Source of gas reservoirs in the fourth member of the Middle Triassic Leikoupo Formation in Western Sichuan Depression

A set of thick charcoal grey dolomicrite and micrite source rocks developed in the Middle Triassic Leikoupo Formation in the Western Sichuan Depression, in addition to four sets of marine source rocks in the Permian and its underlying strata. The gas generation intensity of the source rocks is about (1-4)×10⁹ m³/km², showing a good potential for medium or giant gas fields. According to the gas composition, carbon and hydrogen isotopes, reservoir bitumen and migration conditions, the major sources for the gas reservoirs of the fourth member of the Leikoupo Formation vary in different tectonic zones in the Western Sichuan Depression. The gas from well PZ1 in the Longmenshan piedmont tectonic zone displays high H₂S content, high δ¹³C₂ value (-26.4‰), positive alkane carbon isotopic series and light δD₁ value (-140‰), consistent with characteristics of mixed gas. The thermal evolution degree reflected by natural gas compositions and carbon isotopes falls between those of Permian and Leikoupo source rocks, which indicates that the gas was sourced from both of them, and the deep faults in front of the Longmenshan Mountains provide favorable conditions for gas migration and accumulation from the source rocks. Meanwhile, the gases from well CK1 in the Xinchang tectonic zone display trace to moderate H₂S content, low δ¹³C₂ value (-33.2‰), partial reversal of alkane carbon isotopic series and a light δD₁ value (-147‰), consistent with oil-type gas. The thermal evolution degree reflected by natural gas compositions and carbon isotopes is similar to that of Leikoupo source rocks, which indicates that the gases were mainly from the interior source rocks of the Leikoupo Formation.