Thermochemical sulphate reduction (TSR) versus maturation and their effects on hydrogen stable isotopes of very dry alkane gases

Abstract

Here we report the first study of the effect of thermochemical sulphate reduction (TSR) on the hydrogen isotopes of natural gas. Variably sour (H2S-bearing) and very dry (>97% methane) gas samples from Lower Triassic, Permian and Carboniferous marine carbonate reservoirs in the Sichuan Basin, China, have been analysed. All gases seem to have been sourced from mature marine kerogen and contain H2S that resulted from TSR. The Carboniferous samples are largely unaffected by TSR and were used to assess the effects of normal thermal maturation processes on the carbon and hydrogen isotopes of methane and ethane as a function of gas dryness (a proxy for thermal maturity). Maturation led to heavier carbon isotopes of methane and ethane and hydrogen isotopes of ethane; in contrast methane hydrogen isotopes seem to have little systematic variation with increasing maturity. TSR did not have a systematic effect on the hydrogen isotopes of methane, although the spread of values diminished (ending up at a constant −120‰) as TSR proceeded. This was possibly due to the partial thermochemical sulphate reduction of ethane adding isotopically light methane and thus offsetting the Rayleigh fractionation effects of TSR of methane. In contrast, hydrogen isotopes of ethane became much heavier as TSR proceeded, to values greater than those for samples only influenced by maturation. Under some circumstances, the effects of TSR can be identified and discerned from the effects of normal thermal maturation by plotting the difference between the carbon isotope compositions of methane and ethane and the difference between the hydrogen isotope compositions of methane and ethane.