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Pr and Ph are preferentially oxidized compared to n C 17 and n C 18 , leading to decrease in Pr/ n C 17 and Ph/ n C 18 ratios with thiaadamantanes concentrations, a measure of the extent of TSR in Tazhong oils and condensates. • TSR does not preferentially deplete triterpanes and α,β hopanes over Ts. • Pr and Ph are preferentially oxidized over n C 17 and n C 18. • i C 4 was preferentially depleted over n C 4 only during earlier TSR stage. • Relative oxidization sequence during TSR overlaps significantly. Tazhong and four other well-known TSR-altered petroleum reservoirs from around the world were analyzed for their molecular and carbon isotopic compositions of saturated hydrocarbons, thiaadamantanes concentrations or H 2 S contents to determine the relative stability of saturated hydrocarbons during thermochemical sulfate reduction (TSR). We find that TSR does not preferentially deplete tricyclic terpanes and 17α, 21β hopanes with the "exposed" tertiary carbon atoms compared to 18α (H)-22, 29, 30-trisnorhopane (Ts) and C 29 Ts that have no tertiary carbon atoms, as proposed by Xia and Gong [35]. However, pristane (Pr) and phytane (Ph) seem to have been preferentially oxidized over normal C 17 and C 18 alkanes, respectively, resulting in decreasing Pr/ n C 17 and Ph/ n C 18 ratios with increasing extent of TSR for the Tazhong and Nisku oils and condensates. i C 4 appears to be preferentially depleted relative to n C 4 during early TSR stage. At later TSR stage both n C 3 and n C 4 are oxidized more readily than i C 4 in the Mobile Bay Norphlet Formation condensates. Thus, we tentatively propose that relative oxidization order during TSR may start with long chain iso -alkanes followed by n -alkanes, then cycloalkanes, C 2 -C 4 alkanes and finally methane; however, it seems that oxidization of these compounds overlaps significantly. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]