Abstract:

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 δ^13C 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 temperate 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 Middle-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 superior，which can provide sufficient source for the generation of gas hydrate in shallow part of the northern South China Sea.

Key words: hydrate, biogenic gas, generation condition, northern South China Sea