Coupling mechanism between climate aridification and shale oil shale mineralization during the the Middle Jurassic in the Qaidam Basin

Climate constitutes a crucial factor in the oil shale mineralization, given its pronounced influence on the growth of lacustrine organisms and the depositional environment.. During the late Middle Jurassic, the Qaidam Basin experienced a major climate shift from humid to arid conditions. Amid this transition, a continuously distributed sequence of lacustrine oil shales was deposited in the basin. Focusing on these oil shales, we explore the coevolutionary relationships between aridification and oil shale mineralization using methods such as palynology and geochemistry. The results indicate that the oil shales of the shale interval of the Shimengou Formation were deposited under varying humidity conditions. Specifically, the black, massive oil shales in the lower part are formed in freshwater lakes under a relatively humid climate, while the laminated oil shales in the upper part in saline lakes under arid conditions. During the humid phase, lush terrestrial vegetation and a high flux of terrigenous sediments supplied abundant terrigenous organic matter and clastics to the lakes. This created mixed organic matter sources and a dysoxic environment for organic matter preservation. As a result, medium-quality oil shales with high hydrocarbon-generating potential are created. During the arid phase, reduced terrestrial vegetation and the proliferation of algae led to the formation of algae-dominated organic matter sources. Concurrently, the anoxic, high-salinity sediment-water environment significantly enhances the preservation efficiency of organic matter. Hence, medium- to slightly high-quality oil shales with excellent hydrocarbon-generating potential occur. The salinization of lacustrine basins, induced by the aridification, is identified as the key factor in driving the transition between the two oil shale metallogenic models. This study provides insights into the coupling mechanism between aridification and oil shale mineralization and enrichment while also offering an important basis for predicting high-quality oil shales within the Qaidam Basin.