南海东北部天然气水合物成藏演化地质过程

发布时间：2018-08-11 21:45

【摘要】：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90m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。
[Abstract]:There are many types of gas hydrate in the northeast of the South China Sea, including block, nodular, thin-layer, vein-like and dispersive, which occur alone or coformed with each other and accompanied by carbonate rocks. And distributed in the water depth of 600-900 m, sea bottom and below 200 m strata. The first four hydrates are usually formed in shallow fracture, weak zone between sediment layers, seabed surface and other local areas, the diameter of which is larger than that of normal sediment pores, hydrate can be seen with the naked eye, so it is also called visible hydrate. The dispersed hydrate is filled in the micrometer-nanometer pore sediment, which is difficult to discern by the naked eye, so this kind of hydrate is an invisible hydrate. According to the drilling results, the formation at 90 m below the sea floor is taken as the boundary, and the next two sets of hydrate beds are developed in the study area. The upper part of the hydrate deposit is dominated by the visible hydrate, and the lower part of the hydrate layer is the non-visible hydrate. At the top of each hydrate layer, the authigenic carbonate rocks are developed in the upper part of the hydrate layer, which is reflected in the paleo-cold spring associated with the evolution of hydrate reservoir in the geological history period. The comprehensive analysis shows that the occurrence form of gas hydrate and the evolution of gas hydrate reservoir are controlled by many factors, including lithology, granularity, lithofacies (paleochannel, fan body), neotectonic activity and fault action, fluid and gas migration, etc. And the heat flux, salinity, time and so on, which affect the growth of hydrate crystal nucleation. Among them, the visible hydrate is mainly controlled by tectonic factors, and most of them are formed by local accumulation of active fluid carrying gas in the form of leakage, while dispersed hydrates are mostly influenced by sedimentary factors. The gas diffuses into the sedimentary layer in the appropriate temperature and pressure domain to form hydrate. The geological events since Pliocene are the inducing factors of hydrate formation and decomposition. The changes of glacial and interglacial periods have some coupling relationship with hydrate formation and decomposition. The hydrate formation age should be earlier than that of the overlying authigenic carbonate rock, and later than that of the parent rock deposit. The upper part of the study area was formed in the late Pleistocene, and the lower part was formed during the middle and middle Pleistocene to the late Pleistocene. The formation of the lower hydrates is from the early Middle Pleistocene to the middle of the Middle Pleistocene, and the hydrate deposits have gone through three periods of hydrate reservoir development and two stages of destruction of hydrate deposits. Along with hydrate evolution, two stages of authigenic carbonate rock were developed, the submarine authigenic carbonate rock was formed at the end of Pleistocene, and the top carbonate rock of the lower part of the upper hydrate formation formed in the early Middle Pleistocene. The compound hydrate deposit system composed of multilayer hydrates is the result of the comprehensive action of the diffusion and leakage driven binary reservoir forming process.
【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;中国地质大学(北京)海洋学院;
【基金】：国家自然科学基金面上项目(41372012) 中国地质调查局海域天然气水合物勘查项目
【分类号】：P618.13

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