南海北部陆坡天然气水合物及其赋存沉积物特征

Characterization of natural gas hydrate and its deposits recovered from the northern slope of the South China Sea

南海北部陆坡的天然气水合物样品各具特色,神狐海域天然气水合物样品肉眼不可见,是典型的分散型水合物;珠江口盆地东部海域天然气水合物样品具有块状、脉状、结核状及分散状等多种赋存形式。在实验室内,采用现代分析仪器对这些水合物及其赋存的沉积物样品进行了系统的分析测试,研究了南海北部陆坡神狐、珠江口盆地东部海域天然气水合物的微观结构、水合指数、气体组成等基本特征,探讨了沉积物对天然气水合物赋存形式及微观分布的影响。结果表明,神狐海域沉积物中富含钙质微化石和有孔虫有利于水合物生成,且水合物主要分布在其腔体内;珠江口盆地沉积物颗粒更细,且不含微化石与有孔虫,故难以生成分散状水合物。研究区天然气水合物是典型的Ⅰ型结构,主要组成气体是甲烷,占99.4%以上;神狐海域与珠江口盆地水合物中甲烷分子在大笼的占有率99.3%以上,在小笼中分别为85.7%和91.4%,相应的水合指数分别为5.99和5.90。碳、氢同位素的综合研究表明,研究区天然气水合物的甲烷主要来源于微生物作用下的CO_2还原。

The natural gas hydrate samples recovered from the northern margin of the South China Sea show different characteristics.For instance,hydrate samples from Shenhu area are typical dispersed hydrate invisible by naked eyes. However,hydrate samples from the Pearl River Mouth basin show various occurrences,such as massive,vein,nodule and dispersed.In laboratory,these samples were analyzed systematically by modern analytical instruments.Studies are focused on the structure,hydration number,and gas composition of the gas hydrate samples.The characteristics of the sediments and their effects on the occurrence and distribution of gas hydrate has been investigated.The results show that the sediments from Shenhu area is rich in calcareous microfossils and foraminifera which are beneficial for gas hydrate formation,with gas hydrates occupying the cavities.The sediments without calcareous microfossils and foraminifera from the Pearl River Mouth basin are very fine where dispersed gas hydrate is difficult to form.Gas hydrates recovered from the two areas in the South China Sea demonstrate a typical structure I（sI）hydrate,with more than 99.4% methane as the guest gas molecules.In this paper,the occupancy rates of methane for gas hydrate samples from Shenhu area and the Pearl Mouth basin are more than 99.3%in large cage,whereas 85.7% and91.4% in small cage,respectively,corresponding to hydration numbers of 5.99 and 5.90,respectively.Comprehensive study of carbon and hydrogen isotopes suggests that the methane gases are from the biogenic origin of CO2 reduction.