Studies on Gas Origin and Gas Source Correlation Using Stable Carbon Isotopes — A Case Study of the Giant Gas Fields in the Sichuan Basin,China

Sichuan basin is one of the most gas rich basins in China. After more than 50 years' industrial exploration, about 15 giant gas fields with gas reserves over 300×10⁸m³ have been found in this basin. A number of contributions have demonstrated that stable carbon isotopic composition and molecular compositions have great significance on the determination of gas origin and gas-source correlation of thermogenic gases. We compiled 228 compositional data of gas samples from the 15 giant gas fields in the Sichuan basin to investigate the gas origin and gas-source correlation by the molecular and stable carbon isotopic compositions. Our results demonstrate that natural gases in the Bajiaochang, Qiongxi, Luodai, Hechuan, Guang'an and Xinchang gas fields have positive carbon isotopic distribution pattern between C₁-C₄ alkanes (δ¹³C₁<δ¹³C₂<δ¹³C₃<δ¹³C₄) and are of coal-derived gases sourced from the Upper Triassic Xujiahe Formation, while natural gases from the Feixianguan Formation in the Puguang, the Jialingjiang Formation in the Wolonghe, Dukouhe, Luojiazhai and Tieshanpo gas fields were oil-associated gases mainly sourced from the Permian carbonates and shales. Natural gases in the Moxi gas field can be readily categorized into two types: coal-derived gases sourced from the Upper Triassic Xujiahe Formation and oil-associated gases mainly sourced from the Permian carbonates and shales. Gases from the Wubaiti and Shapingchang gas fields have isotopic reversal among C₁-C₃ alkanes, which is very similar to the gases from the Permian and Carboniferous strata in the Wolonghe gas field. They were oil-associated gases mainly sourced from the Silurian shales and mudstones. Natural gases from the Weiyuan gas field were oil-associated gases sourced from the Cambrian Qiongzhusi Formation.