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

Abstract

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 (IMCC6) and cyclohexane index (ICH) 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. δ13C1<δ13C2<δ13C3<δ13C4), indicating that the gases are primary and have not experienced secondary alteration; a partial reversal of the carbon isotope trend of a few samples is explained by the mixing of gases from the same Xujiahe Fm source rock at different maturity levels. (d) The carbon isotope trends of the C1–C4 alkanes are very similar to those of the gases produced in the western and central Sichuan Basin, and the light hydrocarbon composition is similar to that in the central Sichuan Basin; both findings support that the source rocks of the gases in the Xujiahe Fm in the southern Sichuan Basin are coal measures, as in the western and central Sichuan Basin. (e) The hydrogen isotope values of the alkane gases are higher than those in the western and central Sichuan Basin, indicating a higher salinity of the depositional environment of the Xujiahe Fm in the southern Sichuan Basin. (f) A small part of the natural gas in the Xujiahe Fm in the southern Sichuan Basin is oil-associated gas, which was probably derived from sapropelic source rocks of the Lower Silurian and the Lower Permian.