Mechanism of ultra-deep gas accumulation at thrust fronts in the Longmenshan Mountains, lower Permian Sichuan Basin, China

Giant gas has been discovered in the lower Permian Qixia Formation of the Shuangyushi area in the Long-menshan fold-and-thrust front within dolomite reservoir at depth >6000 m. To improve the understanding of this unique ultra-deep reservoir, we address the mechanism of hydrocarbon accumulation in this area. Gas geochemistry implies that most of the gas is derived from mixing humic and sapropelic kemgens, whereas some gases are derived solely from humic kerogens. The gas reservoirs experienced two main stages of hydrocarbon charging corresponding to oil and gas generation peaks of first the lower Cambrian and second lower-middle Permian source rocks, i.e., Late Triassic and Middle Jurassic, respectively. The weak tectonism and the resistance of the ultra-deep structures to tectonic deformation at thrust fronts provided an ideal preservation environment for hydrocarbons. Our data suggest that deep to ultra-deep oil and gas accumulations in mountain front-foreland basin transition zones is a target of future exploration, providing complement to the traditional focus in shallow formations.