences between these pipe-like structures indicate the dynamic process of gas seepage, which may be controlled by the build-up and dissipation of pore pressure. The 3D seismic data show high saturated gas hydrates with high RMS amplitude tend to cluster on the periphery of the gas chimney. Understanding the fluid migration and hydrate accumulation pattern of the Haima cold seeps can aid in the further exploration and study on the dynamic gas hydrate system in the South China Sea.  相似文献   

13.
南海晚新生代构造运动与天然气水合物资源   总被引：6，自引：0，他引：6  
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.  相似文献   

14.
使用重力取样器、渔网、深潜器等手段,已经在海底及以下浅表层的区域采获天然气水合物样品,但关于浅表层水合物的发育机制、分布规律、与海底地形的关系等问题还缺乏基本认识。根据2006年鄂霍次克海天然气水合物调查航次的调查数据,发现萨哈林东北陆坡区,特别是中、下陆坡区发育大量海底凸起。这些凸起一般呈不对称的丘形,宽几百米,高几十米。与海底沙波、沙脊不同,海底凸起为孤立海底地形,在南北方向上并不连续。海底剖面仪结果清楚地显示古陆坡凸起的发育。现今海底陆坡凸起的幅度普遍地要小于古陆坡凸起的幅度,个别地方古今陆坡凸起的形态有所变化,但大部分古、今陆坡凸起是一一对应的,基本形态没有根本变化。在萨哈林陆坡地区存在两个方向的挤压应力场,分别是由德鲁根盆地向萨哈林陆坡方向的挤压应力场和萨哈林陆坡沿萨哈林走滑断裂向南的挤压应力场,海底陆坡凸起是这两大应力场复合作用的结果。浊反射区中的游离气是底辟构造中的超高压多相物质向上迁移形成的,浊反射区上方对应的海底凸起应该是宏观构造挤压和局部底辟发育叠合的结果,浊反射区上方的海底凸起,在形态等方面应该和其他仅由挤压构造原因形成的凸起有所区别,比如顶部发育裂口等。在底辟构造中,由于游离气体的向上迁移,在整个水合物稳定域中从下到上,直至海底都可能形成水合物。  相似文献   

15.
近期在琼东南盆地超深水区发现了L18气田上新统地层圈闭气田，但在聚气背景、烃源岩、储层沉积成因及天然气输导体系等气田形成条件和成藏模式认识存在争议。通过对该气田形成条件的综合分析，认为上新世轴向古洼槽内地层圈闭、陵水凹陷东洼下渐新统崖城组浅海相烃源岩、上新统限制型重力流砂岩储层和渐新统-中新统断裂垂向沟源通道是形成上新统地层圈闭气田的4个基本条件。中中新世以来盆地中央继承性发育轴向古洼槽和限制型重力流沉积，随着后期地层沉积迁移、差异压实作用，上新统莺歌海组砂岩顶面在轴向洼槽内起伏，并被周边泥岩封盖、封堵，形成了地层圈闭；约3.4 Ma BP，陵水凹陷东洼下渐新统崖城组浅海相烃源岩生成了成熟天然气，沿渐新统-中新统断裂向上运移到上新统莺歌海组重力流沉积砂岩中，再侧向运移至地层圈闭中聚集成藏，具有"烃源岩、圈闭、断裂+砂岩输导层"三要素控藏的上新统地层圈闭成藏模式。  相似文献   

16.
2015~2016年在神狐新钻探区钻遇大量水合物岩心，证实南海北部神狐新钻探区具有较好的水合物成藏环境和勘探前景。结合2008~2009年该区采集的地震资料，我们对晚中新世以来细粒峡谷的沉积特征及其相应的水合物成藏模式进行了分析。通过对大量地震剖面进行解释，发现该区峡谷两侧的隆起上发育大量的滑塌体。本文通过岩心粒度分析，地震相识别分析和水合物测井响应分析等手段综合识别出对水合物成藏有控制作用的三种类型的滑塌体：原生滑塌体、峡谷切割滑塌体、和同生断裂滑塌体。结合沉积速率、流体流速分析和峡谷迁移等沉积学要素对滑塌体成因进行分析，认为峡谷切割滑塌体由于后期峡谷迁移对前期滑塌体切割形成的、同生断裂滑塌体是由于隆起区基底不平引起差异性沉降而形成的。不同类型的滑塌体发育位置不同：原生滑塌体常发育在隆起中坡度较缓的区域、峡谷切割成因滑塌体常发育在不定向迁移的峡谷两侧、同生断裂滑塌体常发育在隆起中坡度起伏较大的区域。三种类型滑塌及其相应的水合物成藏模式不同，其中原生滑塌体有利于水合物成藏，而另外两种类型的滑塌体由于其不能对自由气进行有效封堵而不利于水合物成藏。根据三种滑塌体对水合物成藏的响应指出在粗粒的含有孔虫粉砂岩储层上，覆盖细粒的泥岩对自由气进行封堵有利于水合物成藏，并且多层的泥岩覆盖是造成水合物稳定带中水合物多个分层成矿现象出现的原因。  相似文献   

17.
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   总被引：1，自引：1，他引：0  
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.  相似文献   

18.
Drilling/coring activities onboard JOIDES Resolution for hydrate resource estimation have confirmed gas hydrate in the continental slope of Krishna-Godavari (KG) basin, Bay of Bengal and the expedition recovered fracture filled gas hydrate at the site NGHP-01-10. In this paper we analyze high resolution multi-channel seismic (MCS), high resolution sparker (HRS), bathymetry, and sub-bottom profiler data in the vicinity of site NGHP-01-10 to understand the fault system and thermal regime. We interpreted the large-scale fault system (>5 km) predominantly oriented in NNW-SSE direction near NGHP-01-10 site, which plays an important role in gas hydrate formation and its distribution. The increase in interval velocity from the baseline velocity of 1600 m/s to 1750–1800 m/s within the gas hydrate stability zone (GHSZ) is considered as a proxy for the gas hydrate occurrence, whereas the drop in interval velocity to 1400 m/s suggest the presence of free gas below the GHSZ. The analysis of interval velocity suggests that the high concentration of gas hydrate occurs close to the large-scale fault system. We conclude that the gas hydrate concentration near site NGHP-01-10, and likely in the entire KG Basin, is controlled primarily by the faults and therefore has high spatial variability.We also estimated the heat flow and geothermal gradient (GTG) in the vicinity of NGHP-01-10 site using depth and temperature of the seafloor and the BSR. We observed an abnormal GTG increase from 38 °C/km to 45 °C/km at the top of the mound, which remarkably agrees with the measured temperature gradient at the mound (NGHP-01-10) and away from the mound (NGHP-01-03). We analyze various geological scenarios such as topography, salinity, thermal non-equilibrium of BSR and fluid/gas advection along the fault system to explain the observed increase in GTG. The geophysical data along with the coring results suggest that the fluid advection along the fault system is the primary mechanism that explains the increase in GTG. The approximate advective fluid flux estimated based on the thermal measurement is of the order of few tenths of mm/yr (0.37–0.6 mm/yr).  相似文献   

19.
An analysis of 3D seismic data from the northwestern part of the Ulleung Basin, East Sea, revealed that the gas hydrate stability zone (GHSZ) consists of five seismic units separated by regional reflectors. An anticline is present that documents activity of many faults. The seismic indicators of gas hydrate occurrence included bottom simulating reflector (BSR) and acoustic blanking in the gas hydrate occurrence zone (GHOZ). By the analysis of the seismic characteristics and the gradient of the sedimentary strata, the GHOZ was divided into four classes: (1) dipping strata upon strong BSR, (2) dipping strata below strong BSR, (3) parallel strata with acoustic blanking, and (4) parallel strata below weak BSR. Seismic attributes such as reflection strength and instantaneous frequency were computed along the GHOZ. Low reflection strength and high instantaneous frequency were identified above the BSR, indicating the occurrence of gas hydrate. A remarkably high reflection strength and low instantaneous frequency indicated the presence of free gas below the BSR. Considering the distribution of the gas hydrate and free gas, two gas migration processes are suggested: (1) stratigraphic migration through the dipping, permeable strata and (2) structural migration from below the GHSZ along faults.  相似文献   

20.
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.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号