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

Solid bitumen, H2S, 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 delta S-34 values, to determine if the measurements were consistent with a thermochemical sulfate reduction (TSR) origin of the produced H2S and solid bitumen. The results show that the solid bitumen samples have sulfur contents from 2 6 to 12.5 wt.% and delta S-34 values from +13.4 parts per thousand to +22.1 parts per thousand The delta S-34 values are significantly heavier than those of potential Upper Permian source rock kerogens that average +1.3 parts per thousand (n = 3). suggesting that most of the sulfur in the solid bitumen is derived from isotopically heavy H2S of TSR origin (from +12.0 parts per thousand to +13.7 parts per thousand). Interestingly, two gases produced from gas to water transition zones during drill stem testing (DST) show significantly lighter delta S-34 values (+3 3 parts per thousand and +4 1 parts per thousand) than the H2S from gas-producing intervals. The isotopically light H2S may be genetically linked to fracture-filling isotopically heavy anhydrite with delta S-34 values from +38.5 parts per thousand to +39.6 parts per thousand which are significantly heavier than those of coeval seawater sulfate (+11 8 parts per thousand to +21 7 parts per thousand) 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 delta S-34 values relative to source rock age A dramatic decrease and then increase in delta S-34(kerogen) was found toward the Permian/Triassic boundary for the first time in the East Sichuan Basin. (C) 2010 Elsevier Ltd. All rights reserved.