Carbon isotopic composition of water-soluble gases and their geological significance in the Sichuan Basin

Twenty-two gas and twenty-two water samples were collected from producing wells in the Triassic Xujiahe, Leikoupo and Jialinjiang formations and the Permian Maokou Formation in the Mid, South and Northwest Sichuan gas areas. The difference in the carbon isotopic composition was compared between the free gas and water-soluble gas. The gas concentration and carbon isotopic composition of the free gas showed normal characteristics of organic gas, while hydrocarbon gases exsolved from gas field water had a greater C-13 component, indicating that gas field water plays an important role in the fractionation of carbon isotopes of natural gas. During gas accumulation, if sufficient gas was released from formation waters and mixed into the free gas, the carbon isotopic composition of the free gas may become enriched in C-13. So, the carbon isotopic composition of water-soluble gases provides a new perspective for the study of gas origins and gas accumulation mechanisms. Mixing of gases exsolved from formation waters may be a factor causing carbon isotopic anomalies for natural gases from the Kuqa Depression, Tarim Basin; Jiulongshan region and Weiyuan gas field, Sichuan Basin; and volcanic gas reservoirs of the Songliao Basin.