南海北部天然气水合物钻探区烃类气体成因类型研究

南海北部天然气水合物钻探区水合物气、顶空气样品和沉积物样品烃类气体组分和甲烷同位素特征测试结果表明,气体样品中烃类气体以甲烷为主,含微量乙烷和丙烷,C1/(C2+C3)值均大于或接近1 000。甲烷的碳同位素值为-54.1‰~-62.2‰,氢同位素值为-180‰~-255‰,属于微生物气或是以微生物气为主的混合气,甲烷由CO2还原生成,由原地提供或侧向运移而来。沉积物样品酸解烃分析显示多数样品甲烷丰度大于90%,含一定量的乙烷、丙烷及少量的丁烷,C1/(C2+C3)值均小于50。甲烷的碳同位素值为-29.8‰~-48.2‰,呈现典型的热解气特征,显示由深部运移而来。

Study on Genetic Types of Hydrocarbon Gases from the Gas Hydrate Drilling Area, the Northern South China Sea

Gas composition and isotopes of gas hydrate,headspace gas and sediment samples from the gas hydrate drilling area,the Northern South China Sea,are analysed.Results show that gas hydrate and headspace gas are nearly composed of methane with trace ethane and propane.Their ratios of C1/(C2+C3) are higher than or close to 1,000.δ13C1 values range from-54.1‰ to-62.2‰,and δDCH4 values vary from-180‰ to-255‰.Hence they are mainly bacterial or mixed with bacterial gases,derived from CO2 reduction and produced in situ or by side migration.Results of acid-degassed gases of sediment samples also indicate that methane is mostly higher than 90%,with some ethane,propane and minor butane.The ratios of C1/(C2+C3)are lower than 50 and δ13C1 values change from-29.8‰ to-48.2‰.These features are indicative of typical thermogenic gases migrated up from the depth.