Origin and migration of natural gases in central area of Western Sichuan Depression - Evidence for carbon isotope geochemistry

The chemical and isotopic compositions of nine natural gas samples from Xia oquan, Xinchang and Hexinchang fields in the central part of Western Sichua n Depression were measured. All of the samples consist mainly of CH4 and th e contents of hydrocarbons are > 99% in volume with the gas wetness > 0.5%. The methane delta(13)C(1) and ethane delta(13)C(2) values of these gases a re > - 37 parts per thousand and > - 27 parts per thousand respectively. Th e results indicate that hydrocarbons were formed via thermogenesis. The hyd rocarbons in Xiaoquan field were generated from source rocks at early matur e and mature stages, while the hydrocarbons in Xinchang and Hexinchang fiel ds came from source rocks at mature and late mature stages respectively. In addition, the isotope ratios and the chemical compositions of the gases fr om Xinchang field varied with depth, indicating that the hydrocarbons were derived from the deeply buried and the mature source rocks of Upper Triassi c age and had experienced a long distance migration.