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Although China has abundant lacustrine shale oil resources, their exploitation is challenging. Investigating the various occurrence states of shale oil in shale reservoirs provides value for the exploration and development of shale oil resources. With the shale in the second submember of Da'anzhai Member of Jurassic Ziliujing Formation of the central Sichuan Basin (the Da2 submember) as the research object, the study reveals the occurrence modes of shale oil under different states in various medium pores, through the implementation of the multi-temperature pyrolysis experiment, the observation of pore development characteristics under the scanning electron microscope and the analysis of the pore size distribution before and after oil washing. In the Da2 submember, the shale oil is mainly in the free state (0.42 to 10.88 mg/g), followed by the adsorption state (0.30 to 1.95 mg/g), as revealed by thermal simulation recovery. The reservoir space of shale includes organic pores (pore size: 2 to 600 nm), pyrite intergranular pores (pore size: 10 to 700 nm), shell pores (pore size: 20 to 1 000 nm), quartz/feldspar intergranular pores (pore size: 4 to 500 nm) and clay mineral intergranular pores (pore size: 4 to 500 nm). After oil washing, the results of nitrogen adsorption and high pressure mercury injection demonstrated a significant increase in pores with sizes of 2 to 30 nm and 60 to 1 000 nm, where most of shale oil is stored. Meanwhile, it was demonstrated that the shale oil in the Da2 submember mainly occurs in organic matter and pyrite by establishing a heat map of the relationship between the occurrence state of shale oil and the medium in the rock. Lastly, by fitting the content of shale oil in different occurrence states with the oil volume obtained before and after oil washing, the pore size range for shale oil accumulation in the study area was determined. The free state of shale oil in the Da2 submember primarily accumulates in pores of organic matter and pyrite with pore size of 60 to 700 nm, and the adsorbed state of shale oil mainly accumulates in the organic matter pores with pore size of 2 to 6 nm. In conclusion, this study presents a thorough examination of the occurrence characteristics of shale oil in Da2 submember, and it will support shale oil exploitation efforts in the area.

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Low-maturity shale oil has great resource potential in Jiyang Depression and has become an important field after the successful breakthrough of medium- to high-maturity shale oil. Objective The purpose of this paper is to clarify the pore and fracture characteristics of low-maturity continental shale and their significance to shale oil occurrence. Methods By studying the Es3x and Es4scs members in Qingnan Depression, Jiyang Depression, this study combines thin section observation, TOC content, XRD analysis, solvent extraction, low-temperature N2 adsorption, high-pressure mercury injection, scanning electron microscope observation and EDS analysis to classify the shale lithofacies, characterize the pore and fracture development, and illustrate the importance of shale pores and fractures for shale oil occurrence. Results The TOC content is mostly in the range of 1.0%-4.0%, and the mineral composition mainly includes felsic minerals, followed by clay and carbonate minerals. The major pore types are ink-bottle-shaped pores and plate-parallel-shaped pores, including quartz intergranular pores, clay mineral interlamellar pores, and dolomite intercrystalline pores. The major pore sizes are mostly less than 200 nm and exhibit a multipeak distribution, which is concentrated in the ranges of 2-50 nm, 50-80 nm and 100-200 nm. Horizontal bedding fractures, vertical fractures and network fractures developed in the studied shales and were mostly filled or contacted by bitumen.Felsic-rich shale lithofacies usually have a greater pore volume and specific surface area than clay-rich lithofacies, and felsic minerals positively contribute to pore development. The pore volume and specific surface area first decreased and then increased with increasing thermal maturity. When Ro>0.6%, the shale oil content increases significantly, which is mainly related to the large amount of hydrocarbon generation in organic matters. Conclusion Horizontal bedding fractures, quartz intergranular pores, and dolomite and calcite intercrystalline pores are favourable storage and occurrence spaces for shale oil.

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Shale oil resources are important supplements for the gradually decreasing oil production from conventional reservoirs. Although the exploitation and development of shale oil have achieved considerable progress in the last decade, the commercial extraction of hydrocarbons from shales is still difficult, especially in the lacustrine sedimentary basins of China. One of the key points controlling the successful extraction of hydrocarbons from shale systems is the understanding of the occurrence mechanism of shale oil. This study comprehensively summarizes the theories and techniques to characterize oil occurrence state, occurrence space, oil content, and oil movability in shale systems. Sophisticated instruments, such as high-resolution scanning electron microscopy and high-energy ray imaging, were utilized to qualitatively analyze the pore networks of shales. Advanced physical experiments and numerical simulation techniques, including step-by-step rock pyrolysis, solvent extraction, and NMR, were introduced to characterize shale oil adsorption and movability. By the comparative analysis of the occurrence space, it is found that the image observation technique especially focuses on concentrated pores, such as organic matter-hosted pores. The fluid injection technology yields particular pore size information, which should be calibrated using other information. The 3D digital core, demonstrating the spatial distribution of minerals and pores, is an effective input for shale oil flow simulation. Geological controls analysis about oil retention in organic-rich shales has found that the inorganic matter pores and fractures are probably the "sweet spot" of shale oil, due to the low oil adsorption and high light hydrocarbons content. Many physical experiments measure the total free oil content but neglect the hydrocarbon–rock interaction and the sequential migration of hydrocarbon compounds. Thus, micro-scaled experiments measuring the hydrocarbon adhesion forces are needed to uncover the occurrence mechanism of shale oil in the future. [ABSTRACT FROM AUTHOR]