南海东北部天然气水合物成藏演化地质过程

南海东北部天然气水合物成藏演化地质过程
张光学;陈芳;沙志彬;梁金强;苏新;陆红锋
【期刊名称】《地学前缘》
【年(卷),期】2017(24)4
【摘要】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
prehensive 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 plex hydrate system composed of multilayered hydrate structures is the result of dynamic process driven by methane diffusion and advection.%南海东北部天然气水合物赋存类型多样,包括块状、瘤状、薄层状、脉状及分散状等五种形式,独自出现或相互共生成藏,并伴生碳酸盐岩,且分布在水深600～900m、海底及其以下的200m地层之内.其中前四种水合物常形成于浅断裂隙、沉积物层间薄弱带、海底表面等局部地区,粒径大于正常沉积物孔隙,水合物肉眼可见,故又称可视水合物;分散状水合物充填于微米
纳米级孔隙的沉积物中,肉眼难以分辨,故这类水合物为非可视水合物.据钻井揭示,以海底之下90 m处地层为界,研究区发育上、下两套水合物矿层,上部水合物矿层以可视水合物为主,下部水合物矿层为非可视水合物.其中上部水合物矿层发育上、下两个水合物层段,在各水合物层段的顶部均发育自生碳酸盐岩,反映为地史时期与水合物藏演化相伴生的古冷泉.综合分析认为,天然气水合物赋存形式及水合物藏演化受控于多种因素作用,包括岩性、粒度、岩相(古河道、扇体)等沉积作用,新构造活动及断层作用,流体及气体运移,以及影响水合物结晶成核生长的热流、盐度、时间等.其中可视水合物主要受构造因素控制,多为活跃流体携带气体以渗漏形式局部聚集形成;而分散状水合物则多受沉积因素影响,气体以扩散形式运移至适宜温压域的沉积层内形成水合物.上新世以来的地质事件是水合物形成分解的诱发因素,冰期、间冰期变化与水合物形成分解有一定耦合关系.水合物形成时代应早于上覆的自生碳酸盐岩,晚于其赋存的母岩沉积层,研究区上部水合物矿层的上段形成于晚更新世,下段形成于中更新中期晚更新世,下部水合物矿层形成中更新世早期中更新世中期;水合物矿藏共经历了3次水合物藏发育期和2次水合物藏破坏期.伴随水合物演化,发育两期自生碳酸盐岩,海底自生碳酸盐岩形成于更新世末期,上部水合物矿层之下段的顶部碳酸盐岩形成于中更新世早期.多层水合物构成的复式水合物矿藏系统,是扩散、渗漏驱动的二元成藏动力学过程综合作用的结果.
【总页数】9页(P15-23)
【作者】张光学;陈芳;沙志彬;梁金强;苏新;陆红锋
【作者单位】广州海洋地质调查局国土资源部海底矿产资源重点实验室,广东广州510075;广州海洋地质调查局中国地质调查局海洋石油天然气地质研究中心,广东广州510075;广州海洋地质调查局国土资源部海底矿产资源重点实验室,广东广州510075;广州海洋地质调查局中国地质调查局海洋石油天然气地质研究中心,广东广
州510075;广州海洋地质调查局国土资源部海底矿产资源重点实验室,广东广州510075;广州海洋地质调查局中国地质调查局海洋石油天然气地质研究中心,广东广州510075;广州海洋地质调查局国土资源部海底矿产资源重点实验室,广东广州510075;广州海洋地质调查局中国地质调查局海洋石油天然气地质研究中心,广东广州510075;中国地质大学(北京)海洋学院,北京100083;广州海洋地质调查局国土资源部海底矿产资源重点实验室,广东广州510075
【正文语种】中文
【中图分类】P618.13
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