Burial and thermal history simulation of Paleozoic source rocks in the northwest Sichuan Basin: implications for hydrocarbon generation and charging history

The Paleozoic oil and gas resources in Northwest Sichuan Basin are abundant, but the history of hydrocarbon generation and charging of the main source rocks is not clear, which seriously restricts the evaluation of marine oil and gas resources in this area. This study uses rock pyrolysis, vitrinite reflection, and residual organic carbon data to establish burial thermal history, vitrinite reflection evolution, hydrocarbon generation, and charging history through basin simulation. The results of burial thermal history indicate that the source rocks of the Lower Cambrian and Middle Permian are in the high maturity and overmaturity stages at present. The hydrocarbon generation and charging model indicates that the source rocks of the Lower Cambrian and Middle Permian have a characteristic of sustained hydrocarbon generation. The hydrocarbon generation of the Lower Cambrian source rock started in the late Hercynian period (250 Ma). The peak of liquid hydrocarbon generation was in the Indosinian period (220–170 Ma), and the peak of gaseous hydrocarbon generation was in the Yanshanian period (160 Ma). The Mesozoic-Cenozoic Indosinian movement caused the northwestern Sichuan Basin to experience a strong foreland tectonic evolution and formed a special organic thermal maturation process. The tectonic movement of the foreland basin in northwest Sichuan promotes the transformation of marine source rocks from high maturity to the overmature stage and delays the effect in the transverse direction causing different belts including the thrust napper belt, the hidden front belt, and the depression belt to enter the maturity stage later. The thermal evolution history and natural gas maturity comparison indicated that the high mature gas in the Permian is more likely to come from the Lower Cambrian source rocks.