Abstract: Oil and gas production from Qingshankou Formation in Qijia-Gulong sag of Songliao Basin is closely related to formation overpressure. However, there is a lack of asystematic study for the distribution pattern and genesis of overpressure. Based on the measured formation pressure and that predicted from mud density data, this paper analyzes the pressure distribution characteristics across all the sections of Qingshankou Formation in Qijia-Gulong sag. Based on geological-geophysical log response characteristics, an analysis has been performed on the genesis of overpressure and the factors influencing overpressure preservation, followed by exploring the correlation between overpressure formation and oil accumulation in shale reservoirs. The research results show that Qingshankou Formation in Gulong sag, southern and northern Qijia sag exhibits distinct vertical pressure compartment structures of weak-strong overpressure, normal-weak-strong overpressure, and normal-weak overpressure. The maximum formation pressure coefficients of corresponding structure in the Member 1 of Qingshankou Formation are 1.82, 1.66, and 1.38, while those in the Member 2 and 3 of Qingshankou Formation are 1.79, 1.54, and 1.34, respectively. Different regions show significant differences in overpressure genesis. In Gulong sag, overpressure is as a result of pressurization from hydrocarbon generation. In the southern Qijia sag, overpressure is primarily caused by disequilibrium compaction, pressurization from hydrocarbon generation, and overpressure transfer. In the northern Qijia sag, overpressure is mainly generated from disequilibrium compaction. Further, it is found that the contribution rate of hydrocarbon generation-induced expansion to overpressure in Qijia-Gulong sag from south to north decreases from 97 % to 3 % . Furthermore, the weak-strong overpressure compartment structure formed due to hydrocarbon generation-induced expansion is a mark of high-yield oil layers in Gulong shale. The density of faults at the base of Qingshankou Formation have a limited impact on redistribution of the current formation overpressure. Moreover, low sandstone-to-mudstone ratio, fault stability, as well as development of laminar fractures in the mudstone layers provide good preservation conditions for shale oil retention. The north-south differences in overpressure distribution are highly correlated with the hydrocarbon accumulation of light oil, thin oil and black oil zones, highlighting the importance of understanding the genesis of overpressure in shale. To clarify the distribution and genesis of overpressure in Qingshankou Formation can provide a theoretical support for the prediction of tight oil and shale oil resources and the research of accumulation mechanisms.