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Fine-grained mixed rocks in saline lacustrine basins are important targets for shale oil exploration. However, the controls on shale oil accumulation are complex due to the multi-source mixed deposition. This is a challenging issue in the study of shale oil. Here, we present a case study in the Middle Permian Lucaogou Formation in the Jimusar Sag, Junggar Basin, northwestern China. Results show that the Lucaogou Formation consists mainly of carbonate rocks, dolomitic or calcareous mudstones, tuffaceous or silty mudstones, and siltstones. The dolomitic/calcareous mudstones (TO C average = 6.44 wt. %) and tuffaceous/silty mudstones (TO C average = 4.83 wt. %) have the best hydrocarbon generation potential and contain type I–II1 kerogens that are in the peak oil generation stage. However, the shale oil potential is highest for the carbonate rocks and siltstones with average oil saturation index (OSI) values of 315.03 mg HC/g TOC and 343.27 mg HC/g TOC, respectively. This indicates that hydrocarbon generation potential is not the main factor controlling shale oil potential. Micro-nanoscale pores are the main control. Abundant dissolution pores provide excellent reservoir space for near-source migration and accumulation of shale oil. Different mixing processes between lithofacies control the accumulation of shale oil, and shale oil productivity is the best when multi-facies deposition in transitional zones formed the mixed rocks (facies mixing). In addition, local accumulations of calcareous organisms and adjacent carbonate components on terrigenous sediments (in situ mixing) are also conducive to shale oil enrichment. This is an unusual and special feature of saline lacustrine shale oils, which is different from freshwater lacustrine and marine shale oils. Comprehensive assessment of source rock and reservoir is needed to robustly establish a widely applicable method to determine the shale oil potential in such basins. [ABSTRACT FROM AUTHOR]

[Display omitted] • Multi-step Rock–Eval pyrolysis refines the shale oil potential evaluation. • The amount of adsorbed oil in saline lacustrine sequences was underestimated. • Lithologies and physical properties are joint controls on free oil, while adsorbed oil is mainly controlled by TOC. • Siltstones and carbonate rocks are important shale oil sweet spots. Occurrence state of shale oil in saline lacustrine basins is complex, thus restricting the availability of high-efficiency shale oil exploration and exploitation. To address the complexity, we conducted a study of the middle Permian Lucaogou Formation in the Jimusar Sag, Junggar Basin, China, in a lithofacies perspective, using comprehensively multi-step Rock–Eval pyrolysis (MREP) and mineralogical, petrological, and organic geochemical methods. Results show that the low-mature to mature Lucaogou Formation in this study consists mainly of four lithofacies types, i.e., carbonate rocks, dolomitic/calcareous mudstones, silty/tuffaceous mudstones, and siltstones. The amount of free hydrocarbon (S 1–1 + S 1–2) and revised free hydrocarbon (S 1–1 + S 1–2 + S 2–1) values obtained by MREP exceeds the S 1 value obtained by conventional REP, which refines the evaluation of the shale oil potential. Shale oil in siltstones and carbonate rocks occurs mostly in fractures and pores in the free state, whereas the shale oil in mudstones occurs mainly in the matrix and kerogen, and on mineral surfaces in an adsorbed state. The shale oil potential of the four main lithologies is overall good, with the siltstone having the best potential (i.e., oil saturation index [OSI = S 1 × 100/TOC] OSI avg = 313.22 mg HC/g TOC) and highest free hydrocarbon content ([S 1–1 + S 1–2 ] avg = 13.53 mg HC/g rock). The tuffaceous/silty mudstone has the best hydrocarbon generation potential (TOC avg = 8.01 wt%; PG = [S 1 + S 2 ] avg = 46.11 mg HC/g rock) and the highest content of adsorbed hydrocarbon (S 2–1 avg = 13.78 mg HC/g rock), but the lowest shale oil potential (OSI avg = 105.78 mg HC/g TOC). The controlling factors on the free hydrocarbon content (S 1–1 + S 1–2) are complex but are mainly controlled by petrophysical properties. The amount of adsorbed hydrocarbon (S 2–1) is controlled by the organic matter content. When a rock in the Lucaogou Formation has OSI > 100 mg HC/g TOC, the ratio of free to revised free hydrocarbon is > 50%, and the free hydrocarbon content is > 10 mg HC/g rock, the shale oil is favorable for exploration. Exploration and development of shale oil resources in saline lacustrine basins needs to consider the shale oil potential and its occurrence state and focus on sweet spots with good petrophysical properties, such as siltstones and carbonate rocks. [ABSTRACT FROM AUTHOR]