Carbon isotope fractionation during methane-dominated TSR in East Sichuan Basin gasfields, China: A review

摘要

Thermochemical sulfate reduction is considered to result in H2S >10% and high dryness coefficient values in the Lower Triassic and Upper Permian carbonate reservoirs in the NE Sichuan Basin. The gases pro-1 duced from gas—water transition and water intervals were measured to have H2S higher than 30%, and are calculated to have significantly higher H2S and CO2 contents than a gas produced from a gas interval, ' and thus are not used to reflect TSR extents. Methane and ethane were shown to shift positively in carbon isotopes as a result of TSR. However, the fractionation has not quantitatively described. A logarithmic relationship is found to give the best fit for methane δ~(13)C1 and [1-H2S/(∑C_(1-6)+H2S)] for all the gases with an equation of δ~(13)C1, t =-16.61nx-33.0, indicating a closed system Rayleigh distillation with a kinetic fractionation factor etc of 1.0166. Ethane shows similar δ~(13)C2 shift to methane (6.1 ‰ vs 5.5‰) for H2S/[H2S + ∑C_(1-6)] = 0.2 in the NE Sichuan Basin. The δ~(13)C2 deviation is significantly less than that of Mobile Bay Jurassic Norphlet Fm TSR-altered ethane (+16‰) for H2S/(H2S + 5_∑C(1-6) = 0.1 (Mankiewicz et al, 2009). The ethane in association with high H2S (>10%) in NE Sichuan Basin shows slightly lighter δ~(13)C values than those of the potential source rocks. Thus, it is possible for small amounts of gas derived from cracking of the source rocks to have mixed with TSR-altered gas in the high H2S pools.