日本是能源资源紧缺型国家,能源高度依赖进口.天然气水合物作为一种新型能源,促进其商业化开发已成为日本能源战略的重要部分.20世纪90年代以来,日本通过开展三轮国家级天然气水合物研发计划,实施了两轮海域天然气水合物试采,基本完成了周边海域资源调查评价,形成了一套基于降压法的试采技术体系和环境监测体系.为推动天然气水合物商业化开发,日本通过采取加强顶层设计、设立商业公司、成立研发联盟、加强国际合作等措施,加大了对天然气水合物研发的资金投入和攻关力度,并计划2027年前实现商业化开发.尽管我国在全球天然气水合物研发领域处于领先地位,但吸收借鉴国外相关经验,对推动天然气水合物产业化进程仍具有重要意义.

Studies on gas hydrate phase equilibrium conditions are reported widely in the literature. However, the data available for natural gas hydrates (NGHs) in real marine conditions are limited. In this study, NGH dissociation conditions in bulk brine and marine sediments were measured by using a microdifferential scanning calorimetry (mu-DSC). The sediments used in the experiment were obtained from the Shenhu area of the South China Sea, and the natural gas, as well as part of the water samples, were prepared according to the field data. Additionally, the Chen-Guo model was improved to calculate the NGH equilibrium conditions in the systems containing electrolytes and porous sediments. The experimental results showed that the NGH is relatively more stable than pure CH4 hydrate. The hydrate phase equilibrium curves shift to the left with the increased salinity in bulk water and sediments. In addition, the hydrate decomposition temperatures in marine sediments are obviously lower than those in bulk water. The average deviations of the improved model are 2.32% and 3.7% for bulk brine and sediments, respectively, which indicates great predictability. We consider that besides pore size as the main factor affecting the hydrate phase equilibrium, the water absorption capacity of marine sediments can also affect their decomposition conditions. The hydrate decomposition enthalpies were calculated by the Clausius-Clapeyron equation, and the results showed that the enthalpies increased with the decreased salinity and the addition of marine sediments. These findings are significant to NGH production safety in real marine sediments.

Bottom-simulating reflections (BSRs) in seismic data have been widely accepted to indicate the base of the methane gas hydrate stability zone (MGHSZ), and free gas was thought to exist only below it. However, real geologic systems are far more complex. We have evaluated the results of 3D seismic, logging while drilling, in situ, and coring measurements at a venting gas hydrate system in the Shenhu area of the South China Sea. Our studies reveal that free gas has migrated upward through the thermogenic gas hydrate stability zone into the MGHSZ and become a part of the gas hydrate system. Seismic amplitude anomalies and core results suggest the presence of free gas above the base of MGHSZ at 165 mbsf and the presence of thermogenic gas hydrates below it in well SC-W01. Analyses of P-wave velocity, S-wave velocity, density, and porosity logs reveal that free gas occurs above and below the MGHSZ as well. Integrating log and core analysis with seismic interpretation suggests that the variation in seismic amplitude within the chaotic zone is associated with variable gas saturations, and a large amount of methane and thermogenic gases accumulate near the complex BSRs. We suggest that relative permeability likely plays a significant role in the free-gas distribution and the formation of gas hydrates within a venting gas hydrate system, whereas the effect of dissolved gas short migration is not ignored. Our results have important implications for understanding the accumulation and distribution of gas hydrates and free gas in the venting gas hydrate system and seeps at the seafloor.

天然气水合物分布广、埋藏浅、清洁无污染、储量巨大,是极具发展潜力的清洁能源.为实现天然气水合物商业化开采,急需探索基于多分支井的高效开采技术.在使用TOUGH+HYDRATE模拟器开展数值模拟中,复杂结构井建模是研究工作的难点.为此提出了基于mVIEW的复杂结构井快速建模方法,以多分支井为例简要介绍了建模流程;此外,结合TOUGH+HYDRATE模拟器,以中国地质调查局2017年在南海北部陆坡深水区白云凹陷神狐海域SHSC-4试采井测井曲线数据为基础,建立理想水合物藏分层地质模型,开展单一水平井和多分支井在水合物Ⅱ层中部的降压开采数值模拟.模拟结果表明:该建模方法提高了模拟器在复杂建模方面的能力,对天然气水合物高效开采数值模拟具有较好效果和参考意义;相较于单一水平井降压开采,多分支井开采技术能最大限度地增加天然气水合物藏的裸露面积和深度,有效提高水合物藏储量动用程度,是值得探索的高效开采技术方法.

天然气水合物是未来极具发展潜力的清洁能源,而多分支井开采技术是天然气水合物高效开采的一种值得进一步探索的技术方法.基于南海神狐海域SHSC-4试采井测井曲线数据,建立了理想水合物藏分层地质模型.分析了羽状多分支井布设在水合物Ⅱ层中部时,分支参数对降压开采产能的影响规律.结果表明:①分支参数优化时,应首先考虑分支长度;尤其是当分支长度较短时,分支长度增加的增产效果最强;随着分支长度增加到一定长度,继续增加分支长度的产能增幅逐渐减弱.②其次应考虑分支数目和分支角度;相较于增加分支长度,增加分支数目和分支角度对产能影响较弱且增产效果相近;基于钻井成本考虑,分支数目不宜过多;分支角度大于45°时,产能增幅呈下降趋势,因此分支角度应小于45°.③在充分考虑上述分支参数的基础上,可适当考虑分支间距.相较于其他分支参数,增加分支间距对产能影响最弱,在保证较大控制面积和充分考虑钻井成本的前提下,分支井间距越大越好.

The major historical events not only impact the global pattern in society, politics and economy, but also exsert profound effect over science and technology. Based on researches about the development of theories and technoloy in marine geology during and after the major historical events, such as the past four technological revolutions and World War Ⅰ and Ⅱ, we first divided the development history of the world marine geological survey into 5 periods (emergence period, initial period, development period, mature period and upgrade period). We reviewed the objectives, characteristics and main achievements in the field of marine geological surveys in different periods in the history. Then, we summarized the law of development in world marine geological survey and discussed about the enlightenment. In the end, we put forward our suggestions on the development of China's marine geology in light of its current scientific, political and economic situation.

琼东南海域地震剖面上存在大量的含气特征,同相轴下拉、明显的速度横向变化等,因此水合物之下地层存在成像模糊且归位不准等问题,本文利用基于非线性层析的深度偏移方法提升成像精度.该方法采用全三维体的层析成像反演法建立深度域速度模型,通过对深度偏移道集拾取RMO量,并对其进行反偏移计算运动学不变量;在建立层位、倾角等骨架信息约束的混合模型基础上,利用运动学不变量进行速度层析,使得RMO最小以实现模型更新.该方法避免了常规速度更新的多次迭代偏移,能极大地提升层析效率,并能充分利用剖面骨架和倾角信息,获得高精度的速度模型.在琼东南水合物资料的实际应用中,有效地消除了含气对地层的影响,获得高精度的深度域成像结果.

Ubiquitous pockmarks in continental margins are important to understand the tectonic and sedimentary history and flow activity of their host margins. The study of an extensive field of pockmarks in the South China Sea (SCS) is important both scientifically and economically, for the exploration of oil, gas, and gas hydrate resources and the prevention and control of marine geological disasters. We report our new findings on the characteristics, distribution, and genesis of pockmarks in the SCS based on interpretation of high-resolution multibeam bathymetric data and multichannel seismic profiles collected in the past decade. Three pockmark fields (total area 572,824 km(2)), are recognized in the northern, western, and southern continental margins of the SCS. The pockmarks are highly variable in shape: circular, elliptical, crescent-shaped, elongated or forming a chain in plan view and U-, V- or W-shaped in cross-section. Pockmarks are normally 10-200 m in diameter, with maximum diameters/axial lengths of 7.3 km for circular, 5.5 km for elliptical, 8.6 km for crescent-shaped, 9.5 km for elongated, and 30 km for chain-type pockmarks. Pockmarks are normally less than 35 m deep, with a maximum depth of 341 m at the southwestern SCS margin. These pockmarks might have originated from methane seepages or from pore fluids from deeper sources, which migrated vertically along gas chimneys, mud diapirs, mud volcanoes, faults or buried channels. In addition to possible structural controls, the pockmark morphologies are affected by bottom currents.

通过钻探,在珠江口盆地东部海域获取了天然气水合物实物样品,在5个取心站位目标层段进行了保压取心,获取了水合物岩心释放气样品,同时在13个层段获取了水合物分解气体样品.钻探取心的5个站位都在航次现场选择层段制备了顶空气样品.所有气体均进行了气体组成与同位素分析,结果表明:水合物气体组成以甲烷占绝对优势,甲烷含量96.5％～99.8％;乙烷含量极少,为(175～554)×10-6,未检测出C2+以上烃类气体.水合物气体甲烷碳-氢同位素分析测试结果表明,δ13 C1为-68.4‰～-71.2‰,δDC1为-182‰～-184‰,据此判识水合物气体成因类型为生物成因气.水合物气源成因类型与水合物产出形态没有直接关系,多种产出类型的水合物可能与储层发育及形态特征有密切联系.主要气源位于1000m以内的浅地层中,主要以侧向运移方式运移至稳定域有利部位形成水合物.

天然气水合物是21世纪最具潜力的新型洁净能源之一,同时也是目前尚未开发的储量巨大的一种新能源.全球天然气水合物蕴藏的天然气资源总量约为2.1×1016 m3,相当于全球已探明传统化石燃料碳总量的2倍,主要分布于世界深水海域和永久冻土带中.随着中国在南海神狐海域的试采取得圆满成功,天然气水合物资源的开发利用越来越多地受到世人的关注与重视,但如何安全、经济、高效开采这种新能源,仍需投入大量人力物力进一步开展研究工作.笔者从世界主要国家天然气水合物资源的勘查试采现状入手,分析其开发利用趋势,系统梳理存在问题,提出加快推进天然气水合物勘查试采产业化的启示.

珠江口盆地西部海域发育弱BSR或无BSR的天然气水合物储层,常规叠后反演所获得的参数单一,难以精确预测其天然气水合物分布特征.本文在地震道集优化处理、精细速度分析、岩石物理分析及低频模型精确建立的基础上,针对性地采用叠前同时反演技术,对珠江口盆地西部海域天然气水合物储层进行预测,并利用岩相流体概率分析技术对其进行综合识别,实现了对天然气水合物储层地精细刻画.反演预测结果表明,研究区天然气水合物较为发育,预测结果与钻探结果吻合程度较高,应用效果良好.

This work reassesses two contrasting Late Miocene-Holocene stratigraphic frameworks for the Pearl River Mouth Basin (PRMB), northern South China Sea. Two distinct stratigraphic frameworks based on seismic-stratigraphic (SFI) and biostratigraphic (SFII) data are compared in terms of their applicability and accuracy. In order to stress the differences between both frameworks, major stratigraphic boundaries (T1-base Quaternary and T2-base Pliocene) derived from SFI and SFII were revisited and described. Sedimentation rates estimated for the two frameworks, complemented with published data, indicate that average sedimentation rates and trends for SFI match the regional reference data for Quaternary strata. In terms of well-seismic ties, two allochthonous sedimentary units with similar lithology and grain sizes were identified as comprising fine-grained turbidites at the bottom, and fine-grained sediment failures at the top, revealing widespread mass wasting on the middle to lower continental slope. In such a setting, nannofossil assemblages accumulated on the continental slope were likely reworked because of their relatively minute sizes. Sediment cores with low recovery rates collected during a first hydrate expedition in 2007 have led to important misunderstandings when defining the first and last appearances for nannofossil species. Hence, stratigraphic framework I (SFI) - based on a combination of seismic and sequence stratigraphic data - is suggested as more suitable to attain stratigraphic correlations across the continental slope of the PRMB. Our findings can resolve the ambiguities and uncertainties arising from the two contrasting stratigraphic frameworks, providing a robust foundation to reassess the tectono-sedimentary evolution of the PRMB, South China Sea.

The recent drilling results of marine gas hydrate reveal that the shallow sediments in a hydrocarbon rich depression are the potential places for gas hydrate formation when hydrocarbon rich depression is of preferential temperature,pressure and other conditions for hydrate accumulation.However,our investigation found that the gas compositions and carbon isotope of hydrates in shallow strata show biogenic or mixed bio-thermogenic characteristics,but no thermogenic,although hydrocarbon-rich depressions are of excellent potential of hydrocarbon generation and discharge and enough supply of thermogenic gas,both of which can provide sufficient gases for hydrate formation.Two gas hydrate expeditions (GMGS 01&03) were carried out in the Baiyun Depression,South China Sea (SCS),as organized by Guangzhou Marine Geological Survey in 2007 and 2015,respectively.Compared with the results of no or minor contribution of thermogenic gas to hydrate formation at the drilling sites in 2007,higher contents of ethane and propane (up to 5％) in hydrates were detected at the sites drilled in 2015,providing direct evidence that deep thermogenic gas was a significant source for shallow hydrate formation.Geochemical results of industrial boreholes in the Baiyun depression indicate that part of thermogenic gas would migrate from hydrocarbon source rocks through faults,diapirs and gas chimney to shallow strata to form natural gas hydrate,although deep hydrocarbon gas escaped obviously from the reservoir due to the Dongsha movement in the late Miocene.In this paper we discussed the factors associated with hydrate formation,based on the model of vertical coupling relation of "deep hydrocarbon source rockinterconnecting migration channel-shallow gas hydrate formation" in the two Shenhu hydrate drilling areas,and it is found that the type and permeability of gas migrating path might be the reason for the difference in gas source in the two areas.For the 2007 drilling area,long migration of deep thermogenic gas along low energy channels might cause differentiation of gas compositions and fractionation of carbon isotope,resulting in that the gas shows more "biogenic gas" features.2015 drilling result found that through well developed faulting and diapir structures,the deep gases can be transported efficiently to shallow sediment layers in which gas hydrate formed,without oblivious change in chemical and isotopic composition.

Migration pathways of gas have close relationship with migration and accumulation of natural gas hydrate.Based on high resolution quasi-3D seismic data,and combined with practical drilling results,the authors studied the geological and geophysical features of gas migration pathways and their controlling effects associated with high saturation gas hydrate in GMGS3 drilling sites of Shenhu area.The results show that multiple types of migration pathways are developed in high saturation gas hydrate drilling sites in GMGS3 drilling area,which have favorable corresponding relation with BSR in space.Enhanced reflections occur closely beneath the BSR,and gas charging phenomena are obvious under the enhanced reflection,indicating migrating pathways exist under the gas hydrate stability zone and deep gas has migrated to shallow formations through those pathways.Deep faults,mud diapirs,gas chimneys act as vertical migrating pathways that connect deep thermogenic gases with shallow biogenic gas and pressuretemperature stability zones,and gas hydrate can be formed and accumulated just over those pathways.Shallow slump surface and high continuous sand bodies composed of channel sandstones and submarine fans provide lateral migrating pathways for the migration of shallow biogenic gas and partial thermogenic gas from deep formations,which expands the range of gas supply and increases the gas hydrate distribution scales.It is concluded that tectonic structures and regions with favorable coupling relationship between hydrocarbon migrating system and other elements are prospective targets with high saturation gas hydrate.

The use of bright spots technique for hydrocarbon detection is a feasible and effective method,but it is seldom used in gas hydrate detection.This paper analyzes amplitude behaviors of gas hydrate from stacked seismic data of the Shenhu drilling area,based on rock physics properties of hydrate-,gas-and water-saturated sediments.The results show that there are three hydrate forming models for the diffusion-type hydrate in the Shenhu area:hydrate sediment layer formed at the base of the gas hydrate stability zone,high and low concentration hydrate formed inside the gas hydrate stability zone.Concentrated gas hydrate formed at the base of the gas hydrate stability zone produces BSR;high concentration gas hydrate or gas produces bright spots;and low concentration gas hydrate produces dim out.By synthesis analysis,bright spots and dim out can be used as important indicators for gas reservoir prediction.

The continuous bottom-simulating reflection (BSR) is commonly considered to mark the base of gas hydrate stability zone. Below this depth, gas hydrate gives away to free gas or water filling with pore spaces of sediments. We integrated and analyzed seismic data collected in 2008, and logging-while-drilling (LWD) data and coring results acquired by the Fugro Voyager in 2015 in the Shenhu area on the northern slope of the South China Sea. Based on seismic and well-log correlation, a BSR with typical characteristics of gas hydrates and free gas was identified at 237 m, below the mudline (BML). However, LWD data reveal a 63 m thick hydrate layer from 205 to 268 m BML. Increases in resistivity and velocity at 262 m BML indicate that gas hydrate is likely presented below the BSR. The observed pore-water freshening with depth and infrared image of core samples are consistent with geophysical interpretation. Seismic and well interpretations reveal continuous, discontinuous, and pluming BSRs in the Shenhu area. The continuous BSR indicates the base of the methane gas hydrate stability zone, and structure II gas hydrate is likely presented below the BSR. Deep thermogenic fluid locally entrapped within shallow-buried sediments may reinforce gas-hydrate accumulations near the discontinuous and pluming BSRs. We conclude that seismic responses of structure II gas hydrate can be distinct from structure I gas hydrate. Understanding the seismic characterizations of structures I and II will help in the evaluation of gas-hydrate reservoirs and inferring the presence of deep thermogenic reservoirs.

With the seismic data acquired by the Guangzhou Marine Geological Survey,we studied the types and characteristics of the frequently developed sediments failures in the Shenhu area of the northern South China Sea.The distribution pattern of sediment failures is adopted to reveal the relationship between slopeconfined submarine canyons and sediment failures.The sediments failures in the study area can be grouped to two types.The first type,which is mainly located at the ridges in the lower reach of a canyon and hardly affected by seafloor geomorphology,is characterized by multiple continuous wavy reflectors in a pattern of southwards mass movement.The second type is mainly the products of the mass movement from the ridge to the bottom of a canyon,and is obviously affected by the topography of the canyons.At the head of a canyon,sediment failures always occur as slides with continuous reflectors.And in the middle reach there is intensive deformation and dominated by slumps.From the lower reach to the canyon mouth,however sliding/slumping blocks dominate.Since Quaternary,Great amount of sediment charges come from the Pearl River system towards the north.With the help of steep seafloor topography,the activity of sediment failures is accelerated.Sediment re-deformation may widely occur in submarine canyons.In addition,the vertical migration of gas-bearing fluids coming from gas hydrate is also an important factor for sediment instabilities in the study area.

The first marine gas hydrate expedition in China has been conducted by Guangzhou Marine Geological Survey in the Shenhu Area, northern continental slope of the South China Sea. Previous study has analyzed the P-T conditions, geophysical anomalies and saturation calculations of these gas hydrates, but has not documented in detail the migration of gas-bearing fluids in the study area. Based on the interpretations of 2D/3D seismic data, this work identified two types of migration pathways for gas-bearing fluids in the Shenhu area, i.e., vertical and lateral pathways. The vertical pathways (large-scale faults, gas chimneys and mud diapirs) presented as steep seismic reflection anomalies, which could be traced downward to the Eocene source rocks and may penetrate into the Late Miocene strata. The deeper gases/fluids might be allowed migrating into the shallower strata through these vertical conduits. However, the distributions showed distinct differences between these pathways. Large-scale faults developed only in the north and northeast of the Shenhu area, while in the drilling area gas chimneys were the sole vertical migration pathways. Since the Pliocene, normal faults, detachment faults and favorable sediments have constituted the lateral pathways in the Shenhu gas hydrate drilling area. Although these lateral pathways were connected with gas chimneys, they exerted different effects on hydrate formation and accumulation. Gas-bearing fluids migrated upward along gas chimneys might further migrate laterally because of the normal faults, thereby enlarging the range of the chimneys. Linking gas chimneys with the seafloor, the detachment faults might act as conduits for escaping gases/fluids. Re-deposited sediments developed at the early stage of the Quaternary were located within the gas hydrate stability zone, so hydrates would be enriched in these favorable sediments. Compared with the migration pathways (large-scale faults and mud diapirs) in the LW3-1 deep-sea oil/gas field, the migration efficiency of the vertical pathways (composed of gas chimneys) in the gas hydrate drilling area might be relatively low. Description and qualitative discrimination of migration pathways in the Shenhu gas hydrate drilling area are helpful to further understand the relationship between good-quality deep source rocks and shallow, mainly biogenically-produced, hydrates. As the main source rocks of the Baiyun sag, lacustrine mudstones in the Wenchang and Enping Formations may provide thermogenic methane. Gas chimneys with relatively low migration efficiency created the vertical pathways. Caused by the Dongsha tectonic movement, the release of overpressured fluids might reduce the vertical migration rates of the thermogenic methane. The thick bathyal/abyssal fine-grained sediments since the Late Miocene provided migration media with low permeability. These preconditions may cause carbon isotopic fractionation of thermogenic methane during long-distance vertical migrations. Therefore, although geochemical analyses indicate that the methane forming gas hydrate in the Shenhu area was mainly produced biogenically, or was mixed methane primarily of microbial origin, thermogenic methane still contribute significantly.

Gas hydrates were discovered in the sediment (seafloor to around 200 mbsf)in the northeastern South China Sea at water depth between 600 and 900 m.Gas hydrates show different morphology,including massive,nodular,laminated,veins and disseminated,occurring either independently or along with methane derived authigenic carbonate.The first four types of hydrates,whose sizes are normally larger than the sediment pore size and could be observed by naked eyes,are usually formed in the shallow fractures and on the seafloor,while disseminated gas hydrates usually fill the micrometer to nanometer scale pore space and could not be observed by naked eyes.Drilling results show that there are two gas hydrate reservoirs separated by sediments at 90 mbsf.The upper one is dominated by massive hydrates,while the lower one mainly consists of disseminated hydrates.The upper one could be further divided into upper and lower hydrate intervals.Both of the gas hydrate intervals contain authigenic carbonate,reflecting the activity of ancient cold seeps influenced by gas hydrate evolution in the geological history.Comprehensive analysis reveals that the hydrate morphology and evolution of hydrate reservoirs were controlled by many factors,including lithology,grain size,sediment facies,tectonic activity,faults,fluid and gas transport,heat flow,salinity and time.Massive gas hydrates were mainly determined by tectonic activities and were the consequence of gas accumulation induced by active fluid advection,while disseminated hydrate were mainly controlled by sedimentation process and were the results of fluid diffusion.A series of geological events since the Pliocene,such as changes between glacial and interglacial periods,were the triggers of gas hydrate formation and decomposition.The time of hydrate formation is earlier than the overlaying authigenic carbonate and later than the mother rock sediment.The upper hydrate interval of upper hydrate reservoir was formed in the late Pleistocene and the lower hydrate interval was formed in the middle middle Pleistocene-late Pleistocene,while lower hydrate reservoir was formed in the early middle Pleistocene-middle middle Pleistocene.Hydrate reservoirs underwent three formation periods and two decomposition periods.Authigenic carbonate developed in two periods along with hydrate evolution.Carbonates at seafloor were formed in the late Pleistocene,while carbonates below upper hydrate reservoir were formed in the early middle Pleistocene.Complex hydrate system composed of multilayered hydrate structures is the result of dynamic process driven by methane diffusion and advection.

Based upon the high-resolution seismic data acquired by the Guangzhou Marine Geological Survey and the cores collected by the First Hydrate Drilling Expedition (GMGS01),a kind of fine-grained turbidite is identified in the Shenhu Area of the northern South China Sea.According to the seismic profiles crossing hydrate drilling sites,two seismic units are recognized above the BSR (bottom simulating reflectors):the Unit 1 at the bottom consisting of thin-bedded lenticular chaotic seismic reflectors and the Unit 2 at the top consisting of thick continuous moderate-amplitude seismic reflectors with wavy structures.Grain size analysis illustrates that the deposits are composed of fine-grained silt or silty clay ranging 4-63 μm in grain size.Both the lithological features and grain size parameters are consistent from bottom to top without significant changes.It implies that all the deposits should be the results of a similar depositional process.Moreover,on the C-M diagram,the samples from the hydrate bearing sediments show a distribution pattern parallel to the C=M baseline,indicating an origin of fine-grained turbidites.Regional survey suggests that these fine-grained turbidites might be associated with some small-scale channels in the north of the Shenhu Area.The sediments provided by these small channels move downslopewards and re-deposi-ted in the middle to lower slope as fine-grained turbidites.Based upon the above interpretation,a model is proposed to reveal the relationships between fine-grained tuebidites and hydrates,It says that the heterogeneous distributions of gas hydrates in GMGS01 of the Shenhu Area,northern South China Sea probably owes its origin to the uneven distribution of fine grained turbidites.