南海北部陆坡区神狐海域构造特征及对水合物的控制

通过对南海北部陆坡区神狐海域高精度2D和3D地震资料的精细解释,在研究区共识别出4种构造类型,分别为气烟囱(流体底辟)、区域大尺度断层、深水扇中的正断层和滑移体中的滑脱断层。气烟囱具有直立的通道形态,其内部结构可划分为杂乱反射带、模糊反射带和顶部强振幅区域。大尺度断层位于水合物钻探区的西北部和东北部,断层规模大,对深部地层表现出明显的控制作用。深水扇中的正断层广泛发育于上新世的深水扇中,特别是在水合物钻探区西部进积特征明显的深水扇中,正断层的数量更多。滑移体中的滑脱断层在神狐海域的第四纪地层中非常常见,在剖面上呈雁列式分布。研究结果表明,大尺度断层由于和水合物钻探区的距离较远,对于水合物的成藏可能不起控制作用。气烟囱和规模小数量多的断裂体系为含气流体的运移提供了垂向和侧向的输送通道,构成了水合物的流体运移体系。当富含甲烷气体的流体通过这些垂向-侧向的运移通道时,在合适的温压条件下,被适于水合物聚集的沉积体所捕获,就有可能形成水合物。水合物钻探区内东西部构造特征的差异,使得研究区内形成了不同的流体运移体系,这可能是控制钻探区水合物不均匀性分布的一个关键因素。     

南海北部陆坡高饱和度天然气水合物气源运聚通道控藏作用

气源运聚通道与天然气水合物富集成藏关系密切。利用准三维地震资料并结合钻探成果,深入研究了神狐海域GMGS3钻探区高饱和度水合物站位气源运移疏导通道地质地球物理特征及其控藏作用。结果表明:高饱和度水合物产出站位发育多种类型运移疏导通道,且与BSR空间耦合关系较好;紧邻BSR之下为强振幅反射,强振幅下部游离气体充注现象明显,表明水合物稳定域下部存在气体运移的通道,且深部气体向浅层发生了运移。深大断裂、底辟及气烟囱构成了沟通深部热解气及浅层生物气与浅层温压稳定域的垂向通道,在这些通道上方可以直接形成水合物;浅部滑塌面、水道砂及海底扇构成的高孔渗连续性砂体为浅层生物气及深部运移而来的部分热成因气横向运移通道,气体的侧向运移扩大了气体供给范围,增加了矿体横向展布规模。文章认为,天然气运移疏导系统与其他成藏要素匹配良好的构造和区域是勘探高饱和度水合物的有利目标。     

The formation mechanism of mud diapirs and gas chimneys and their relationship with natural gas hydrates: insights from the deep-water area of Qiongdongnan Basin,northern South China Sea

ABSTRACT

Mud diapirs and gas chimneys are widely developed in continental slope areas, which can provide sufficient gas for hydrate formation, and they are important for finding natural gas hydrates. Based on the interpretation and analysis of high-resolution 2D and 3D seismic data covering the deep-water area in the Qiongdongnan Basin (QDNB), northern South China Sea, we studied the formation mechanism of mud diapirs and gas chimneys and their relationship with natural gas hydrates. Mud diapirs and gas chimneys are columnar and domelike in shape and the internal regions of these bodies have abnormal reflections characterized by fuzzy, chaotic, and blanking zones. The reflection events terminate at the rims of mud diapirs and gas chimneys with pull-up reflections and pull-down reflections, respectively. In addition, ‘bright spots’ and diapiric-associated faults occur adjacent to mud diapirs and gas chimneys. The rapidly deposited and deeply buried fine sediments filling in the Tertiary in deep-water areas of the QDNB and overpressure potential derived from undercompacted mudstones, as well as from the pressurization of organic matter and hydrocarbon generation, provide abundant materials and intensive driving forces for the formation of mud diapirs and gas chimneys. Bottom simulating reflectors (BSRs) with strong amplitude and high or poor continuity were recognized atop the mud diapirs and gas chimneys and in the structural highs within the same region, indicating that they have a close relationship with each other. The mud diapirs and gas chimneys and associated high-angle faults provide favourable vertical pathways for the hydrocarbons migrating from deep strata to shallow natural gas hydrate stability zones where natural gas hydrates accumulate; however, some BSRs are characterized by weak amplitude and poor continuity, which can be affected by high temperature and overpressure in the process of the mud diapir and gas chimney activities. This mutually restricting relationship must be taken into consideration in the process of gas hydrate exploration in QDNB.     

神狐钻探区天然气水合物成藏地质条件分析

以天然气水合物成藏系统理论为指导,对神狐钻探区天然气水合物成藏地质条件进行了系统研究。研究结果表明：神狐钻探区具有优越的烃类生成体系和流体运移体系。天然气水合物气源以生物气-热成因气混合气为主,气源岩生烃潜力大;断层、气烟囱以及断层滑脱面可以为含烃流体在纵向和横向上的运移提供优势运移通道。地温特征和成藏就位体系-沉积物岩性及其岩性组合特征是控制该区水合物层在空间尺度上分布不均匀的主要原因,地温和地温梯度越低,沉积物粒度越粗,且具备“上细下粗”的沉积物岩性组合更有利于水合物的形成。     

琼东南盆地天然气水合物与浅层气共生体系成藏特征

继我国在神狐海域两次天然气水合物试采成功之后,近几年来在琼东南盆地的勘探证实了天然气水合物的存在,而且钻探表明其与浅层气具有复杂的共生关系.为揭示琼东南盆地深水区天然气水合物与浅层气共生体系成藏特征,结合岩心、测井及三维地震数据,阐明了天然气水合物与浅层气的空间分布特征,研究结果表明,天然气水合物主要赋存在海底以下200 m范围内的沙质沉积物中,且其形成过程与浅层气的垂向运移有关.对天然气水合物与浅层气共生体系成藏特征的深入分析表明,深部热成因气和浅部生物成因气是其重要的气体来源,第四系未固结沙层是良好的储层,且天然气水合物和浅层气共生体系的分布主要受深部气烟囱和断层的控制.浅层气藏为天然气水合物提供稳定的气源条件;第四系块体流沉积与含天然气水合物地层能有效地封堵浅层气的纵向运移,进一步促进浅层气的成藏.因此,天然气水合物的形成与浅层气的发育具有正反馈的相互作用关系,有利于形成更大规模的天然气水合物矿体和浅层气藏,具有良好的商业开发潜力.      

Gas Sources of Natural Gas Hydrates in the Shenhu Drilling Area, South China Sea: Geochemical Evidence and Geological Analysis

The Shenhu gas hydrate drilling area is located in the central Baiyun sag, Zhu Ⅱ depression, Pearl River Mouth basin, northern South China Sea. The gas compositions contained in the hydrate-bearing zones is dominated by methane with content up to 99.89% and 99.91%. The carbon isotope of the methane (δ13C1 ) are 56.7‰ and 60.9‰, and its hydrogen isotope (δD) are 199‰ and 180‰, respectively, indicating the methane from the microbial reduction of CO2 . Based on the data of measured seafloor temperature and geothermal gradient, the gas formed hydrate reservoirs are from depths 24-1699 m below the seafloor, and main gas-generation zone is present at the depth interval of 416-1165 m. Gas-bearing zones include the Hanjiang Formation, Yuehai Formation, Wanshan Formation and Quaternary sediments. We infer that the microbial gas migrated laterally or vertically along faults (especially interlayer faults), slump structures, small-scale diapiric structures, regional sand beds and sedimentary boundaries to the hydrate stability zone, and formed natural gas hydrates in the upper Yuehai Formation and lower Wanshan Formation, probably with contribution of a little thermogenic gas from the deep sedments during this process.     

Migration and accumulation characteristics of natural gas hydrates in the uplifts and their slope zones in the Qiongdongnan Basin,China

Various factors controlling the accumulation of natural gas hydrates (NGHs) form various enrichment and accumulation modes through organic combination. This study mainly analyzes the geological and geophysical characteristics of the NGHs occurrence in the uplifts and their slope zones within the deep-water area in the Qiongdongnan (QDN) Basin (also referred to as the study area). Furthermore, it investigates the dominant governing factors and models of NGHs migration and accumulation in the study area. The results are as follows. (1) The uplifts and their slope zones in the study area lie in the dominant pressure-relief direction of fluids in central hydrocarbon-rich sags in the area, which provide sufficient gas sources for the NGHs accumulation and enrichment through pathways such as gas chimneys and faults. (2) The top and flanks of gas chimneys below the bottom simulating reflectors (BSRs) show high-amplitude seismic reflections and pronounced transverse charging of free gas, indicating the occurrence of a large amount of gas accumulation at the heights of the uplifts. (3) Chimneys, faults, and high-porosity and high-permeability strata, which connect the gas hydrate temperature-pressure stability zones (GHSZs) with thermogenic gas and biogenic gas, form the main hydrate migration system. (4) The reservoir system in the study area comprises sedimentary interlayers consisting of mass transport deposits (MTDs) and turbidites. In addition, the reservoir system has developed fissure- and pore-filling types of hydrates in the pathways. The above well-matched controlling factors of hydrate accumulation enable the uplifts and their slope zones in the study area to become the favorable targets of NGHs exploration.©2022 China Geology Editorial Office.     

泥底辟输导流体机制及其与天然气水合物成藏的关系

摘要：详细阐述不同成因的泥底辟流体输导模式,探讨了泥底辟输导体系的演化与天然气水合物成藏之间的关系,并分析神狐海域泥底辟输导体系对天然气水合物成藏的影响。底辟核外部伴生断裂、底辟核内部流体压裂裂缝和边缘裂缝带均可作为输导流体的通道。根据运移通道和动力等差异性,提出泥底辟输导流体的2种端元模式：超压-流体压裂输导型和边缘构造裂缝输导型。在此基础上,讨论了泥底辟(泥火山)的不同演化阶段对水合物的形成、富集和分解的影响。早期阶段,泥底辟形成的运移通道可能未延伸到水合物稳定带,导致气源供给不够充分;中期阶段,水合物成藏条件匹配良好,利于天然气水合物生成;晚期阶段,泥火山喷发引起水合物稳定带的热异常,可能导致水合物分解,直至泥火山活动平静期,水合物再次成藏。神狐海域内泥底辟分为花冠状和穹顶状两类,花冠状泥底辟以超压-流体压裂输导型为主;穹顶状泥底辟以底辟边缘裂缝输导型为主。泥底辟输导体系的差异性可能是神狐海域天然气水合物非均质分布的影响因素之一。 关键词：泥底辟;输导体系;天然气水合物;成藏机制;神狐海域     

晚期泥底辟控制作用导致神狐海域SH5钻位未获水合物的分析

海底泥底辟构造与天然气水合物成藏关系密切,泥底辟既能为水合物提供充分的气源物质,同时又能促使地层温度场改变进而影响水合物成藏稳定性。南海北部神狐海域SH5站位虽然BSR明显,但钻探证实不存在天然气水合物。该钻位温度剖面异常高,温度场上移,同时在其下伏地层中发现泥底辟构造和裂隙通道。根据上述事实并结合泥底辟发育各个阶段中的特点,认为泥底辟构造对天然气水合物成藏具有控制作用。泥底辟发育早期和中期阶段,低热导率和低热量有机气体有利于天然气水合物生成;而在晚期阶段,高热量液体上侵稳定带底界,导致水合物分解迁移。SH5站位很可能由于受到处于晚期阶段的泥底辟上侵而未能获取天然气水合物。     

南海东沙西南海域冷泉碳酸盐岩特征及其意义

海底冷泉流体或自生碳酸盐沉积可为碳氢化合物(主要指常规油气或天然气水合物)、冷泉生物群落的调查和研究提供有利线索。对东沙群岛海区、神狐海区和ODP184-1146钻孔碳酸盐沉积的特征进行了对比和研究,结果表明这些海区碳酸盐岩的岩性特征、碳氧同位素组成等存在明显差异。东沙群岛海区和神狐海区的碳酸盐岩的化学成分如TFe、A l2O3、TiO2、K2O、Na2O、MnO、P2O5等不同,表明它们可能分别受到了粘土矿物和铁锰氧化物影响。在神狐海区至少发生了一次冷泉流体活动,形成了早晚两期冷泉碳酸盐岩,而在东沙群岛海区发生了至少3次冷泉流体活动,形成了多期冷泉碳酸盐岩,反映了这两个海区碳酸盐沉积的甲烷成因不同,且形成环境也受不同的因素控制。研究揭示,在东沙群岛南部海区可能发育热成因甲烷汇聚的天然气水合物,其海底可能存在冷泉生物群落,这为东沙西南海域天然气水合物和冷泉生物群落的进一步寻找和研究提供了新思路。     

南海北部神狐海域天然气水合物成藏特征

天然气水合物作为一种新型、洁净、潜在的新能源,越来越引起世界各国科学家的重视,对天然气水合物研究也进一步深入,但天然气水合物作为一种能源矿产,对其成藏机制的研究相对较少。针对我国天然气水合物调查研究相对较为详细的神狐海域,从其物质来源、气体运移通道、成藏条件等角度探讨神狐海域天然气水合物的成藏特征,指出白云凹陷古近纪埋藏的巨厚烃源岩是其成藏的主要物质基础;底辟构造发育区是形成水合物流体向上运移的主要通道;新近纪晚期大面积发育的滑塌体是水合物的主要赋存区。神狐海域具备天然气水合物成藏的优越条件,是进一步勘探水合物的远景区。     

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.     

Gas hydrate systems at Hydrate Ridge offshore Oregon inferred from molecular and isotopic properties of hydrate-bound and void gases

We report and discuss molecular and isotopic properties of hydrate-bound gases from 55 samples and void gases from 494 samples collected during Ocean Drilling Program (ODP) Leg 204 at Hydrate Ridge offshore Oregon. Gas hydrates appear to crystallize in sediments from two end-member gas sources (deep allochthonous and in situ) as mixtures of different proportions. In an area of high gas flux at the Southern Summit of the ridge (Sites 1248-1250), shallow (0-40 m below the seafloor [mbsf]) gas hydrates are composed of mainly allochthonous mixed microbial and thermogenic methane and a small portion of thermogenic C₂₊ gases, which migrated vertically and laterally from as deep as 2- to 2.5-km depths. In contrast, deep (50-105 mbsf) gas hydrates at the Southern Summit (Sites 1248 and 1250) and on the flanks of the ridge (Sites 1244-1247) crystallize mainly from microbial methane and ethane generated dominantly in situ. A small contribution of allochthonous gas may also be present at sites where geologic and tectonic settings favor focused vertical gas migration from greater depth (e.g., Sites 1244 and 1245). Non-hydrocarbon gases such as CO₂ and H₂S are not abundant in sampled hydrates. The new gas geochemical data are inconsistent with earlier models suggesting that seafloor gas hydrates at Hydrate Ridge formed from gas derived from decomposition of deeper and older gas hydrates. Gas hydrate formation at the Southern Summit is explained by a model in which gas migrated from deep sediments, and perhaps was trapped by a gas hydrate seal at the base of the gas hydrate stability zone (GHSZ). Free gas migrated into the GHSZ when the overpressure in gas column exceeded sealing capacity of overlaying sediments, and precipitated as gas hydrate mainly within shallow sediments. The mushroom-like 3D shape of gas hydrate accumulation at the summit is possibly defined by the gas diffusion aureole surrounding the main migration conduit, the decrease of gas solubility in shallow sediment, and refocusing of gas by carbonate and gas hydrate seals near the seafloor to the crest of the local anticline structure.     

Seismic characteristics and mechanism of fluid flow structures in the central depression of Qiongdongnan basin,northern margin of South China Sea

ABSTRACT

Understanding fluid flow structures in a rifted basin may enhance our knowledge of their origination and evolution. Through geochemical analysis and seismic interpretation, different fluid flow features are identified in the central depression of Qiongdongnan basin, northern South China Sea. These structures include mud diapir, gas chimney, hydrothermal pipes, faults, blowout pipes, and associated extrusions. Mud diapirs are primarily located on the slope belts, whereas gas chimneys are on the basement highs in the southwest of the study area. Their distribution appears closely controlled by tectonic stress field and overpressure, the later is caused by hydrocarbon generation and compaction disequilibrium. High sediment overloading, weak post-rift tectonic activity, and high average geothermal gradient may contribute to the compaction disequilibrium. The occurrence of gas chimneys on the basement high suggests that lateral transportation and relief of overpressure is a significant factor. Distribution of broad hydrothermal pipes is related with the thinning continental crust and pre-existing boundary faults in the central depression. They are probably attributed to intruded sills dissolution and were caused by hydrothermal fluids vertically. Geochemical data from gas reservoirs analysis indicates that mud diapirs and gas chimneys are critical pathways for thermogenic gases, whereas hydrothermal pipes and part of the faults may act as pathways of both thermogenic and inorganic gases. The blowout pipes mainly occur in the northwestern central depression near the continental slope, where fluid flows ascend gradually from a series of Pliocene-current prograding wedge-formed units with a hydraulic fracture in shallow. Hundreds of seafloor pockmarks and mounds associated with blowout pipes located above the NE-SW elongated Pliocene-Quaternary slope-break belts. These extrusive structures indicate that fluids ascend through blowout pipes and were expelled at the present seabed. Our results indicate that fluid flow structures are probably responsible for fluid activities and must be taken into account when assessing the hydrocarbon potential, geologic hazard, and benthic ecosystem.     

海底天然气水合物分解与甲烷归宿研究进展

综述了近年来天然气水合物分解与甲烷归宿等方面的研究成果。天然气水合物的汇聚与地质构造或地层圈闭有关,其溶解受物质转换控制,分解则受热转换控制。水合物释放甲烷的运移方式包括分散式、中心式和大规模排放式。缺氧氧化和耗氧氧化是甲烷在海洋环境中的2种主要转化方式。天然气水合物释放甲烷的最终归宿主要为：①重新形成天然气水合物;②形成化能自养生物群落和沉淀出碳酸盐沉积;③与氧发生氧化后转变为CO2;④直接排放进入到大气中。沉积物中的微构造、化能自养生物群落、自生碳酸盐矿物及其碳氧同位素组成是水合物释放事件的指纹记录。     

南海神狐海域有孔虫与高饱和度水合物的储存关系

通过对神狐海域沉积物组分与水合物成藏关系的研究, 得到SH7B孔含水合物层(155~177 m)有孔虫丰度以及有孔虫壳体微结构与水合物饱和度的关系.结果表明, 有孔虫丰度与水合物饱和度有良好的对应关系, 有孔虫丰度高, 水合物饱和度也高; 反之亦然.有孔虫丰度与水合物饱和度二者的相关系数为0.72, 说明有孔虫与水合物的分布和富集有关.扫描电镜研究表明, 有孔虫成岩作用不明显, 有孔虫为有效孔隙, 有孔虫独特的壳体结构增加了沉积物的孔隙空间, 有利于水合物的储存和富集.大部分有孔虫壳体大小相当于砂粒级, 它的存在一方面增加沉积物粗组分砂的含量, 另一方面增加沉积物的孔隙度.沉积物中生物组分——有孔虫, 是南海神狐海域水合物富集的重要因素之一.      

南海神狐海域X区块天然气水合物的控制因素

为了探讨南海神狐海域X区块天然气水合物的控制因素,开展了高分辨地震资料精细解释和天然气水合物地震相研究。结果显示,X区块存在3条主控断层和众多滑塌断层;预测的稳定带内水合物呈多层分布,稳定带底部水合物层厚度较大、连续性较好。结合地震反射特征和可能的气源讨论初步认为,X区块天然气水合物的成藏模式主要有两种类型:在断裂或滑塌构造发育区,水合物的气源以热解成因气为主,水合物呈多层分布;在构造平缓地区,水合物的气源以生物成因气为主,水合物呈单层分布,且主要分布在似海底反射(BSR)之上一个狭窄的范围内。     

波阻抗反演在南海北部神狐海域天然气水合物勘探中的应用

针对神狐海域的地质构造和天然气水合物的赋存特征,以重点测线三维地震数据为基础,分析讨论了基于宽带约束的模拟退火波阻抗反演方法、流程和关键技术问题,定量获得了含天然气水合物沉积物的波阻抗特征。结果表明:基于宽带约束的模拟退火波阻抗反演数据具有较高的有效垂向分辨率和较好的横向连续性;神狐海域高波阻抗异常反映了含天然气水合物沉积层,而不连续异常低波阻抗层是水合物层之下游离气的表现,这与钻探结果吻合。由此可见,基于宽带约束的模拟退火波阻抗反演可为天然气水合物层识别和预测、勘探目标圈定、钻探井位选择提供重要依据。     

Seismic fine imaging and its significance for natural gas hydrate exploration in the Shenhu Test Area,South China Sea

Shenhu area in South China Sea includes extensive collapse and diapir structures, forming high-angle faults and vertical fracture system, which functions as a fluid migration channel for gas hydrate formation. In order to improve the imaging precision of natural gas hydrate in this area, especially for fault and fracture structures, the present work propose a velocity stitching technique that accelerates effectively the convergence of the shallow seafloor, indicating seafloor horizon interpretation and the initial interval velocity for model building. In the depth domain, pre-stack depth migration and residual curvature are built into the model based on high-precision grid-tomography velocity inversion, after several rounds of tomographic iterations, as the residual velocity field converges gradually. Test results of the Shenhu area show that the imaging precision of the fault zone is obviously improved, the fracture structures appear more clearly, the wave group characteristics significantly change for the better and the signal-to-noise ratio and resolution are improved. These improvements provide the necessary basis for the new reservoir model and field drilling risk tips, help optimize the favorable drilling target, and are crucial for the natural gas resource potential evaluation.     

Formation of gas hydrate deposits in the Siberian Arctic shelf

Natural gas hydrate deposits have been estimated to store about 10% of gas in hydrate form (even with regard to a higher concentration of gas in hydrates), proceeding from the known ratio of dissolved-to-deposited gas. This high percentage is largely due to the fact that the buffer factor in natural gas hydrate deposits is lower than that for free gas because of less diverse structural conditions for gas accumulation. Therefore, the available appraisal of world resources of hydrated gas needs a revision.Hydrates in rocks are either syngenetic or epigenetic. Syngenetic hydrates originate from free or dissolved gas which was present in rocks in situ at the time when PT-conditions became favorable for gas hydrate formation. Epigenetic hydrates are derived from gas which came by migration into rocks with their PT-conditions corresponding to formation of gas hydrates.In addition to the optimum PT-conditions and water salinity, economic gas hydrate accumulation requires sustained supply of natural gas into a specific zone of gas hydrate formation. This condition is feasible only in the case of vertical migration of natural gas along faults, fractured zones, and lithologic windows, or, less often, as a result of lateral migration.Of practical importance are only the gas hydrate deposits produced by vertical or lateral gas migration.