Petrophysical Properties of Natural Gas Hydrates-Bearing Sands and Their Sedimentology in the Nankai Trough

Abstract. The Nankai Trough parallels the Japanese Island, where extensive BSRs have been interpreted from seismic reflection records. High resolution seismic surveys and drilling site-survey wells conducted by the MTI in 1997, 2001 and 2002 have revealed subsurface gas hydrate at a depth of about 290 mbsf (1235 mbsl) in the easternmost part of Nankai Trough. The MITI Nankai Trough wells were drilled in late 1999 and early 2000 to provide physical evidence for the existence of gas hydrate. During field operations, continuous LWD and wire-line well log data were obtained and numerous gas hydrate-bearing cores were recovered. Subsequence sedimentologic and geochemical analyses performed on the cores revealed important geologic controls on the formation and preservation of natural gas hydrate. This knowledge is crucial to predicting the location of other hydrate deposits and their eventual energy resource. Pore-space gas hydrates reside in sandy sediments from 205 to 268 mbsf mostly filling intergranular porosity. Pore waters chloride anomalies, core temperature depression and core observations on visible gas hydrates confirm the presence of pore-space hydrates within moderate to thick sand layers. Gas hydrate-bearing sandy strata typically were 10 cm to a meter thick. Gas hydrate saturations are typically between 60 and 90 % throughout most of the hydrate-dominant sand layers, which are estimated by well log analyses as well as pore water chloride anomalies.
It is necessary for evaluating subfurface fluid dlow behavious to know both porosity and permeability of gas hydrate-bearing sand to evaluate subsurface fluid flow behaviors. Sediment porosities and pore-size distributions were obtained by mercury porosimetry, which indicate that porosities of gas hydrate-bearing sandy strata are approximately 40 %. According to grain size distribution curves, gas hydrate is dominant in fine- to very fine-grained sandy strata.     

Subsurface Occurrence of Natural Gas Hydrate in the Nankai Trough Area: Implication for Gas Hydrate Concentration

Abstract. The Nankai Trough runs along the Japanese Islands, where extensive BSRs have been recognized in its forearc basins. High resolution seismic surveys and site-survey wells undertaken by the MITI have revealed the gas hydrate distribution at a depth of about 290 mbsf. The MITI Nankai Trough wells were drilled in late 1999 and early 2000. The highlights were successful retrievals of abundant gas hydrate-bearing cores in a variety of sediments from the main hole and the post survey well-2, keeping the cored gas hydrate stable, and the obtaining of continuous well log data in the gas hydrate-dominant intervals from the main hole, the post survey well-1 and the post survey well-3. Gas-hydrate dominant layers were identified at the depth interval from 205 to 268 mbsf. Pore-space hydrate, very small in size, was recognized mostly filling intergranular pores of sandy sediments. Anomalous chloride contents in extracted pore water, core temperature depression, core observations as well as visible gas hydrates confirmed the presence of pore-space hydrates within moderate to thick sand layers. Gas hydrate-bearing sandy strata typically were 10 cm to a meter thick with porosities of about 40 %. Gas hydrate saturations in most hydrate-dominant layers were quite high, up to 90 % pore saturation.
All the gas hydrate-bearing cores were subjected to X-ray CT imagery measurements for observation of undisturbed sedimentary textures and gas-hydrate occurrences before being subjected to other analyses, such as (1) petrophysical properties, (2) biostratigraphy, (3) geochemistry, (4) microbiology and (5) gas hydrate characteristics.     

The Geochemical Context of Gas Hydrate in the Eastern Nankai Trough

Abstract. Geochemical studies for gas hydrate, gas and organic matter collected from gas hydrate research wells drilled at the landward side of the eastern Nankai Trough, offshore Tokai, Japan, are reported. Organic matter in the 2355 m marine sediments drilled to Eocene is mainly composed of Type III kerogen with both marine and terrigenous organic input. The gas hydrate-bearing shallow sediments are immature for hydrocarbon generation, whereas the sediments below 2100 mbsf are thermally mature. The origins of gases change from microbial to thermogenic at around 1500 mbsf.
Carbon isotope compositions of CH₄ and CO₂, and hydrocarbon compositions consistently suggest that the CH₄ in the gas hydrate-bearing sediments is generated by microbial reduction of CO₂. The δ¹³C depth-profiles of CH₄ and CO₂ suggest that the microbial methanogenesis is less active in the Nankai Trough sediments compared with other gas hydrate-bearing sediments where solid gas hydrate samples of microbial origin were recovered. Since in situ generative-potential of microbial methane in the Nankai Trough sediments is interpreted to be low due to the low total organic carbon content (0.5 % on the average) in the gas hydrate-bearing shallow sediments, upward migration of microbial methane and selective accumulation into permeable sands should be necessary for the high concentration of gas hydrate in discrete sand layers.     

Detection of Methane Hydrate-Bearing Zones from Seismic Data

Abstract. The MITI Nankai Trough wells were drilled offshore Japan in the Tokai area in 1999 and 2000. The occurrence of methane hydrate was confirmed by various indicators in the borehole logs and from core data. These findings have a large impact on potential future Japanese energy resources and other related-scientific interests.
We first tried to find the methane hydrate-bearing zones using interval velocities derived from NMO velocity analysis. However, this analysis produced poor resolution. To achieve a more detailed delineation of the gas hydrate- and gas-bearing zones, we executed a seismic impedance inversion calibrated by the logs from two of the MITI Nankai Trough wells. Although these two wells are only about 90 m apart, we were able to produce an impedance section with fine detail by adopting a simple initial model and incorporating physical properties of the methane hydrate-bearing zones. The locations of the methane hydrate-bearing zones are readily apparent in the final section.     

基于统一硬化参数的深海能源土本构模型

深海能源土是指含天然气水合物（俗称"可燃冰"）的深海沉积物,其本构特性的模拟对可燃冰的安全开采至关重要。首先分析了水合物对能源土强度、剪胀和软化等力学特性的影响机理,水合物饱和度越大,对能源土力学特性影响越显著。然后在修正剑桥模型的基础上,通过引入水合物的饱和度和统一硬化参数来修正屈服函数,以反映水合物对能源土强度、剪胀、软化等特性的影响,建立了能考虑天然气水合物胶结作用形成及退化影响的深海能源土弹塑性本构模型,推导了相应的弹塑性矩阵。最后,通过模拟结果与已有能源土三轴试验数据对比分析,表明模型能很好地预测能源土强度、剪胀和软化等特性,验证了模型的合理性和有效性。     

Progress in Global Gas Hydrate Development and Production as a New Energy Resource

Natural gas hydrates have been hailed as a new and promising unconventional alternative energy, especially as fossil fuels approach depletion, energy consumption soars, and fossil fuel prices rise, owing to their extensive distribution, abundance, and high fuel efficiency. Gas hydrate reservoirs are similar to a storage cupboard in the global carbon cycle, containing most of the world’s methane and accounting for a third of Earth’s mobile organic carbon. We investigated gas hydrate stability zone burial depths from the viewpoint of conditions associated with stable existence of gas hydrates, such as temperature, pressure, and heat flow, based on related data collected by the global drilling programs. Hydrate-related areas are estimated using various biological, geochemical and geophysical tools. Based on a series of previous investigations, we cover the history and status of gas hydrate exploration in the USA, Japan, South Korea, India, Germany, the polar areas, and China. Then, we review the current techniques for hydrate exploration in a global scale. Additionally, we briefly review existing techniques for recovering methane from gas hydrates, including thermal stimulation, depressurization, chemical injection, and CH4–CO2 exchange, as well as corresponding global field trials in Russia, Japan, United States, Canada and China. In particular, unlike diagenetic gas hydrates in coarse sandy sediments in Japan and gravel sediments in the United States and Canada, most gas hydrates in the northern South China Sea are non-diagenetic and exist in fine-grained sediments with a vein-like morphology. Therefore, especially in terms of the offshore production test in gas hydrate reservoirs in the Shenhu area in the north slope of the South China Sea, Chinese scientists have proposed two unprecedented techniques that have been verified during the field trials: solid fluidization and formation fluid extraction. Herein, we introduce the two production techniques, as well as the so-called “four-in-one” environmental monitoring system employed during the Shenhu production test. Methane is not currently commercially produced from gas hydrates anywhere in the world; therefore, the objective of field trials is to prove whether existing techniques could be applied as feasible and economic production methods for gas hydrates in deep-water sediments and permafrost zones. Before achieving commercial methane recovery from gas hydrates, it should be necessary to measure the geologic properties of gas hydrate reservoirs to optimize and improve existing production techniques. Herein, we propose horizontal wells, multilateral wells, and cluster wells improved by the vertical and individual wells applied during existing field trials. It is noteworthy that relatively pure gas hydrates occur in seafloor mounds, within near-surface sediments, and in gas migration conduits. Their extensive distribution, high saturation, and easy access mean that these types of gas hydrate may attract considerable attention from academia and industry in the future. Herein, we also review the occurrence and development of concentrated shallow hydrate accumulations and briefly introduce exploration and production techniques. In the closing section, we discuss future research needs, key issues, and major challenges related to gas hydrate exploration and production. We believe this review article provides insight on past, present, and future gas hydrate exploration and production to provide guidelines and stimulate new work into the field of gas hydrates.     

美国天然气水合物研究计划介绍

以美国近年来提出的天然气水合物研究计划和项目申请书为基础,介绍美国科学家在天然气水合物研究领域中所关心的关键科学与技术问题和研究焦点,供我国天然气水合物研究者在项目设计和开展研究工作时参考。美国天然气水合物研究关注的重点科学问题主要集中在 4个方面：天然气水合物的物理与化学特性研究;天然气水合物开采技术研究;天然气水合物灾害-安全性与海底稳定性研究;天然气水合物在全球碳循环中的作用研究。在研究方法上主要采取天然气水合物区的现场地质地球化学观测、实验室合成和测定及计算模拟,特别关注与水合物和油气冷泉相关的生命过程及与水合物的相互作用研究。     

青海木里地区天然气水合物三维地震探测

首次在中国祁连山冻土区青海木里地区实施了针对天然气水合物的三维地震探测。根据工区内冻土层厚度、地层岩性、水合物储集空间类型和赋存深度等特 点,设计了三维地震数据采集方案,制定了地震数据处理流程,获得了良好的三维地震探测结果。结果表明,工区内天然气水合物赋存层段地震响应特征与国外冻土区的天然气水合物具有明 显差异;冻土层厚度、岩性、水合物储集空间类型等因素,对该区地震资料振幅、频率、速度等影响显著;水合物层段的地震资料存在低频、中等杂乱振幅等明显的特点,这些特征为该区天然气水合物探测和识别提供了依据。     

Application of AVO Analysis to Seismic Data for Detection of Gas below Methane Hydrate Stability Zone in Nankai Trough Area

Abstract. For the purpose of development of methane hydrate, occurring in the deep marine subsurface, as a resource, the most important issue is to understand the methane hydrate system (generation, migration and accumulation) as well as to delineate the methane hydrate reservoir properties. We have applied the Amplitude Versus Offset (AVO) analysis to the seismic data acquired in the Nankai Trough, offshore Japan, in order to confirm the occurrence of gas just below the methane hydrate-bearing zone, assuming that gas will show a so-called Class-3 AVO response. Knowledge of the amount and occurrence of gas in the sediment below methane hydrate-bearing zone is one of the keys to understand the methane hydrate system.
We have utilized the qualitative analysis of AVO methodology to delineate how gas is located below the BSR, which is thought to be the reflection event from the interface between the methane hydrate-bearing zone and the underlying gas-bearing zone. In the region of MITI Nankai Trough Well PSW-3, we observe two BSRs separated by 25 ms. After AVO modeling using well data, we applied AVO attribute analysis and attribute crossplot analysis to the seismic data. Finally we applied an offset-amplitude analysis to CMP gather data at specific locations to confirm the results of AVO attribute analysis. The AVO analysis shows that there is very little gas located in the underlying sediment below methane hydrate-bearing zone. This result supports the fact that we could not obtain any clear evidence of gas occurrence just below the methane hydrate-bearing zone in the Nankai Trough well drilling.     

考虑天然气水合物上覆层不排水抗剪强度深度变化的海底斜坡稳定性影响分析

天然气水合物分解可以诱发海底斜坡失稳对海底工程设施产生造成破坏影响。因此,海底斜坡稳定性状态评价对海底工程设施选址、安全运行具有重要意义。文章根据南海北部神狐海域水合物富集区工程地质特征,采用有限元强度折减法分析了斜坡几何参数、土层强度变化,以及水合物储层特征等因素对水合物分解前后海底斜坡稳定性的影响规律。结果表明,未考虑水合物分解时,海底斜坡稳定性主要受控于斜坡坡度和土体强度,且主要表现为浅层滑坡。考虑水合物的分解时,水合物层强度降低会对斜坡的整体稳定性产生影响,但同等上覆层条件下最危险滑动面位置受水合物层埋深影响较大,且存在受地形几何特征与上覆土层强度控制的临界埋深。埋深大于临界埋深时,水合物分解对斜坡稳定性的影响较小,最危险滑动面位置位于上部浅层,表现为浅表层破坏。小于临界埋深时,最危险滑动面位置则经过水合物层,表现为深层滑坡。根据目前模型中的水合物层埋深条件,水合物分解后的深层滑动面安全系数仍高于浅部地层,意味该海域水合物开采仍需要关注浅层海底滑坡灾害的影响。     

Lithology, Biostratigraphy, and Magneto stratigraphy of Gas Hydrate-Bearing Sediments in the Eastern Nankai Trough

Abstract: Stratigraphic controls on the formation and distribution of gas hydrates were examined for sediments from a BH-1 well drilled in the landward slope of the Nankai Trough, approximately 60 km off Omaezaki, Japan. Three lithologic units were recognized in the 250 m-thick sequence of sediments: Unit 1 (0–70 mbsf) consists of calcareous silt and clay with thin volcanic ash layers, Unit 2 (70–150 mbsf) consists of calcareous silt and clay with volcanic ash and thin sand layers, and Unit 3 (150–250 mbsf) consists of weakly consolidated calcareous silt and clay with thick and frequent sand layers. Soupy structures and gas bubbles in the sediments indicate the presence of two hydrate zones between 40 and 130 mbsf and below 195 mbsf. Nannofossil biostratigraphy and magnetostratigraphy indicate that the sequence recovered at the BH-1 well is mostly continuous and represents sediments deposited from 0 to 1.5 Ma. Calculation of the sedimentation rate reveals a condensed section between 65 and 90 mbsf. The inferred distribution of gas hydrates in the BH-1 well appears to be strongly controlled by the stratigraphy and lithology of the sediments. Thick, gently inclined sand layers in Unit 3 provide a conduit for the migration of gases from deeper regions, and are considered responsible for the formation of the hydrate zone below 195 mbsf. At shallower levels, thin, gently inclined sand layers are also considered to allow for the migration of gases, leading to the formation of the upper hydrate zone between 40 and 130 mbsf. The overlying sub-horizontal silt and clay of the condensed section, truncating the underlying gently inclined sand and silt/clay layers, may provide an effective trap for gases supplied through the sand layers, further contributing to hydrate formation in the upper hydrate zone.     

祁连山冻土区天然气水合物及其基本特征

2008年11月5日, 由中国地质科学院矿产资源研究所、勘探技术研究所和青海煤炭地质局105勘探队施工的“祁连山冻土区天然气水合物科学钻探工程”DK-1孔取得重大突破, 成功钻获天然气水合物实物样品。这是我国冻土区首次钻获并检测出的天然气水合物实物样品, 也是世界上第一次在中低纬度高原冻土区发现的天然气水合物, 具有重要的科学、经济和环境意义。目前钻获的天然气水合物均产于冻土层之下, 产出深度133~396 m, 其层位属于中侏罗统江仓组。水合物以薄层状、片状、团块状赋存于粉砂岩、泥岩、油页岩的裂隙中, 或以浸染状赋存于细粉砂岩的孔隙中。祁连山冻土区天然气水合物具有埋深浅、冻土层 薄、气体组分复杂、以煤层气为主等特征, 应是一种新类型水合物。     

南海东沙海域末次冰期异常沉积事件与水合物分解

对南海东沙海域陆坡区973-3柱状样开展沉积学和年代学、有孔虫同位素和壳体B/Ca比值、碳酸钙和黄铁矿含量的分析, 发现该岩芯浮游有孔虫Globigerinoides ruber和底栖有孔虫Uvigerina peregrina的δ13C在末次冰期同时在多个层位发生负偏, 最大负偏达-2.03‰; 有孔虫δ13C负偏层位环境[CO₃2-]浓度相对偏低; 负偏层位同时出现大量黄铁矿, 最高含量达17%.在负偏层位以下沉积的碳酸盐溶解作用强烈, CaCO₃含量最低, 沉积物颜色偏深.究其成因与水合物分解关系较大.甲烷渗漏事件发生在末次冰期, 说明末次冰期的海平面下降是水合物分解的主要诱因.根据有孔虫δ13C负偏的次数和程度推断至少发生过4次甲烷渗漏, 渗漏的强度基本相当.      

热释光：一种冻土区天然气水合物地球化学勘查新技术

开发不受沼泽微生物影响的地球化学勘查技术是提高中纬度冻土区天然气水合物探井预测成功率的重要课题之一。选择在祁连山聚乎更天然气水合物已知区进行土壤热释光勘查技术实验。实验区为高寒沼泽景观,面积150 km2,采样密度2点/km2,采样深度60 cm,采集土壤样品300件,应用热释光测量仪对土壤样品进行了热释光分析。实验结果表明,天然气水合物矿藏上方出现天然热释光高值异常,与烃类异常浓度范围一致,为顶部异常模式。结合地质和地球化学勘查成果对异常进行了综合解释,认为祁连山聚乎更地区天然气热释光异常与天然气水合物矿藏关系密切,源于深部水合物矿藏。研究对天然气水合物的进一步调查具有重要的参考价值。     

祁连山冻土区天然气水合物现场识别方法

天然气水合物是一种赋存在低温、高压条件下,陆上永久冻土区和海底沉积物中的规模巨大的新型能源。在冻土区的天然气水合物研究过程中,钻探取样和天然气水合物岩芯研究仍是识别和推断天然气水合物最直接有效的方法。因此,如何在钻探现场快速有效地识别出天然气水合物及相关异常特征就显得极其重要。近几年在祁连山天然气水合物勘探过程中,探索性地总结出适用于冻土区的天然气水合物现场识别方法,主要包括肉眼观测、孔口气涌观测、岩芯红外测温、岩芯裂隙孔隙水盐度测定、岩芯气体解析与组分测定和岩芯次生构造与伴生矿物鉴别等方法。利用该套现场识别方法和随钻岩芯编录,有效地查明了祁连山冻土区天然气水合物在岩芯中的产状和分布特征,为该区天然气水合物资源评价和试开采试验提供了重要依据。     

青海祁连山冻土区发现天然气水合物

祁连山冻土区位于青藏高原北缘,多年冻土面积约10×10~4km~2,具有良好的天然气水合物形成条件和找矿前景.2008～2009年间中国地质调查局在青海省天峻县木里煤田聚乎更矿区施工"祁连山冻土区天然气水合物科学钻探工程",迄今共完成钻探试验井4口,总进尺2059.13m,分别在DK-1、DK-2和DK-3钻井中钻获天然气水合物实物样品,取得了找矿工作的重大突破.天然气水合物产于冻土层之下,埋深133～396m.水合物呈白色、乳白色晶体,点火能燃烧,红外热像仪测温后呈明显的低温异常,放进水里强烈冒泡,水合物分解后能不断冒出气泡和水滴,并残留下特征的蜂窝状构造.激光拉曼光谱仪检测呈现特征的水合物光谱曲线,测井曲线也具有较明显的高电阻率和高波速标志.祁连山天然气水合物具有冻土层薄、埋深浅、气体组分复杂、以煤层气成因为主等明显特征,是一种新类型水合物.这是我国冻土区首次钻获的天然气水合物实物样品,也是全球首次在中低纬度高山冻土区发现天然气水合物实物样品,具有重要的科学意义和经济意义.     

多孔介质中天然气水合物稳定性的实验研究进展

勘探表明天然气水合物多产出于细碎屑沉积物中,其分布和赋存形式受温度、压力、水化学条件等多种物理化学因素的影响。前人的实验研究表明不同孔径尺度中的甲烷水合物稳定性有别于块状、层状水合物,同时孔隙表面的润湿性也是影响因素之一。在综合分析前人研究成果的基础上,系统阐述了孔隙的孔径、孔隙内表面润湿性对所含天然气水合物稳定性的影响规律,总结了可能的内在机理;并指出了当前应当尽快建立包括空间效应、温度、压力和组分等因素的综合天然气水合物相图,查明含天然气水合物沉积物的孔隙结构和表界面特征,建立天然气水合物的稳定性模型,将有助于精确预测天然气水合物的分布和规模,对于水合物开发和甲烷存储技术的研发也有着重要的意义。     

Coexistence of natural gas hydrate,free gas and water in the gas hydrate system in the Shenhu Area,South China Sea

Shenhu Area is located in the Baiyun Sag of Pearl River Mouth Basin, which is on the northern continental slope of the South China Sea. Gas hydrates in this area have been intensively investigated, achieving a wide coverage of the three-dimensional seismic survey, a large number of boreholes, and detailed data of the seismic survey, logging, and core analysis. In the beginning of 2020, China has successfully conducted the second offshore production test of gas hydrates in this area. In this paper, studies were made on the structure of the hydrate system for the production test, based on detailed logging data and core analysis of this area. As to the results of nuclear magnetic resonance (NMR) logging and sonic logging of Well GMGS6-SH02 drilled during the GMGS6 Expedition, the hydrate system on which the production well located can be divided into three layers: (1) 207.8–253.4 mbsf, 45.6 m thick, gas hydrate layer, with gas hydrate saturation of 0–54.5% (31% av.); (2) 253.4–278 mbsf, 24.6 m thick, mixing layer consisting of gas hydrates, free gas, and water, with gas hydrate saturation of 0–22% (10% av.) and free gas saturation of 0–32% (13% av.); (3) 278–297 mbsf, 19 m thick, with free gas saturation of less than 7%. Moreover, the pore water freshening identified in the sediment cores, taken from the depth below the theoretically calculated base of methane hydrate stability zone, indicates the occurrence of gas hydrate. All these data reveal that gas hydrates, free gas, and water coexist in the mixing layer from different aspects.     

水合物沉积物力学性质的三维离散元分析

水合物沉积物力学特性研究是天然气水合物开采领域中的热点问题。为深入了解水合物对沉积物力学特性的影响,在提出一个新的水合物沉积物离散元数值试样制备方法的基础上,模拟了不同水合物饱和度沉积物试样的三轴排水试验,并从其应力-应变关系、体变特性、弹性模量及峰值强度等方面对模拟结果及已有室内三轴试验结果进行了对比,然后利用该方法对具有不同微观胶结参数的水合物沉积物样进行了三轴离散元数值试验。研究结果表明：所提出的离散元模拟方法能较好地反映水合物沉积物的主要力学特性;天然气水合物与土颗粒间胶结性能的改变会对水合物沉积物的力学响应产生一定的影响;水合物沉积物强度和模量的增加是孔隙填充水合物和粒间胶结水合物共同作用的结果。     

Geochemical insights into contribution of petroleum hydrocarbons to the formation of hydrates in the Taixinan Basin,the South China Sea

Methane hydrate in the South China Sea(SCS)has extensively been considered to be biogenic on the basis of itsδ13C and δD values.Although previous efforts have greatly been made,the contribution of thermogenic oil/gas has still been underestimated.In this study,biomarkers and porewater geochemical parameters in hydrate-free and hydrate-bearing sediments in the Taixinan Basin,the SCS have been measured for evaluating the contribu-tion of petroleum hydrocarbons to the formation of hydrate deposits via a comparative study of their source inputs of organic matters,environmental conditions,and microbial activities.The results reveal the occurrence of C14-C16 branched saturated fatty acids(bSFAs)with relatively high concentrations from sulfate-reducing bacteria(SRBs)in hydrate-bearing sediments in comparison with hydrate-free sediments,which is in accord with the positive δ13C values of dissolved inorganic carbon(DIC),increasing methane concentrations,decreasing alka-linity,and concentration fluctuation of ions(Cl-,Br,SO2-,Ca2+,and Mg2+).These data indicate the relatively active microbial activities in hydrate-bearing sediments and coincident variations of environmental conditions.Carbon isotope compositions of bSFAs(-34.0％o to-21.2％o),n-alkanes(-34.5％o to-29.3％o),and methane(-70.7％o to-69.9％o)jointly demonstrate that SRBs might thrive on a different type of organic carbon rather than methane.Combining with numerous gas/oil reservoirs and hydrocarbon migration channels in the SCS,the occurrence of unresolved complex mixtures(UCMs),odd-even predominance(OEP)values(about 1.0),and biomarker patterns suggest that petroleum hydrocarbons from deep oil/gas reservoirs are the most probable carbon source.Our new results provide significant evidence that the deep oil/gas reservoirs may make a contribution to the formation of methane hydrate deposits in the SCS.