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

• A gas exsolved from H2S-saturated water is calculated to have much higher H2S.
• The model δ13C1, t = −16.6lnx − 33.0 is developed for methane oxidization by TSR.
• The ethane may have mixed with gas not altered by TSR.

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 produced 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 δ13C1 and [1 − H2S/(∑C1–6+H2S)] for all the gases with an equation of δ13C1, t = −16.6lnx − 33.0, indicating a closed system Rayleigh distillation with a kinetic fractionation factor αC of 1.0166. Ethane shows similar δ13C2 shift to methane (6.1‰ vs 5.5‰) for H2S/[H2S + ∑C1–6] = 0.2 in the NE Sichuan Basin. The δ13C2 deviation is significantly less than that of Mobile Bay Jurassic Norphlet Fm TSR-altered ethane (+16‰) for H2S/[H2S + ∑C1–6] = 0.1 (Mankiewicz et al., 2009). The ethane in association with high H2S (>10%) in NE Sichuan Basin shows slightly lighter δ13C 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.

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