Oil shale formation in the Upper Cretaceous Nenjiang Formation of the Songliao Basin (NE China): Implications from organic and inorganic geochemical analyses

Abstract

Two oil shale layers are present in the Upper Cretaceous Nenjiang Formation of the Songliao Basin, representing excellent hydrocarbon source rocks. Their biomarker compositions provide evidence for a major contribution by aquatic organisms within the photic zone of the water column. Phytoplankton blooms were promoted by warm–humid climate, and lake eutrophication. Salinity stratification and anoxic bottom water conditions are indicated in the lower oil shale layer, whereas decreased salinity and partial oxygenation of the water column are evident in the upper oil shale layer. Moreover, an increase in sizes of alginite from the lower to the upper oil shale layer and enhanced telalginite concentrations is detected. Therefore, a stratified water column with high salinity and anoxic bottom water conditions contributes to organic matter (OM) preservation in the lower oil shale layer. In contrast, high bioproductivity in combination with OM preservation favored by enhanced algae sizes and telalginite concentrations are suggested as the OM enrichment mechanisms in the upper oil shale layer. In addition, factors such as clay minerals, microbial activity, and detrital matter input cannot be ignored for their influence on OM enrichment.In this study, a preservation model within the lower oil shale layer and a productivity model within the upper oil shale layer are established. The models imply that excellent preservation is the major controlling factor for OM enrichment in the lower oil shale layer, whereas the high bioproductivity is the major controlling factor for OM enrichment in the upper oil shale layer. However, the combination of both factors for oil shale deposition must be considered.