Origin and carbon isotope fractionation of CO2 in marine sour gas reservoirs in the Eastern Sichuan Basin

According to gas chemical and carbon isotopic compositions for 75 gas samples from the Eastern Sichuan Basin, C1-C3 gaseous hydrocarbons are dominant with high dryness coefficient (C1/C1-3 = 0.99-1.0). Despite the presence of various contents of H2S and CO2, a reasonably good positive correlation is exhibited when both contents are > 5%. The carbon dioxide in the natural gas in the Eastern Sichuan Basin mainly originated from carbonate dissolution by acid gases and thermochemical sulfate reduction (TSR). The content of CO2 produced at the early stage of acid gas dissolution is commonly < 5%, with δ13CCO2 value < −3.0‰. However, when the CO2 content exceeds 5%, the δ13CCO2 values range from −8‰ to +2‰, and the CO2 and H2S contents demonstrated a positive correlation. The majority of CO2 is generated via TSR as well as acid gas dissolution during the late uplift of the basin. Particularly, the CO2 with δ13CCO2 value < −3‰ is primarily the residual CO2 generated via the TSR after it was partially precipitated to form calcite, while the CO2 with δ13CCO2 value > −3‰ is a mixture of the CO2 produced early via TSR and the CO2 produced by the subsequent carbonate rock dissolution by acid gases.