Abstract: The Shahejie Formation shale in Dongying Sag is one of the key formations in shale oil exploration and development. However, its complex mineral composition and diverse laminae types result in strong heterogeneity in the distribution of favorable shale oil zones, posing significant challenges for prediction and leading to highly variable development outcomes. This study focuses on the organic-rich lacustrine shale of the lower third member of the Shahejie Formation (Sha-3 lower member) as a case study. By employing techniques such as core observation, thin section analysis, mineral composition testing, stepwise pyrolysis, low-temperature nitrogen adsorption, and field emission scanning electron microscopy (FE-SEM), the study characterizes variations in reservoir quality and oil-bearing potential among different shale types. The analysis clarifies the pore size distribution for shale oil occurrence and identifies the lower pore size limit for movable oil, while exploring factors influencing shale oil content. The Shahejie Formation shale features diverse pore types, including laminar fractures and various intergranular pores. Under ambient temperature and pressure, residual shale oil is mainly stored in the pores of 15-100nm in the nanoscale range, with a lower pore size limit of approximately 3 nm for free oil occurrence. Organic matter abundance (TOC) provides the material foundation for shale oil generation and enrichment, while sufficiently large reservoir spaces ensure the retention of significant hydrocarbon quantities. Among the studied shale lithofacies, organic-rich laminated calcareous shale and laminated mixed shale with high TOC content and high porosity demonstrate superior oil-bearing potential and higher free oil content, making them the most favorable lithofacies in the study area. These findings offer critical insights for advancing the understanding of lacustrine shale oil enrichment mechanisms and optimizing development strategies.