南沙海槽潜在天然气水合物的地质环境及其指标特征

Geological settings and indicators of potential gas hydrates in the Nansha Trough area,South China Sea

南沙海槽的构造和沉积受控于南海的构造运动和加里曼丹西北大陆边缘的演化,具有适于天然气水合物形成的物源基础、温压条件、输导系统和储藏场所。似海底反射层（BSR）出现在水深650～2 800 m、海底下65～350 m深的晚中新世沉积物中,与褶皱、逆冲推覆构造及穹窿构造有关;沉积物中的甲烷含量和孔隙水的SO24-含量表现出异常变化特征,硫酸盐-甲烷界面（SMI）深度仅为8～11 m;表层沉积的自生石膏和黄铁矿的成岩环境与甲烷流体排溢引起的厌氧甲烷氧化（AOM）有关,这些地球物理和地球化学指标均指示南沙海槽发育天然气水合物。研究表明,南沙海槽沉积物的甲烷以二氧化碳还原型微生物成因为主,少量为混合气,海槽东南部可能是最有潜力的天然气水合物远景区。

The Nansha Trough （formerly known as Northwest Borneo Trough） lies in the southeast of South China Sea （SCS）. Its structural settings and sedimentary history are controlled by the collision and evolution of the SCS and North West Borneo Margin, hence the Nansha Trough has abundant organic materials, suitable temperature-pressure, conduit systems and geological setting for gas hydrate formation. BSRs （bottom simulating reflectors） occur in the post-Miocene sediments between 65 and 350 m, beneath the seafloor at water depths of 650-2 800 m, are associated with folded, thrust, and ridge structures. The SO4^2- concentrations of pore water strongly decreased within several meters in sedimentary columns, while the methane content increased. The depth of SMI （sulfate methane interface） varies in a range of 8 to 11 m, resulting from methane venting from deep sedimentary columns. The diagenesis environlnent of gypsum and pyrite in surface sediments was associated with AOM （anaerobic oxidation of methane） of methane fluids. All of the above evidence shows that gas hydrates likely occur beneath the seafloor in the Nansha Trough. The results here suggest that CH4 in sediments was probably of microbial origin, produced by CO2 reduction, and partially from mixed gases, and the southeastern Nansha Trough could be a most promising area for gas hydrates.