H₂S-generation by methane-dominated TSR and carbon isotope fractionation in Lower Triassic Feixianguan Formation, Northeast Sichuan Basin

Melhane-dominaled TSR has been proposed lo occur in Lower Triassic Feixianguan Formation oolilic dolomite reservoirs on ihe northeast side lo the Kaijiang-Liangping basin (Cai et al., 2004), but not in oolilic limestone on the southwest side where there is no significant H₂S and low porosity. TSR-calcile is shown lo have δ¹³C values mainly from - 10‰ to - 29‰ and coexists with solid bitumen, elemental sulfur, pyrile and quartz, and has replaced dolomite, anhydrite and celeslile. However, present CO₂ gas has δ¹³C values mainly from -2‰ to +2‰, suggestive of dissolution of carbonates (likely dolomite) after TSR-calcile precipitation, Malural gases from Changxing Fm, Feixianguan Fm. and Jialingjiang Fm. from the Fast lo Puguang gasfield show a logarithmic relationship between methane δ¹³C₁ and residual hydrocarbons (HCs). Based on this relationship, it can be calculated that about 15% methane of well Du4 gas from the Feixianguan Fm. may have been oxidized as the result of TSR.