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

The Yuanba gas field in the Permian Changxing Formation (P(2)c), which exhibits wide variations in its hydrogen sulfide (H2S) concentration (1.20-12.16%), is a typical sour gas field in the northern Sichuan Basin. The sulfur-rich reservoir's solid bitumen (atomic SAC ratios are 0.032-0.142), and late calcite cement delta C-13 values, which are smaller than the delta C-13 values of the host dolostone, indicate that the H2S 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 delta C-13 values increase as the GSI increases, although no obvious increase was observed in the methane delta C-13 values. The calcite cements' delta C-13 values (-1536 to +4.56 parts per thousand) 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 (SO42-) were mainly enriched during dolomitization. Insufficient dissolved Sat most likely caused the lower H2S 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 Sat- in residual water in paleo-oil zones, SOT from bottom water may also be involved in TSR; therefore, oil reservoirs with bottom water have more SO42- and can produce more H2S than pure oil reservoirs. This phenomenon may be the main cause of the great difference in the H2S concentrations between reservoirs, while gravitational differentiation during late uplift most likely creates differences in H2S concentrations in a single reservoir. Carbon dioxide (CO2), which has a relatively heavy delta C-13 value (-3.9 to-0.3 parts per thousand), may be the combined result of TSR, the balance between CO2 and inorganic fluid systems, and carbonate decomposition. (C) 2016 Elsevier Ltd. All rights reserved.