Understanding the occurrence state of shale oil is crucial for the effective development of shale oil resources. Although the second member of the Kongdian Formation (Ek₂) is a key interval for lacustrine shale oil production in the Cangdong Sag, Bohai Bay Basin, the occurrence state and controlling factors of shale oil in this formation remain poorly understood. This study established a multi-step programmed pyrolysis, combined with a light hydrocarbon recovery scheme, to quantitatively characterize the shale oil in different occurrence states. An integrated approach utilizing Rock-Eval pyrolysis, pyrolysis-gas chromatography, and crude oil gas chromatography was employed. Factors influencing the shale oil occurrence state were analyzed from petrology and organic geochemistry perspectives. The study revealed significant variations of shale oil occurrence states within the Ek₂, attributed to differences in sedimentary organic matter, mineral compositions, sedimentary structures, and thermal maturity. Felsic laminae are the primary reservoir space for oil in laminated shales, and the frequent interbedding of felsic and organic-rich laminae facilitates the retention of free oil. The contents of free and adsorbed oil are primarily influenced by organic matter content and shale storage capacity, both of which exhibit distinct occurrence patterns. Based on the shale reservoir quality classification using the pyrolysis values of S_1-1 + S_1-2 and (S_1-1 + S_1-2) × 100/TOC, the Ek₂ shale demonstrates significant exploitation potential, with the first-level reservoirs comprising 66%, second-level reservoirs 11%, and third-level reservoirs 23%. These findings provide new insights into the geological accumulation and production of shale oil.

The 3rd member of the Shahejie Formation (Es³) in the Qibei Sub-sag, Huanghua Depression, Bohai Bay Basin holds enormous potential for shale oil exploration and exploitation. Using systematic experiments and analyses including core and thin section observations, scanning electron microscopy (SEM), X-ray diffraction (XRD), high-pressure mercury injection, total organic carbon (TOC) content analysis, and rock pyrolysis analysis, we explore the geological conditions and controlling factors for shale oil enrichment in the Es³⁽¹⁾. Six typical lithofacies of the Es³⁽¹⁾ are identified: laminated medium-grained calcareous-dolomitic shale, laminated fine-grained mixed shale, lamellar fine-grained mixed mudstone, lamellar medium-grained mixed mudstone, lamellar coarse-grained felsic mudstone, and massive medium-grained mixed mudstone. These mudstones/shales exhibit a TOC content ranging from 0.23 % to 2.57 %, with organic matter dominated by Types Ⅱ₁ to Type Ⅲ kerogen. The Es³⁽¹⁾ shales display peak pyrolysis temperatures (T_max) varying from 435 ℃ to 463 ℃, indicating that these shales are generally in the mature stage of thermal evolution. Major reservoir space types identified in the study area include intergranular pores, intercrystalline pores, inter-clay-flake pores, intragranular dissolution pores, organic pores, and microfractures. Shale oil predominantly occurs in inter-clay-flake and dissolution pores with small pore-throats, as well as intercrystalline pores and microfractures, both with large pore throats. Major factors controlling shale oil enrichment in the Es³⁽¹⁾ include TOC content, organic matter type, reservoir type, and sedimentary structures, which together form microscopic source rock-reservoir assemblages. Among the various lithofacies, the laminated medium-grained calcareous-dolomitic shale, laminated fine-grained mixed shale, and the lamellar fine-grained mixed mudstone exhibit effective source rock-reservoir assemblages, establishing themselves as favorable lithofacies for shale oil enrichment in the study area.

The Ordovician-Silurian Wufeng and Longmaxi Shale in the Sichuan Basin were studied to understand the genesis and diagenetic evolution of carbonate minerals and their effects on reservoir quality. The results of geochemical and petrological analyses show that calcite grains have a negative Ce anomaly indicating they formed in the oxidizing environment of seawater. The high carbonate mineral contents in the margin of basin indicate that calcite grains and cores of dolomite grains appear largely to be of detrital origin. The rhombic rims of dolomite grains and dolomite concretions with the δ¹³C of –15.46‰ and the enrichment of middle rare earth elements were formed during the sulfate-driven anaerobic oxidation of methane. The calcite in radiolarian were related to the microbial sulfate reduction for the abundant anhedral pyrites and δ¹³C value of –11.34‰. Calcite veins precipitated in the deep burial stage with homogenization temperature of the inclusions ranging from 146.70 ℃ to 182.90 ℃. The pores in shale are mainly organic matter pores with pore size mainly in the range of 1–20 nm in diameter. Carbonate minerals influence the development of pores through offering storage space for organic matter. When calcite contents ranging from 10% to 20%, calcite grains and cement as rigid framework can preserve primary pores. Subsequently, the thermal cracking of liquid petroleum in primary pores will form organic matter pores. The radiolarian were mostly partially filled with calcite, which combining with microcrystalline quartz preserved a high storage capacity.