南海北部陆坡水合物初步研究:ODP钻孔证据

Preliminary Study of Gas hydrate in the Northern Slope of the South China Sea: Evidences from ODP

海洋水合物以其潜在的巨大资源价值和环境效应近年来引起地学界的极大关注,南海北部陆坡是我国重点研究的海域之一。ODP184航次在南海北部实施的钻孔为该区水合物研究提供了宝贵的第一手资料。通过对钻孔资料的研究表明,东沙东侧沉积速率比西侧高得多,对水合物的形成更有利。1144和1146站位甲烷浓度很高,其中1144站位最高达64982×10^(-6),以生物气为主,1146站位最高达85205×10^(-6),以热解气为主；1148站位甲烷浓度较低,以热解气为主；而1145站位甲烷浓度极低,因此从甲烷浓度来看1144、1146站位附近最有利于水合物的形成。孔隙水成分来看,1144站位没有发现氯浓度、盐度等指标明显变化,表明没有水合物存在；1146站位476m以下氯浓度和盐度明显下降,并与烃类气体含量明显增大相对应。可能与水合物有关。从烃类气体浓度变化和孔隙水成分变化来看,1148站位458～492m的滑塌层可能是气体和其它一些元素运移的盖层。

Recently, Gas hydrate is drawing increasing attention of geologists at home and abroad, as its high energy potential resource and environmental impact on global change. The northern slope of the South China Sea is one of target areas for gas hydrate study in China. The ODP Leg 184 located in the area provides useful first-hand information for gas hydrate research. The study of drilling data reveals that the sedimentation rate in the sea area adjacent to eastern Dongsha Islands is much higher than the sea area located at west side of the Dongsha Islands and the sea area at the eastern Dongsha has the better environment for gas hydrate formation. Both Sites 1144 and 1146 have a high record of the methane concentrations, in which, the Site 1144's was found to be dominated by biogenetic gas and it reaches 64982ppmv in maximum; a maximum of 85205ppmv was recorded at Site 1146, which is dominated by thermal decomposition gaseogenetic gas. The methane concentration in site 1148 is lower and it shows domination of thermal decomposition gas, and at the Site 1145, the methane concentration is extremely low. According to the analysis of methane concentration, it infers that the vicinities of Site 1144 and Site 1146 have the conditions favorable for formation of gas hydrate. The compositions of porous water in Site 1144, such as chloride (Cl^-) and salinity, do not have an obvious variation; it is an indicator of no gas hydrate. But the phenomena decreasing abruptly in chloride and salinity below 476m of Site 1146, which coincided with an increase of hydrocarbon gases, indicates possible relation with gas hydrate. The slumped layer between 458 to 492m in Site 1148 is inferred to be a cover of migration of the hydrocarbon gases and other elements based on the analysis of concentration of methane gases in core and the elements in porous water.