First Report of Thermochemical Sulfate Reduction Reaction in the Upper Paleozoic Carbonate in South-east of Ordos Basin, China

Thermochemical sulfate reduction (TSR) is the reaction between anhydrite and petroleum fluids at elevated temperatures to produce H2S and calcite. TSR has been studied in a number of sedimentary basins such as Sichuan basin... [ view full abstract ]

Thermochemical sulfate reduction (TSR) is the reaction between anhydrite and petroleum fluids at elevated temperatures to produce H2S and calcite. TSR has been studied in a number of sedimentary basins such as Sichuan basin and Tarim basin in China because it can have a profound impact on the commercial viability of petroleum resources, with H2S typically being undesirable. However, few TSR has been reported in Ordos basin with similar geological background. Analyses have been carried out to the gas, anhydrite, pyrite, calcite and bulk rock composition and isotope ratios together with petrography and fluid inclusion to determine if TSR is developed in Ordos basin, China. The results show that anhydrite is well developed in Majiagou formation of Upper Paleozoic from 2800m, which provides materials foundation for TSR reaction. In the third Member of Majiagou formation of about 2900m, H2S began to be produced with pyrite found in the same section. The replacive calcite commonly has low δ13C values (ranging from -18.4‰ to -10.3‰(PDB)) and reduced fluid inclusion salinity which indicates TSR origin. H2S and CO2 produced from TSR is significant sour gas, which can greatly improve the physical property of reservoir when dissolved in water.