This study investigates the shale of the upper sub-member of Member 4 of Shahejie Formation in the Minfeng subsag,Dongying sag,integrating core and thin-section observations with elemental geochemistry to reconstruct paleoenvironmental conditions. An innovative multi-method analytical framework was employed—including scanning electron microscopy coupled with cathodoluminescence(SEM-CL),focused ion beam-scanning electron microscopy(FIB-SEM),laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS),stable isotope(C,O,Sr)analysis,fluid inclusion microthermometry,and U-Pb geochronology—to systematically characterize diagenetic mineral evolution and elucidate the reservoir formation mechanism. Results indicate that deposition of the target interval occurred under an arid to semi-humid climate,within a fine-grained,brackish-deep lacustrine setting characterized by strong redox stratification. High-frequency vertical cyclicity is evident,reflecting a progressive facies transition from carbonate-rich mixed shale → mixed shale → feldspar-rich mixed shale. Total organic carbon(TOC)ranges predominantly between 1.8wt% and 6.0 wt%,with abundant organic matter accumulation attributed primarily to high sedimentation rates and robust terrigenous and/or endogenous organic input—rather than prolonged bottom-water anoxia alone. Granular calcite and fibrous calcite exhibit distinct geochemical signatures: divergent rare earth element(REE)patterns,contrasting δ¹³C and δ¹⁸O values,and significantly different U-Pb ages. Notably,columnar-to-fibrous calcite laminae oriented near-horizontally yield a broad U-Pb age spectrum(31.2±4.3 Ma to 25.1±4.2 Ma),spanning the full early-to-middle diagenetic interval. Illite comprises 80%~100% of the clay mineral assemblage;its formation is dominantly controlled by montmorillonite-to-illite transformation,while feldspar dissolution represents a major silica source for authigenic quartz precipitation. Authigenic quartz is ubiquitously observed within argillaceous laminae and calcite laminae. Under overpressured conditions,co-precipitation of authigenic quartz and recrystallized calcite effectively buffered porosity loss during cementation—thereby preserving critical pore space and facilitating subsequent pore enhancement via acidic fluid dissolution during later diagenesis. This interplay constitutes a key reservoir-quality preservation and improvement mechanism in deeply buried lacustrine shales.

fine-grained sedimentary environment  /  diagenetic evolution  /  mixed shale  /  Minfeng subsag  /  Dongying sag