Sulfur isotopic fractionation and mechanism for Thermochemical Sulfate Reduction genetic H2S

Thermochemical Sulfate Reduction (TSR) is the dominant mechanism of H2S genesis in deep carbonate reservoirs and more than 50 large and medium gas fields have been discovered with TSR-genetic H2S around the world. The paper focuses on sulfur isotope geochemistry and fractionation in the process of TSR based on isotopic analyses of sulfide samples acquired from H2S-bearing gas fields in the Sichuan Basin, China and studies on sulfur isotopes in H2S-bearing gas fields around the world. The research findings suggest fractionated sulpur isotopes from H2S and sulfate samples were less than 15 parts per thousand in TSR-genetic natural gases with a high content of H2S, which mainly ranged 2.5 parts per thousand similar to 13.82 parts per thousand with an average 10 parts per thousand. The sulfur isotope in a marine gypsum rock in the Sichuan Basin would cover a wide spectrum with stepped trend and that in H2S shows similar tendency, which indicates sulfur element in H2S samples from each H2S-bearing gas field would come from sulfate in the same measures and TSR process may occur in each reservoir bed. Samples from different gas fields in the Sichuan Basin have similar sulfur isotopic fractionation due to similar TSR temperatures. Sulfur isotopic fractionation is dependent on the intensity of TSR instead of the sulfur isotope in a sulfate sample itself. The sulfur isotope of H2S would be close to that in in-situ sulfate with intensified TSR process.