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The Sichuan Basin in China is a sour petroleum province. In order to assess the origin of H₂S and other sulphur compounds as well as the cause of petroleum alteration, data on H₂S, thiophene and thiol concentrations and gas stable isotopes (δ³⁴S and δ¹³C) have been collected for predominantly gas phase petroleum samples from Jurassic, Triassic, Permian and Upper Proterozoic (Sinian) reservoirs. The highest H₂S concentrations (up to 32%) are found in Lower Triassic, anhydrite-rich carbonate reservoirs in the Wolonghe Field where the temperature has reached >130 °C. δ³⁴S values of the H₂S in the Wolonghe Triassic reservoirs range from +22 to +31‰ and are close to those of Triassic evaporitic sulphate from South China. All the evidence suggests that the H₂S was generated by thermochemical sulphate reduction (TSR) locally within Triassic reservoirs. In the Triassic Wolonghe Field, both methane …