Thick dark source rocks were developed in the Qingshankou Formation (Late Cretaceous, Songliao Basin). According to sedimentary characteristics and systematical geochemical tests, the geological conditions of shale oil and gas accumulation are studied. The thick dark mudstone succession (TOC > 2%) is represented by deep lake environment and their thickness could reach 91 m in the center depression, Songliao Basin. Organic matter is mainly composed by type I and II1 kerogen. Vitrinite reflectance (0.5-0.7% Ro) and RockEval parameter T-max (425-445 degrees C) indicate that the source rock is marginal mature to mature. Abundant brittle minerals (average 51.1%) and the 4.1-14.0% porosity are developed in the mudstone. Based on the SEM and microscope observation, the micron level to nanometer level pores are the main reservoir for shale gas and oil. Through comprehensive analysis of these geological conditions, it is concluded that the lower part in the Qingshankou Formation has the potential to form shale oil and gas reservoirs, and the deep region of the Southeast Uplift near the basin center side is the best favorable zone of shale oil and gas accumulation.