Geochemical characteristics of natural gases in the Upper Triassic Xujiahe Formation in the southern Sichuan Basin, SW China

The chemical and stable (carbon and hydrogen) compositions of gases in the Upper Triassic Xujiahe Formation (Xujiahe Fm) in the southern Sichuan Basin were analysed and compared with those of gases of the Xujiahe gas pools in other areas of the Sichuan Basin. The geochemical characteristics of the Xujiahe Fm gases in the southern Sichuan Basin are summarized as follows: (a) Gases are dominated by alkane gases, and the majority of the natural gas is wet gas. (b) The methylcyclohexane index (I-MCC6) and cyclohexane index (I-CH) values of the gases are high, indicating that the natural gases originated from humic organic matter. (c) The stable carbon and hydrogen isotopes of methane and its homologues in most of the gases display a normal trend (i.e. delta C-13(1)