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The investigation of the shale oil development potential of the lower third section–upper fourth section (ES33–ES41) of the Eocene–Oligocene Shahejie Formation in the Dongying Sag, Bohai Bay Basin, eastern China, continues to be a scientific challenge. A total of 23 shale samples was collected from these strata, and the organic petrology, organic geochemistry, mineral composition, porosity, and pore structure of these samples and their relationships with the retained oils were investigated. The results indicated that these shales with type I–IIa kerogen are rich in lamalginite and its debris, and the Ro values of these shales range from 0.70% to 1.00%. The non-micropores (>2 nm) that are mainly developed from inorganic minerals are greater than the micropores (<2 nm) that largely contributed from the organic matter of the shale. The retained oil contents presented by the free hydrocarbons (S1) and extracted organic matter (EOM) exhibited significantly positive relationships with the total organic carbon (TOC) contents and micropore volumes, which may indicate that the retained oils are largely stored in organic matter micropores resulting from the volume swelling of kerogen. The total oils and their light compositions, as well as the S1/TOC and EOM/TOC values, increase with the burial depth of the shales, indicating that the content and mobility of the retained oils are largely controlled by the maturity of shales. This study predicts that the burial depth of favorable shale oil reservoirs in the Dongying Sag should be greater than 3500 m (Ro > 0.90%), and the siltstone or carbonate rock interlayer, especially with laminated or layered textures, will further control the sweet spot intervals of shale oil. This study provides new geological evidence for revealing the retention mechanism of shale oils and has practical significance for shale oil exploration and development in the Dongying Sag, Bohai Bay Basin.