Thermochemical sulfate reduction is considered to result in H 2 S >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 H 2 S higher than 30%, and are calculated to have significantly higher H 2 S and CO 2 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 C 1 and [1 − H 2 S/(∑C 1–6 +H 2 S)] for all the gases with an equation of δ 13 C 1, t = −16.6lnx − 33.0, indicating a closed system Rayleigh distillation with a kinetic fractionation factor α C of 1.0166. Ethane shows similar δ 13 C 2 shift to methane (6.1‰ vs 5.5‰) for H 2 S/[H 2 S + ∑C 1–6 ] = 0.2 in the NE Sichuan Basin. The δ 13 C 2 deviation is significantly less than that of Mobile Bay Jurassic Norphlet Fm TSR-altered ethane (+16‰) for H 2 S/[H 2 S + ∑C 1–6 ] = 0.1 (Mankiewicz et al., 2009). The ethane in association with high H 2 S (>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 H 2 S pools.