南海北部水合物中天然气成因及形成条件

Genesis and Generation Condition of Natural Gas in Gas Hydrate in the Northern South China Sea

南海北部神狐海域钻探获得的水合物中天然气组分以甲烷为主，为典型干气，气体甲烷碳氢同位素组成揭示天然气为典型的生物成因，为二氧化碳还原形成。南海北部地区在硫酸盐一甲烷还原界面（SMI）以下进入生物甲烷生成阶段，盐度适中，适宜产甲烷菌等菌群的生存和生物甲烷气的生成，埋深200—1500m层段是生物甲烷的主要生成阶段。中新世中晚期、上新世和第四纪沉积物以泥为主，部分层段为砂泥岩互层，有机质丰度较高，类型好，热演化程度低，生物气生成条件优越，可为浅部天然气水合物的形成提供充足的气源。

The composition of the natural gas in hydrates in the drilling holes of the sediments in the Shenhu sea area is mainly methane,which is the typical dry gas.According to the values of methane δ13 C and δD,the gas hydrates of Shenghu sea area are biogenic gas,originating from carbon dioxide reduction.The northern South China Sea entered the biogenic gas generation stage under the Sulfate Methane Interface（SMI）.The tem-perate salinity condition is favourable for methanogens survivance and biogenic methane generation.The burial depth from 200 to 1 ,500 m is suitable for the biogenic methane generation.The main marine sediments of Mid-dle-Late Miocene,Pliocene and Quaternary are mainly mudstone,partially interbed with sand and shale,which contain abundant,good-typed and immature organic matter.The condition of biogenic gas generation is superi-or,which can provide sufficient source for the generation of gas hydrate in shallow part of the northern South China Sea.