Mechanisms for lacustrine shale oil enrichment in Chinese sedimentary basins

By analyzing the tectonic and sedimentary environments for the formation of organic-rich shales in China’s continental lacustrine basins, we identify significant differences in the development of high-quality continental source rocks across various types of lacustrine basins. For shale sequences deposited in fresh-water lacustrine basins, the main lithofacies are felsic and clayey shales, as observed from the 1^st member of the Upper Cretaceous Qingshankou Formation (K₂qn¹ section) in the Songliao Basin and the 7^th member of the Triassic Yanchang Formation (T₃yc⁷ section) in the Ordos Basin. For shale sequences developed in a saline lacustrine environment, however, carbonates and evaporites are dominant lithofacies, as represented by the Paleogene Shahejie Formation in the Jiyang Depression. There are three types of lithofacies assemblages for Chinese lacustrine shales, that is, the shale interbedded/intercalated with sand, mixed shale, and clayey shale. These lithofacies assemblages determine the hydrocarbon source-reservoir coupling characteristics, differential evolution of hydrocarbon generation, and property differences of in-situ fluids in the lacustrine organic-rich shales. The shale interbedded/intercalated with sand assemblage is characterized by source-reservoir separation and near-source migration. The mixed shale assemblage shows macroscopic integration and microscopic separation between source rock and reservoir. In contrast, the clayey shale acts as both the source and reservoir of in-situ generated hydrocarbons, featuring pervasive oil distribution. As revealed by evidence, inorganic pores provide the most favorable storage space for lacustrine shale oil in medium-low maturity, and form effective pore-fracture networks for hydrocarbon transport together with multi-type and multi-scale microfractures. Self-sealing capacity of shale is conducive to the in-situ or proximal preservation of shale oil and gas. Comparison of typical continental shale sequences in the Chinese sedimentary basins indicates that favorable source-reservoir coupling, suitable thermal maturity level, and self-sealing capacity of shale are the major controls for oil enrichment in lacustrine shale. This study also presents a preliminary model for differential enrichment of lacustrine shale oil in China. Therefore, the laminated shales in medium-low maturity in gentle slope zones and the clayey shale-rich strata in medium-high maturity in deep sags should be prioritized in lacustrine shale oil exploration in downfaulted lacustrine basins. Moreover, both the shale interbedded/intercalated with sand and the clayey shale in medium-high maturity are crucial to making breakthroughs in lacustrine shale oil exploration therein.