Thermal instability of gas hydrate bearing sediments: Some issues

The overall stability of marine strata holding gas hydrates is dependent on their shear strength characteristics. These characteristics, in turn, are dependent on thermal flux that is imposed for dissociation of the hydrates for the safe and efficient extraction of methane gas from the hydrate bearing sediments. Due to the imposition of thermal flux on these sediments, their fabric structure and pore space hydrate saturation changes, which impacts the overall stability of the sea bed. Estimating stability conditions in such a ‘multiphase and dynamic system’ necessitates collection of undisturbed samples without compromising their in-situ thermodynamic conditions. This is a daunting task given the huge cost of procuring samples and the challenge of maintaining an undisturbed sample with in-situ thermodynamic conditions till it is brought to the laboratory. Synthesizing hydrate bearing sample sediments in laboratory for conducting studies to identify heat migration mechanisms and thermal property measurements and linking them to the shear strength characteristics provides an affordable solution to this problem. With this in view, a critical review of the available literature, dealing with laboratory synthesis of hydrate bearing sediments, their thermal and strength characteristics, the coupled phenomenon of heat and fluid migration, and its impact on the overall stability of marine sediments, has been conducted and presented in this paper. This will facilitate understanding the factors governing and the mechanism of heat transfer in a multiphase system, the changes in the system brought about by the hydrate dissociation front, and the overall impact on the stability of seabed.     

用地球化学方法勘查中国南海的天然气水合物

天然气水合物是一种未来新型能源,赋存于低温高压环境下的海洋沉积物中,但也可形成于大陆永久冻土带中。天然气水合物资源量巨大,具有经济和环境上的研究意义。近年来,国际上己对天然气水合物的产况、分布和形成机理开展了大量研究,但国内这方面的工作还刚刚开展。对中国南海的调查表明该区存在天然气水合物赋存的有利地质条件、温压条件和富含有机质的沉积条件。在南海的许多海区还发现了指示天然气水合物存在的地震标志(BSR)。介绍了在南海天然气水合物勘查中的地球化学异常标志。这些地球化学异常的产生可能与天然气水合物的形成或分解过程有关。研究内容包括沉积物中气体含量(主要为甲烷和乙烷),甲烷的碳同位素,孔隙水中阴离子(Cl^-、SO4^2-等)、阳离子(Ca^2 、Mg^2 、Ba^2 、Sr^2 ,B^3 和NH4^ 等)浓度和δ^18,δD,δ^11B,及^87Sr／^86Sr等同位素组成,此外还对海底沉积物的热释光特征和紫外、可见、近红外反射光谱特征开展了探索性研究。通过进一步加强理论和实验研究,结合地球物理和地球化学资料,在不远的将来将会在南海发现和圈定天然气水合物矿藏。     

海洋天然气水合物开采方法及产量分析

海洋天然气水合物的巨大储量刺激了世界各国能源部门努力研究如何从天然气水合物储层生产天然气。根据水合物形成的条件，只有当水合物处在其相平衡条件以外，水合物才能分解。因此，水合物的开采方法只能为热熔法、抑制剂刺激法、减压法和地面分解法。为了对天然气水合物储层中气体的生产有个定量的评估，本文以水合物开采井为例，运用数学方法推导了水合物井中气体的产生量。结果表明，在天然气水合物储层中，天然气释放量是井内水合物分解温度、压力及水合物层气体渗透性的敏感函数。该函数可以用于天然气水合物井气体开采量的计算及对水合物储层可开采性评价。     

天然气水合物的重要特征与评价及其在中国海域的勘探前景

从勘探技术和资源评价的角度综述了甲烷水合物生成和聚集的重要特征, 如地震反射剖面、测井曲线资料、地球化学特点等以及对未知区的地质勘探和选区评价 .甲烷水合物在地震剖面上主要表现为BSR(似海底反射)、振幅变形(空白反射)、速度倒置、速度-振幅结构(VAMPS)等,大规模的甲烷水合物聚集可以通过高电阻率(＞100欧姆.米)声波速度、低体积密度等号数进行直接判读.此项研究实例表明,沉积物中典型甲烷水合物具有低渗透性和高毛细管孔隙压力特点,地层孔隙水矿化度也呈异常值,并具有各自独特的地质特征.现场计算巨型甲烷水合物储层中甲烷资源量的方法可分为:测井资料计算法公式为:SW＝(abRw/φm.Rt)1/n;地震资料计算法公式为:ρp＝(1-φ)ρm+(1-s)φρw+sφρh、VH＝λ.φ.S.对全球甲烷水合物总资源量预测的统计达20×1015m3以上.甲烷水合物形成需满足高压、低温条件,要求海水深度＞300 m.因此,甲烷水合物的分布严格地局限于两极地区和陆坡以下的深水地区,并具有3种聚集类型:1.永久性冻土带;2.浅水环境;3.深水环境.深海钻探计划(DSDP)和大洋钻探计划(ODP)已在下述10个地区发现大规模的甲烷水合物聚集,他们是:秘鲁、哥斯达黎加、危地马拉、墨西哥、美国东南大西洋海域、美国西部太平洋海域、日本海域的两个地区、阿拉斯加和墨西哥湾地区.在较浅水沉积物岩心样中发现甲烷水合物的地区,包括黑海、里海、加拿大北部、美国加里福尼亚岸外、墨西哥湾北部、鄂霍茨克海的两个地区.在垂向上,甲烷水合物主要分布于海底以下2 000 m以浅的沉积层中.最新统计表明又主要分布于二个深度区间:200～450 m和700～920 m,前者是由ODP995～997站位发现的;后者在加拿大麦肯齐河三角洲马立克2L-38号井中897～922 m处发现.中国海域已发现多处甲烷水合物可能赋存地区,包括东沙群岛南部、西沙海槽北部、西沙群岛南部以及东海海域地区.姚伯初报道了南海地区9处地震剖面速度异常值的发现,海水深度为420～3 920 m,海洋地质研究所则在东海海域解释了典型BSR反射的剖面,具有速度异常、弱振幅、空白反射、与下伏反射波组具不整合接触关系(VAMPS)等,大致圈定了它们的分布范围,表明在中国海域寻找甲烷水合物具有光明的前景.     

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。     

沉积物中水合物含气量测定方法

建立了沉积物中水合物含气量的测定方法;测定了人工松散沉积物中甲烷水合物、南海神狐海域及祁连山冻土区天然气水合物样品的含气量;计算了样品的表观水合指数(水与气体的摩尔比);探讨了水合物含气量的影响因素.测试结果表明,人工甲烷水合物样品表观水合指数与晶体水合指数相近,样品中水合物的浓度大,含气量较高;南海神狐海域及祁连山冻...     

Experimental research on the mechanical properties of methane hydrate-bearing sediments during hydrate dissociation

This paper describes studies of the effect of hydrate dissociation on the safety and stability of methane hydrate-bearing sediments. Methane hydrates within the sediments were dissociating under the conditions of a confining pressure of 0.5 MPa, 1 MPa, 2 MPa and a temperature of −5 °C. After 6 h, 24 h, or 48 h, a series of triaxial compression tests on methane hydrate-bearing sediments were performed. The tests of ice-clay and sediments without hydrate dissociation were performed for comparison. Focusing on the mechanical properties of the sediments, the experimental results indicated that the shear strength of the ice-clay mixtures was lower than that of the methane hydrate-bearing sediments. The strength of the sediments was reduced by hydrate dissociation, and the strength tended to decrease further at the lower confining pressures. The secant modulus E_S of the sediments dropped by 42.6% in the case of the dissociation time of the hydrate of 48 h at the confining pressure of 1 MPa; however, the decline of the initial yield modulus E₀ was only 9.34%. The slower hydrate dissociation rate contributed to reducing the failure strength at a declining pace. Based on the Mohr–Coulomb strength theory, it was concluded that the decrease in strength was mainly affected by the cohesive reduction. Moreover, the mathematical expression of the M–C criterion related to the hydrate dissociation time was proposed. This research could be valuable for the safety and stability of hydrate deposits in a permafrost region.     

南海台西南盆地天然气水合物区沉积物的地球化学和微生物特征分析

The Taixinan Basin is one of the most potential gas hydrate bearing areas in the South China Sea and abundant gas hydrates have been discovered during expedition in 2013. In this study, geochemical and microbial methods are combinedly used to characterize the sediments from a shallow piston Core DH_CL_11(gas hydrate free) and a gas hydrate-bearing drilling Core GMGS2-16 in this basin. Geochemical analyses indicate that anaerobic oxidation of methane(AOM) which is speculated to be linked to the ongoing gas hydrate dissociation is taking place in Core DH_CL_11 at deep. For Core GMGS2-16, AOM related to past episodes of methane seepage are suggested to dominate during its diagenetic process; while the relatively enriched δ18O bulk-sediment values indicate that methane involved in AOM might be released from the "episodic dissociation" of gas hydrate.Microbial analyses indicate that the predominant phyla in the bacterial communities are Firmicutes and Proteobacteria(Gammaproteobacteria and Epsilonproteobacteria), while the dominant taxa in the archaeal communities are Marine_Benthic_Group_B(MBGB), Halobacteria, Thermoplasmata, Methanobacteria,Methanomicrobia, Group C3 and MCG. Under parallel experimental operations, comparable dominant members(Firmicutes and MBGB) are found in the piston Core DH_CL_11 and the near surface layer of the long drilling Core GMGS2-16. Moreover, these members have been found predominant in other known gas hydrate bearing cores, and the dominant of MBGB has even been found significantly related to gas hydrate occurrence. Therefore,a high possibility for the existing of gas hydrate underlying Core DH_CL_11 is inferred, which is consistent with the geochemical analyses. In all, combined geochemical and microbiological analyses are more informative in characterizing sediments from gas hydrate-associated areas in the South China Sea.     

Comparison of marine gas hydrates in sediments of an active and passive continental margin

Two sites of the Deep Sea Drilling Project in contrasting geologic settings provide a basis for comparison of the geochemical conditions associated with marine gas hydrates in continental margin sediments. Site 533 is located at 3191 m water depth on a spit-like extension of the continental rise on a passive margin in the Atlantic Ocean. Site 568, at 2031 m water depth, is in upper slope sediment of an active accretionary margin in the Pacific Ocean. Both sites are characterized by high rates of sedimentation, and the organic carbon contents of these sediments generally exceed 0.5%. Anomalous seismic reflections that transgress sedimentary structures and parallel the seafloor, suggested the presence of gas hydrates at both sites, and, during coring, small samples of gas hydrate were recovered at subbottom depths of 238m (Site 533) and 404 m (Site 568). The principal gaseous components of the gas hydrates wer methane, ethane, and CO₂. Residual methane in sediments at both sites usually exceeded 10 mll^?1 of wet sediment. Carbon isotopic compositions of methane, CO₂, and ΣCO₂ followed parallel trends with depth, suggesting that methane formed mainly as a result of biological reduction of oxidized carbon. Salinity of pore waters decreased with depth, a likely result of gas hydrate formation. These geochemical characteristics define some of the conditions associated with the occurrence of gas hydrates formed by in situ processes in continental margin sediments.     

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。     

Numerical Study on Dissociation of Gas Hydrate and Its Sensitivity to Physical Parameters

The natural gas hydrate resource is tremendous.How to utilize the gas from hydrates safely is researchers' concern.In this paper,a one-dimensional model is developed to simulate the hydrate dissociation by depressurization in hydrate-bearing porous medium.This model can be used to explain the effects of the flow of multiphase fluids,the endothermic process of hydrate dissociation,the variation of permeability,the convection and conduction on the hydrate dissociation.Numerical results show that the hydrate dissociation can be divided into three stages:a rapid dissociation stage mainly governed by hydrate dissociation kinetics after an initially slow dissociation stage governed mainly by flow,and finally a slow dissociation stage.Moreover,a numerical approach of sensitivity analysis of physical parameters is proposed,with which the quantitative effect of all the parameters on hydrate dissociation can be evaluated conveniently.     

Estimation of methane fluxes from bottom sediments of Lake Baikal

Methane bubble fluxes in gas flares from bottom sediments in Lake Baikal were estimated for the first time using hydroacoustic methods. Earlier work has demonstrated the occurrence of gas seeps both inside and outside of areas where bottom simulating reflectors were identified in seismic profiles. Fluxes ranged from 14 to 216 tons per year, with the flux for the entire area of the central and southern basins ranging from 1,400 to 2,800 tons per year. Comparison with other water bodies showed that fluxes from the most intensive Baikal flares were similar to those in the Norwegian and Okhotsk seas. Gas hydrates decompose at the lower boundary of the gas hydrate stability zone due to sedimentation. Calculation of the amount of methane produced due to sedimentation gave a total of between 2,600 and 14,000 tons per year for the central and southern basins of the lake. Based on rough estimation, the total flux from shallow- and deep-water gas seeps is similar to the amount of methane produced due to sedimentation. This suggests that gas hydrates possibly occupy much more than 10?% of the pore volume near the base of the gas hydrate stability zone, or that there are other reasons for gas hydrate dissociation and bubble flux from these bottom sediments.     

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.     

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.     

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.     

Experimental investigation of gas-water relative permeability for gas-hydrate-bearing sediments from the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope

Permeability of hydrate reservoirs found in nature is likely to be heavily influenced by the percent of the pore volume occupied by hydrates. The quantification of how hydrate saturation affects permeability is of key interest for reservoir engineering studies. In this study, an experimental setup was modified to test permeability characteristics of unconsolidated core samples containing various saturations of methane hydrates. Hydrates were formed in the unconsolidated samples using a refrigerated core holder connected to a brine and methane injection system. Studies of this type conducted to date have rarely been performed on core samples recovered from actual hydrate-bearing sedimentary sections from natural hydrate intervals. Samples from the Mount Elbert site on the Alaska North Slope (ANS) were used for this study.Relative permeability measurements using hydrate constituent components (e.g. water and methane) are not very desirable due to difficulties in preventing additional hydrate formation during displacement experiments. Relative permeability measurements performed with hydrate constituent components (e.g. water and nitrogen) can help to significantly mitigate issues with additional hydrate formation. However, unsteady state relative permeability experiments produce piston like displacement results suggesting that steady state experiments might be preferable.It was observed that as in previous work using consolidated core samples, permeability of both brine and gases was reduced in unconsolidated hydrate-bearing core samples. Experimental results show that low to moderate hydrate saturations (1.5 to 36%) can significantly reduce permeability of porous media. These saturations, in fact, are lower than hydrate saturations observed in the natural hydrate systems at Mount Elbert.     

Potential for gas hydrate formation at the northwest New Zealand shelf margin - New insights from seismic reflection data and petroleum systems modelling

In this study we provide evidence for methane hydrates in the Taranaki Basin, occurring a considerable distance from New Zealand's convergent margins, where they are well documented. We describe and reconstruct a unique example of gas migration and leakage at the edge of the continental shelf, linking shallow gas hydrate occurrence to a deeper petroleum system. The Taranaki Basin is a well investigated petroleum province with numerous fields producing oil and gas. Industry standard seismic reflection data show amplitude anomalies that are here interpreted as discontinuous BSRs, locally mimicking the channelized sea-floor and pinching out up-slope. Strong reverse polarity anomalies indicate the presence of gas pockets and gas-charged sediments. PetroMod™ petroleum systems modelling predicts that the gas is sourced from elevated microbial gas generation in the thick slope sediment succession with additional migration of thermogenic gas from buried Cretaceous petroleum source rocks. Cretaceous–Paleogene extensional faults underneath the present-day slope are interpreted to provide pathways for focussed gas migration and leakage, which may explain two dry petroleum wells drilled at the Taranaki shelf margin. PetroMod™ modelling predicts concentrated gas hydrate formation on the Taranaki continental slope consistent with the anomalies observed in the seismic data. We propose that a semi-continuous hydrate layer is present in the down-dip wall of incised canyons. Canyon incision is interpreted to cause the base of gas hydrate stability to bulge downward and thereby trap gas migrating up-slope in permeable beds due to the permeability decrease caused by hydrate formation in the pore space. Elsewhere, hydrate occurrence is likely patchy and may be controlled by focussed leakage of thermogenic gas. The proposed presence of hydrates in slope sediments in Taranaki Basin likely affects the stability of the Taranaki shelf margin. While hydrate presence can be a drilling hazard for oil and gas exploration, the proposed presence of gas hydrates opens up a new frontier for exploration of hydrates as an energy source.     

海洋天然气水合物研究新进展——2006西太平洋地球物理会议

简介了2006年西太平洋地球物理会议中海洋天然气水合物研究的最新成果,主要集中在水合物勘探技术,新的水合物积聚区预测,水合物成因、性质分析,水合物评估手段、开发计划,水合物模拟实验研究等几个方面。在对以上几方面分类总结的基础上,提出了几点关于海洋天然气水合物未来研究方向的认识。     

Contribution of thermogenic organic matter to the formation of biogenic gas hydrate: Evidence from geochemical and microbial characteristics of hydrate-containing sediments in the Taixinan Basin,South China Sea

High-saturation (40–100%), microbial gas hydrates have been acquired by expedition GMGS2 from the Taixinan Basin. In this study, geochemical and microbial features of hydrate-containing sediments from the drilling cores (GMGS2-09 and GMGS2-16) were characterized to explore their relationships with gas hydrate formation. Results showed that the average TOC content of GMGS2-09 and GMGS2-16 were 0.45% and 0.63%, respectively. They could meet the threshold for in situ gas hydrate formation, but were not available for the formation of high-saturation gas hydrates. The dominant members of Bacteria at the class taxonomic level were Alphaproteobacteria, Bacilli, Bacteroidia, Epsilonproteobacteria and Gammaproteobacteria, and those in Archaea were Marine_Benthic_Group_B (MBGB), Miscellaneous_Crenarchaeotic_Group (MCG), Group C3, Methanomicrobia and Methanobacteria. Indicators of microbes associated with thermogenic organic matter were measured. These include: (1) most of the dominant microbes had been found dominant in other gas hydrates bearing sediments, mud volcanos as well as oil/coal deposits; (2) hydrogenotrophic methanogens and an oilfield-origin thermophilic, methylotrophic methanogen were found dominant the methanogen community; (3) hydrocarbon-assimilating bacteria and other hyperthermophiles were frequently detected. Therefore, thermogenic signatures were inferred existed in the sediments. This deduction is consistent with the interpretation from the seismic reflection profiles. Owing to the inconsistency between low TOC content and gas hydrates with high saturation, secondary microbial methane generated from the bioconversion of thermogenic organic matters (oil or coal) was speculated to serve as enhanced gas flux for the formation of high-saturation gas hydrates. A preliminary formation model of high-saturation biogenic gas hydrates was proposed, in which diagenesis processes, tectonic movements and microbial activities were all emphasized regarding to their contribution to gas hydrates formation. In short, this research helps explain how microbial act and what kind of organic matter they use in forming biogenic gas hydrates with high saturations.