南海北部陆坡大型气田区天然气水合物的成藏地质构造特征

南海北部被动大陆边缘是板块活动相对较弱的地区,业已证实是巨大天然气田形成的有利场所,同时也是水合物发育的有利地区。对被动大陆边缘水合物的富集规律研究表明:热解成因天然气水合物在被动大陆边缘占有重要地位,被动陆缘的活动断裂、底辟构造、滑塌堆积、断裂坡折带和海底扇砂体对天然气水合物的富集过程具有重要的控制作用。从南海北部陆坡的研究实例看,断裂坡折带作为重要的疏导系统,有利于天然气向水合物稳定带运移,同时控制着海底扇和滑塌堆积等水合物有利成矿相带。断裂的幕式活动促进热解气体向上渗漏,并在水合物稳定带中形成水合物。在天然气田与天然气水合物的共存区域,可以形成两种类型的水合物:一种是构造渗漏型,它是以高浓度的深部热解气为主、以浅层生物气为辅的水合物;另一种是地层扩散型,这类水合物以低浓度的浅层生物气为主,以深部热解气为辅。

Geological structures for forming gas hydrate reservoir in the huge deepwater gas field of the northern South China Sea

The plate activity is weaker in the passive continental margin of the northern South China Sea, which is favorable for forming the large scale gas field and gas hydrate reservoirs. The study on gas hydrate enrichment in the passive continental margin showed the importance of thermogenic gas for forming gas hydrate. The active faults, diapire structure, slump deposits, faults break and coarse submarine fan in the slope controlled the concentration of gas hydrate. The study case of the northern South China Sea showed that fault break was an important migration system and favorable for gas to transport into the stable zone of gas hydrate and also controlled the profitable sedimentary facies zones, such as submarine fan and slump deposits. Episodic activities of fauls could prompt thermogenic gas to leak upwards and form gas hydrate in the stable zone of gas hydrate. In the coexisting zone of gas and gas hydrate, there were two models of gas hydrate reservoir. One model was the tectonic discharge dominated by high concentration thermogenic gas with a little shallow biogenetic gas, and another was the strata diffusion mainly containing the low concentration shallow biogenetic gas and a little thermogenic gas.