Biomarkers, carbon and mercury isotopes constrain the natural gas sources in the Middle Permian Maokou Formation, northwestern Sichuan Basin, China

The identification of gas source in deep and ultra-deep gas reservoirs formed by multiple sets of potential source rocks has always been a challenging issue, primarily attributed to the parameter failures resulting from the high-overmaturity gas reservoirs. The Sichuan Basin, China, contains abundant natural gas within the Middle Permian Maokou Formation in the northwestern part of the basin. However, the gas source remains controversial due to the presence of three potential source rocks: the Cambrian Qiongzhusi Formation; the Member one of the Maokou Formation; and the Gufeng Member of the Maokou Formation. To identify the gas sources and accumulation processes, the analyses of biomarkers and carbon and mercury (Hg) isotopes were conducted. From west to east, across the Moxi, Bajiaochang, and Nanchong regions of the Sichuan Basin, there is an increasing contribution from Cambrian strata and a decreasing contribution from the Member one of the Maokou Formation. In the Shuangyushi and Jiulongshan areas, natural gas is primarily derived from the Gufeng Member of the Maokou Formation, as indicated by the Hg and organic C isotopic compositions of the gas. In contrast, in the Hewanchang area, natural gas is exclusively generated from Cambrian strata, influenced by local tectonism. In summary, variations in gas sources across the northwestern Sichuan Basin reflect the distribution of source rocks in different strata and heterogeneous tectonism within the Longmenshan Fault Zone.