Geochemistry and origin of continental natural gas in the western Sichuan basin, China

The molecular compositions and stable carbon and hydrogen isotopes of natural gases of Jurassic and Upper Triassic strata in western Sichuan Basin were investigated to assess the origin, maturity, sources. Then the results were compared with those of central Sichuan Basin. Results show that the natural gas in Western Sichuan basin is wet gas and the dry coefficient is higher than that of central Sichuan basin. The stable carbon and hydrogen isotopes represent a positive carbon isotopic series of alkanes (i.e. δ13C1<δ13C2<δ13C3). Continental natural gases in Sichuan Basin mainly belong to coal-derived gases, and only the gases from Jurassic strata in central Sichuan basin are oil-derived gases. Carbon dioxide is mainly abiogenic, with some biogenic. The abiogenic carbon dioxide is a result of the thermal metamorphism of carbonates. The natural gas in the Sichuan Basin mainly come from mature and high mature stage, while only the gases of Jurassic strata in central Sichuan Basin were generated in the mature stage. The Jurassic natural gas in western Sichuan basin possibly originated from a mixture of T3X3 and T3X5 black mudstones with low thermal maturity. However, the oil-derived gases of Jurassic strata in central Sichuan basin were found to come from Ziliujing-baitianba source rock. The deep Triassic natural gases in Western and Central Sichuan basin originated from the nearby or in situ source rocks of Xujiahe formation.