Preliminary Discussion on Geochemical Characteristic of Methane in Shenhu Gas Hydrate Field, North Slope of the South China Sea

摘要

Hydrocarbon gas compositions and isotopic compositions ofmethane in samples collected in Shenhu gas hydrate field aremeasured and analysed. Results show that, in gas hydrate andheadspace gas samples, the dominant hydrocarbon gases aremethane with trace ethane and propane, and their volumetricratios C_1/(C_2+C_3) are higher than or close to 1000. δ~(13)C_1 (PDB,‰) are likely to fall within the range of -54.1‰ to -62.2‰ andδDCH4 (VSMOW, ‰) range from -180‰ to -255‰. They arediagnostic of microbial or mixed with microbial origin andmethane is mainly derived by reduction of carbon dioxide,probably produced in situ or by lateral migration. In contrast,analysis of acidolysis hydrocarbon gas in sedimentary samplesindicates that most of them are composed of >90% methane,with minor ethane, propane and few butane. C_1/(C_2+C_3) arelower than 50, and δ~(13)C_1 range from -29.8‰ to -48.2‰. Thesefeatures are indicative of typical thermal gas migrated up fromthe depth.