The Evolution and Influence of Pore-Fluid Pressure on Hydrocarbon Generation of Organic Matter in the Lower Cretaceous Shahezi Formation, Xujiaweizi Fault Depression, Songliao Basin, China

The Shahezi Formation is the main source rock formation in the deep part of the Xujiaweizi Fault Depression. In this study, firstly, based on measured pressure data and logging data, analyzed with methods such as fluid inclusion testing and basin numerical simulation, the pressure evolution history of the Shahezi Formation was systematically reconstructed. Secondly, hydrocarbon generation simulation experiments were carried out under pressures of 45, 70, and 100 MPa to reveal the influence of pressure on the hydrocarbon generation and expulsion process of organic matter. The results showed that the evolution of the pore pressure could be divided into four stages. Prior to the end of the early Cretaceous (~104 Ma), the Xujiaweizi Fault Depression was in a normal-pressure state. In the early Late Cretaceous (104–78 Ma), overpressure was generated in the source rocks due to undercompaction and hydrocarbon generation, which was a stage of rapid pressure increase. In the Late Cretaceous (78–65 Ma), a large amount of gas was generated in the source rocks, and the pressure reached the highest value. From the early Paleogene to the present (65–0 Ma), it has been a stage of slow pressure decrease. The strata have experienced slow uplift, and the pressure has gradually decreased. The effect of high pressure can promote the conversion of heavy hydrocarbon gases in the C1–C5 components to methane, but the promoting effect is limited. In contrast, high pressure has a certain effect on the preservation of liquid hydrocarbon components, and this preservation effect may be caused by the conversion of unstable saturated hydrocarbons to aromatic hydrocarbons.