Origin and migration of natural gases in central area of Western Sichuan Depression

The chemical and isotopic compositions of nine natural gas samples from Xiaoquan, Xinchang and Hexinchang fields in the central part of Western Sichuan Depression were measured. All of the samples consist mainly of CH, and the contents of hydrocarbons are > 99 % in volume with the gas wetness > 0.5 % . The methane 8" C, and ethane S^C; values of these gases are > - 37%c and > - 27%e respectively. The results indicate that hydrocarbons were formed via thermogenesis. The hydrocarbons in Xiaoquan field were generated from source rocks at early mature and mature stages, while the hydrocarbons in Xinchang and Hexinchang fields came from source rocks at mature and late mature stages respectively. In addition, the isotope ratios and the chemical compositions of the gases from Xinchang field varied with depth, indicating that the hydrocarbons were derived from the deeply buried and the mature source rocks of Upper Triassic age and had experienced a long distance migration.