SourceReservoir Coupling Mechanism and Shale Oil Enrichment Patterns of Mixed SiliciclasticCarbonate Shales in the Lower Ganchaigou Formation, Western Qaidam Basin, Driven by HighFrequency Climatic Cycles

To clarify the source–reservoir configuration characteristics of saline lacustrine shale oil systems and their control on shale oil enrichment, this study focuses on the mixed shale of the Upper Xiaganchaigou Formation (E₃²) in the Yingxiongling Sag, Qaidam Basin. Based on core observations, thin-section identification, well logging data and laboratory analyses, the lithofacies succession, source–reservoir assemblages and their control on shale oil enrichment under high-frequency climatic cycles were systematically investigated. The results show that: (1) the climatic cycle boundaries of the mixed shale in the Yingxiongling Sag are typically marked by normally graded sandstone erosion surfaces overlying evaporite-bearing layered dolomite. Two lithofacies, namely limestone–dolomite interbedded lithofacies and sandstone–dolomite mixed lithofacies, are developed in the proximal provenance area, whereas three lithofacies, including limestone–dolomite interbedded lithofacies, mudstone–dolomite mixed lithofacies and sandstone–dolomite mixed lithofacies, are developed in the distal provenance area. (2) The source–reservoir qualities of the three lithofacies differ significantly. The mudstone–dolomite mixed lithofacies has the highest organic matter abundance and the greatest hydrocarbon generation potential, making it the main source rock in the study area. In contrast, the limestone–dolomite interbedded lithofacies has the highest porosity and the most favorable pore-throat structure, and therefore represents the most favorable reservoir lithofacies. (3) Climatic cycles and paleogeomorphic differentiation jointly control lithofacies successions and source-reservoir configurations. Two types of source–reservoir assemblages are identified in the study area: an integrated source-reservoir type and a separated source–reservoir type. The latter can be further subdivided into overlying and sandwich-style configurations. The integrated source–reservoir type is mainly controlled by fracture development, the overlying type is characterized by near-source charging, and the sandwich-style type has the advantage of dual-source hydrocarbon supply. Together, these assemblages control the differential enrichment of shale oil in the study area. This study clarifies the source-reservoir assemblage characteristics of the shale interval in the Yingxiongling Sag and their influence on shale oil enrichment, providing a geological basis for target interval selection in similar saline lacustrine shale oil systems.