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

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 delta C-13(1) and [1 - H2S/(Sigma C1-6+H2S)] for all the gases with an equation of delta C-13(1.t) = -16.61nx - 33.0, indicating a closed system Rayleigh distillation with a kinetic fractionation factor alpha c of 1.0166. Ethane shows similar delta C-13(2) shift to methane (6.1 parts per thousand vs 5.5 parts per thousand) for H2S/[H2S + Sigma C1-6] = 0.2 in the NE Sichuan Basin. The delta C-13(2) deviation is significantly less than that of Mobile Bay Jurassic Norphlet Fm TSR-altered ethane (+16 parts per thousand) for H2S/[H2S + Sigma C1-6] = 0.1 (Mankiewicz et al., 2009). The ethane in association with high H2S (> 10%) in NE Sichuan Basin shows slightly lighter delta C-13 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. (c) 2013 Elsevier Ltd. All rights reserved.