In the Songliao Basin, northeastern China, the oil shale-bearing succession in the Upper Cretaceous Qingshankou Formation contains excellent source rocks. Oil shales with different total organic carbon (TOC) contents and oil yields developed in the lower member of the formation (K(2)qn(1)). In this study, we apply gas chromatography-mass spectrometry (GC-MS) to determine the geochemical characteristics, organic matter (OM) sources and depositional environments of various grades of oil shale. Rock-Eval pyrolysis indicates that type I kerogen is the predominant organic matter in the K(2)qn(1) oil shale, though variability in n-alkanes, steranes and hopanoids contents implies that organic matter from a variety of sources is present. High-quality oil shales are dominated by phytoplanktonic/algal and bacterial organic matter, while lower-quality oil shales are dominated by planktonic kerogen with a minor contribution from land plants. Organic matter types can indicate a high shale oil conversion rate, and increased prospects for oil shale utilization. Oxygen-deficient bottom water conditions, related to salinity stratification, are evidenced by biomarker ratios (Pr/Ph, gammacerane index (GI)). We propose that the highest-quality oil shales were deposited under anoxic conditions, with strong salinity stratification of the water column. OM sources, redox conditions and water column salinity stratification were the key factors controlling the accumulation of high-quality oil shale in the southeastern Songliao Basin.