Thermochemical sulphate reduction and the generation of hydrogen sulphide and thiols (mercaptans) in Triassic carbonate reservoirs from the Sichuan Basin, China

Abstract

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 and ethane seem to be involved in thermochemical sulphate reduction since their δ13C values become less negative as TSR proceeds. Thiol concentrations correlate positively with H2S in the Triassic Wolonghe gas field, suggesting that thiol production is associated with TSR. In contrast, elevated thiophene concentrations are only found in Jurassic reservoirs in association with liquid phase petroleum generated from sulphur-poor source rocks. This may suggest that thiophene compounds have not come from a source rock or cracked petroleum. Rather they may have been generated by reaction between localized concentrations of H2S and liquid range petroleum compounds in the reservoir. However, in the basin, thiophene concentrations decrease with increasing vitrinite reflectance suggesting that source maturity (rather than source type) may also be a major control on thiophene concentration.