Abstract: A series of fine-grained sediments with mixed origins of mudstone-dolomite and clastic rock are developed in the Cretaceous of Qingxi Sag in the Jiuquan Basin, providing favorable conditions for shale oil enrichment. Samples of different lithofacies were selected as research objects, and a series of analytical tests were conducted, including rock pyrolysis, total organic carbon, X-ray diffraction, scanning electron microscope (SEM) analysis, rock thin section observation, low-temperature nitrogen adsorption, and high-pressure mercury intrusion experiments. Through the comprehensive applications of mineral petrology, organic geochemistry, and reservoir physical property analyses, detailed reservoir characterization and oil-bearing evaluation were conducted for shale oil in the sag. The research results showed that: (1) The shale mineral compositions and lithologies in the sag were complex, and the main lithofacies types were dolomitic shale, fine siltstone, gravelly sandstone, and mudstone. The corresponding shale oil types were mixed sedimentation (in a broad sense) and interbedded types. The fine siltstone exhibited the best oil-bearing property, followed by dolomitic shale, and both of them were favorable reservoirs for shale oil. (2) The pore sizes of shale reservoirs were distributed in the range of 10 nm to 100 μm, among which fine siltstone had a relatively uniform pore size distribution, conducive to the enrichment of shale oil. (3) Based on total organic carbon (TOC) content, S₁ content, and oil saturation index (OSI), the oil-bearing properties of shale oil were evaluated and classified into ineffective resources, low-efficiency resources, medium-efficiency resources, and high-efficiency resources. (4) Based on shale lithofacies, geochemical characteristics, reservoir physical property characteristics, microscopic pore structure, and oil-bearing characteristics, the shale oil in the sag was divided into Class Ⅰ, Class Ⅱ, and Class Ⅲ sweet spots, in descending order of resource quality. This study clarified the reservoir and oil-bearing characteristics of the shale oil in the Qingxi Sag and categorized shale oil sweet spots, which is of great significance for shale oil exploration in the sag.