TSR origin of sulfur in Permian and Triassic reservoir bitumen, East Sichuan Basin, China

Abstract

Solid bitumen, H 2S, pyrite and anhydrite samples from the Lower Triassic Feixianguan Formation and Upper Permian Changxing Formation, and source rock kerogen samples from the Cambrian, Ordovician, Silurian and Permian in the East Sichuan Basin were analyzed for their sulfur content and δ 34S values, to determine if the measurements were consistent with a thermochemical sulfate reduction (TSR) origin of the produced H 2S and solid bitumen. The results show that the solid bitumen samples have sulfur contents from 2.6 to 12.5 wt.% and δ 34S values from +13.4‰ to +22.1‰. The δ 34S values are significantly heavier than those of potential Upper Permian source rock kerogens that average +1.3‰ ( n = 3), suggesting that most of the sulfur in the solid bitumen is derived from isotopically heavy H 2S of TSR origin (from +12.0‰ to +13.7‰). Interestingly, two gases produced from gas to water transition zones during drill stem testing (DST) show significantly lighter δ 34S values (+3.3‰ and +4.1‰) than the H 2S from gas-producing intervals. The isotopically light H 2S may be genetically linked to fracture-filling isotopically heavy anhydrite with δ 34S values from +38.5‰ to +39.6‰, which are significantly heavier than those of coeval seawater sulfate (+11.8‰ to +21.7‰) as measured for bedded anhydrite. The link may have resulted from reduction of part of the dissolved sulfate by hydrocarbons under sulfate-limited conditions. Source rock kerogens show a secular change in δ 34S values relative to source rock age. A dramatic decrease and then increase in δ 34S kerogen was found toward the Permian/Triassic boundary for the first time in the East Sichuan Basin.