Two oil shale sequences in the Upper Cretaceous Qingshankou and Nenjiang Formations of the Songliao Basin, NE China, representing organic matter (OM)-rich hydrocarbon source rocks, are investigated. According to bulk and inorganic geochemical data, there are significant differences in OM enrichment conditions between the first member of the Qingshankou Formation (K(2)qn(1)) and the second member of the Nenjiang Formation (K(2)n(2)). The oil shale in K(2)qn(1) was deposited in a warm-humid paleoclimate, with strong water salinity stratification and dysoxic conditions, combined with high bioproductivity and low detrital matter input. In contrast, the oil shale in K(2)n(2) was accumulated in a warm semi-humid paleoclimate, with weak water salinity stratification and partially oxic conditions, when the bioproductivity and detrital matter input were medium. In addition, the origin of OM influenced its enrichment. In summary, high bioproductivity and strong water salinity stratification were the major controlling factors for OM enrichment in K(2)qn(1), while the origin of OM and dysoxic conditions were the major governing factors for OM enrichment in K(2)n(2). Thus, the OM enrichment models of these two members are established.