To explore the mechanism of reservoir formation in the mixed shale of the Dongying Depression, a series of techniques including thin section observation, X-ray diffraction for whole rock mineral content analysis, scanning electron microscopy, nuclear magnetic resonance, small-angle X-ray scattering, electron probe microanalysis, and in-situ rare earth element analysis were employed to quantitatively characterize the reservoir space and reservoir performance. The mineral types were systematically observed and analyzed, and the mineral genesis and its impact on reservoir properties were dissected. The results show that the vitrinite reflectance (Ro) of the mixed shale in the upper 4th member of Shahejie Formation of Minfeng Sag is on average 6.3%, and the total organic carbon (TOC) content is on average 2.9%, which belongs to typical organic-rich shale with medium to low thermal maturity. The formation pressure coefficient is 1.2 to 1.8. The reservoir space types are diverse, with clay mineral micropores, intergranular pores of quartz and feldspar, intercrystalline pores of carbonate minerals, dissolution pores, intercrystalline pores of pyrite, and organic matter pores all developed. The pore connectivity is good, and the average porosity of the mixed shale is about 8%. The laminated quartz-feldspar mixed shale has the best reservoir physical properties, followed by the calcareous mixed shale, and the clayey mixed shale has the highest porosity below 300 nm. The authigenic quartz accounts for more than 70% of the total quartz in the shale, mostly formed by the dehydration transformation, dissolution, and precipitation of silica from clay minerals under the action of organic acids in a high organic matter background. The calcite laminae within the mud are mostly formed in the early diagenetic stage, while the fine and coarse crystalline granular calcite is mostly a product from the early to late diagenetic stage. The dolomite sources are diverse, and most are the products of dolomitization of calcite in the mud laminae. The early diagenetic stage is the key period for diagenesis and reservoir formation. The synergetic effect of overpressure formation background and framework mineral support is the most crucial mechanism for pore preservation in the mixed shale of the Minfeng Sag.