Three Episodes of Hydrocarbon Generation and Accumulation of Marine Carbonate Strata in Eastern Sichuan Basin, China

Abstract: It is concluded that there are three hydrocarbon generation and accumulation processes in northeastern Sichuan on the basis of the characteristics of solid bitumen, gas‐light oils‐heavy oils, homogenization temperature of fluid inclusions and diagenesis for beach‐ and reef‐facies dolomite gas‐bearing reservoirs in the Puguang Gas Field, northeastern Sichuan Basin, southern China. The first hydrocarbon generation and accumulation episode occurred in the Indosinian movement (late Middle Triassic). The sapropelic source rocks of the O₃w (Upper Ordovician Wufeng Formation)‐S₁l (Lower Silurian Longmaxi Formation) were buried at depths of 2500 m to 3000 m with the paleogeothermal temperature ranging from 70°C to 95°C, which yielded heavy oil with lower maturity. At the same time, intercrystalline pores, framework pores and corrosion caused by organic acid were formed within the organic reef facies of P₂ch (Upper Permian Changxing Formation). And the first stage of hydrocarbon reservoir occurred, the level of surface porosity of residual solid bitumen {solid bitumen/ (solid bitumen + residual porosity)} was higher than 60%. The second episode occurred during the Middle Yanshanian movement (late Middle Jurassic). During that period, the mixed organic source rocks were deposited in an intra‐platform sag during the Permian and sapropelic source rocks of O₃w‐S₁l experienced a peak stage of crude oil or light oil and gas generation because they were buried at depths of 3500 m to 6800 m with paleogeothermal temperatures of 96–168°C. At that time, the level of surface porosity of residual solid bitumen of the T₁f shoal facies reservoirs was between 25% and 35%, and the homogenization temperatures of the first and second stages of fluid inclusions varied from 100°C to 150°C. The third episode occurred during the Late Yanshanian (Late Cretaceous) to the Himalayan movement. The hydrocarbon reservoirs formed during the T₁f and P₂ch had the deepest burial of 7700 m to 8700 m and paleogeotemperatures of 177°C to 220°C. They could be cracked into dry gas (methane), and the same with the source rocks of the Permian and O₃w‐S₁l because they all reached the pyrolysis stage under such conditions. Consequently, the present natural gas (methane) reservoirs were developed.