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Diagenesis typically exerts a crucial impact on the formation of high-quality sandstone reservoirs in the Eocene Shahejie Formation, Dongying Depression. To better understand the formation process of petrophysical properties, this research conducts petrographic and geochemical analyses to investigate the nature of diagenetic fluids. Petrographic observations suggest that the dominant cements are carbonate, authigenic quartz, and clay minerals, accompanied with the dissolution of feldspar and calcite. The homogenization temperature of aqueous inclusions in quartz overgrowth usually exceeds 90°C corresponding to the maturity of organic matter. Quartz overgrowths contain higher amounts of CaO and Al2O3 than detrital quartz. This indicates that the siliceous fluid mainly originates from the dissolution of feldspar. Moreover, the conversion of clay minerals also provides trace amounts of silica into pore water during the burial process. Carbonate cements consist of early-stage calcite as well as late-stage Fe-calcite and ankerite. Calcite with relatively higher MnO proportions shows yellow luminescence and dissolution signs. Fe-calcite and ankerite cements have a higher homogenization temperature than that of quartz overgrowth and mainly concentrate in FeO and MgO as well as contain a small amount of Na+, K+, and Sr2+. The rare earth element (REE) pattern of bulk mudstone and carbonate cements as well as C–O isotopic evidences indicate that the diagenetic fluids of carbonate cementation are primarily controlled by the adjacent mudstone, whereas mineral dissolution and altered clay minerals in sandstone provide additional cations for the local reprecipitation of late-stage carbonate. Therefore, diagenetic fluids within sandstone reservoirs are typically subject to alkaline–acid–alkaline conditions and are influenced by internal sources in a closed system. Compaction significantly reduces the pore space of sandstone reservoirs in the Boxing Sag. Carbonate cementation further increases the complexity of pore structure and obeys the principle of mass balance. [ABSTRACT FROM AUTHOR]

Calcite veins, which developed parallel to the bedding, are widespread in laminated source rocks in the Eocene Dongying Depression. However, there is a lack of systematic description and classification of the veins. This study presents a systematic characterization of the calcite veins, host rocks, and micritic carbonate laminae by applying petrographic and geochemical methods to understand vein-forming mechanisms. Antitaxial and syntaxial veins are examined. Antitaxial veins contain typical fibrous crystals with the most intense fluorescence, and the median zone of these veins is often the micritic carbonate. Calcite crystals in syntaxial veins develop a blocky morphology of various sizes, indicating obvious growth competition. Data of rare earth elements and trace elements obtained from the micritic laminae, host rocks, and calcite veins are very similar. This indicates that the vein-forming nutrients originated from the carbonate in the host rocks and micritic laminae. The minor difference in C and Sr isotopes between calcite veins and micritic carbonate within the host rock and the negative shift in O isotopes in the veins are caused by ion exchange and dehydration of swelling clay minerals in the burial environment. This further proves that the calcite veins are formed in a closed system. Geochemical analysis suggests that the rocks are in the oil window and have good hydrocarbon potential. Thermal evolution of the acidic fluids generated from organic matter (OM) resulted in the dissolution of carbonate and formed fluid overpressure in the rocks. Fluid overpressure induced the formation of fractures in the interlayer and expanded the veins with the force of crystallization due to fibrous calcite growth. Blocky crystals grow in the fractures from the margins toward the center. Hydrocarbon expulsed via OM maturation in the host rock fills the intercrystalline pores. Moreover, shale with bedding-parallel calcite has the characteristics of high-quality shale oil reservoirs. These characteristics will probably provide guidance for shale oil exploration. [ABSTRACT FROM AUTHOR]

Shale oil is attracting increasing attention in the petroleum industry as an essential target of unconventional oil and gas exploration. Recent exploration efforts have indicated that laminar carbonates are closely related to the favorable depositional strata of shale oil. This research focused on inter-bedded layers of laminar carbonates as well as mudstone and shale in the Dongying depression of the Bohai Bay Basin in eastern China, and attempted to reveal the genesis and evolution of the carbonates by means of drill core observations, thin sections, high resolution scanning electron microscopic observations, and fluorometric, cathodoluminescence and geochemical analyses. The results demonstrated that: (1) the carbonates could be categorized into two groups, crystalline carbonates and micritic carbonates; (2) the crystalline carbonates were well crystallized and greenish under fluorescent light; and (3) the micritic carbonates were not well crystallized and had biotic textures. It was discovered that the crystalline carbonates were mainly developed in the shale layer and were formed from the late-filling effects of the diagenetic cracks along the shale fissility planes; shale layers with crystalline carbonates are usually favorable for shale oil reserves. The micritic carbonates were formed from direct deposition controlled by biological effects in the stratified flows of the seasonal lake basins and are unfavorable for shale oil reserves. [ABSTRACT FROM AUTHOR]