Origin and distribution of hydrogen sulfide in the Yuanba gas field, Sichuan Basin, Southwest China

Abstract The Yuanba gas field in the Permian Changxing Formation (P 2 c), which exhibits wide variations in its hydrogen sulfide (H 2 S) concentration (1.20–12.16%), is a typical sour gas field in the northern Sichuan Basin. The sulfur-rich reservoir's solid bitumen (atomic S/C ratios are 0.032–0.142), and late calcite cement δ 13 C values, which are smaller than the δ 13 C values of the host dolostone, indicate that the H 2 S originated from thermal sulfate reduction (TSR) and oil was involved in TSR. The gas souring index (GSI) of P 2 c's gases is generally lower than 0.1. The ethane δ 13 C values increase as the GSI increases, although no obvious increase was observed in the methane δ 13 C values. The calcite cements' δ 13 C values (−15.36 to +4.56‰) in dolostone are heavier than the typical reported values, which implies that only limited heavy hydrocarbon gases were involved in TSR. No anhydrites developed in P 2 c's reservoirs, and dissolved sulfate anions (SO 4 2− ) were mainly enriched during dolomitization. Insufficient dissolved SO 4 2− most likely caused the lower H 2 S concentrations in the Permian to Triassic reservoirs in the northeastern Sichuan Basin compared to the Permian Khuff Formation in Saudi Arabia and the Jurassic Smackover Formation in Mississippi. Except for the SO 4 2− in residual water in paleo-oil zones, SO 4 2− from bottom water may also be involved in TSR; therefore, oil reservoirs with bottom water have more SO 4 2− and can produce more H 2 S than pure oil reservoirs. This phenomenon may be the main cause of the great difference in the H 2 S concentrations between reservoirs, while gravitational differentiation during late uplift most likely creates differences in H 2 S concentrations in a single reservoir. Carbon dioxide (CO 2 ), which has a relatively heavy δ 13 C value (−3.9 to −0.3‰), may be the combined result of TSR, the balance between CO 2 and inorganic fluid systems, and carbonate decomposition.