Fluid evolution and hydrocarbon accumulation model of ultra-deep gas reservoirs in Permian Qixia Formation of northwest Sichuan Basin, SW China

The fluid evolution and reservoir formation model of the ultra-deep gas reservoirs in the Permian Qixia Formation of the northwestern Sichuan Basin are investigated by using thin section, cathodoluminescence, inclusion temperature and U-Pb isotopic dating, combined with gas source identification plates and reservoir formation evolution profiles established based on burial history, thermal history, reservoir formation history and diagenetic evolution sequence. The fluid evolution of the marine ultra-deep gas reservoirs in the Qixia Formation has undergone two stages of dolomitization and one phase of hydrothermal action, two stages of oil and gas charging and two stages of associated burial dissolution. The diagenetic fluids include ancient seawater, atmospheric freshwater, deep hydrothermal fluid and hydrocarbon fluids. The two stages of hydrocarbon charging happened in the Late Triassic and Late Jurassic–Early Cretaceous respectively, and the Middle to Late Cretaceous is the period when the crude oil cracked massively into gas. The gas reservoirs in deep marine Permian strata of northwest Sichuan feature multiple source rocks, composite transportation, differential accumulation and late finalization. The natural gas in the Permian is mainly cracked gas from Permian marine mixed hydrocarbon source rocks, with cracked gas from crude oil in the deeper Sinian strata in local parts. The scale development of paleo-hydrocarbon reservoirs and the stable and good preservation conditions are the keys to the forming large-scale gas reservoirs.