The abundance and composition of total organic carbon, aliphatic and aromatic hydrocarbons including biomarkers, and the δ¹³C composition of total organic matter and individual compounds in core samples of the Nenjiang Formation from the SK-1 borehole of the Songliao Basin provide information on the paleoenvironmental conditions of lacustrine sediments formed during the Upper Cretaceous. The distribution of n-alkanes, and their δ¹³C composition, and 4-methylsteranes and dinosteranes in the first member of the Nenjiang Formation (K₂n¹) indicate that the organic matter largely originated from algal organisms and is dominated by types I and II kerogen. The occurrence of gammacerane, aryl isoprenoids isorenieratane and low pristane/phytane and diasterane/sterane ratios further suggest a salinity stratified water column associated with anoxic bottom waters, as well as possible photic zone oxygen depletion conditions during major source rock deposition of the K₂n¹. The organic geochemical data also indicate significantly different depositional environments for the second member of the Nenjiang Formation (K₂n²) sediments, which are interpreted to have been deposited under a uniform salinity (non-stratified) oxic water column. Consequently, although the organic matter in the K₂n² sediments was largely derived from algal and bacterial sources, the kerogen types are dominantly IIb and III due to the oxidative degradation of organic matter, combined with a gradual increase in land-plant input. Furthermore, the occurrence and shift of two saw-toothed distribution patterns of the δ¹³C composition of C₂₉ to C₃₃ n-alkanes, with "W" and "M" patterns being isotopically heavy odd- and even-numbered alkanes, respectively, is suggestive of organic input shift or an unknown mechanism of carbon uptake or isotopic fractionation. In particular, the heavy even carbon numbered high molecular weight n-alkanes is rare and needs further investigation.