Shale oil in the Lower Cretaceous of the Erennaoer Depression, Erlian Basin: Insights from in-situ fluid content and hydrocarbon micromigration

Understanding the occurrence and enrichment mechanisms of shale oil is essential for the effective exploration of continental basins with complex reservoir characteristics. In this study, an integrated approach combining nuclear magnetic resonance (NMR) with fluid restoration techniques, Rock-Eval pyrolysis, total organic carbon (TOC) analysis, X-ray diffraction (XRD), crude oil composition analysis, and microscopic observation was used to investigate the shale in the Erennaoer Depression of the Erlian Basin. Based on the innovative in-situ fluid content characterization method, the shale micromigration characteristics were quantitatively analyzed by micromigrated hydrocarbons (ΔQ). The coupling relationship between shale pore fluid content, pore structure, and mineral composition was analyzed, elucidating the shale pore fluid occurrence mechanisms. Three dominant shale lithofacies, felsic-rich, felsic, and calcareous, were identified, with interparticle pores at quartz grain edges serving as the primary storage space. Shale oil occurrence is jointly governed by lithofacies, pore structure, and hydrocarbon micromigration. While most samples exhibit minimal micromigration (-200 mg/g TOC < ΔQ < 107 mg/g TOC), interlaminar redistribution is common. In organic-lean shales (TOC < 0.7%), hydrocarbon generation capacity limits oil content, and no extra-micromigration occurs. In organic-rich shales (TOC > 0.7%), excess hydrocarbons are expelled and stored in interparticle pores of quartz laminae, with free oil content positively correlated with meso- to macropores development. Notably, felsic-rich shales with low clay content show evidence of intra-micromigration and preferential accumulation of light-saturated hydrocarbons (C 14 –C 18 ), resulting in ΔQ < –200 mg/g TOC, OSI > 200 mg/g TOC, and T max < 425°C. These hydrocarbons form multi-scale source–reservoir coupling systems through selective micromigration into adjacent quartz laminae and felsic-rich shale interbeds with low clay, while heavier fractions remain in organic-rich layers. These findings provide new insights into the spatial distribution of shale oil and identify favorable sweet spots in the Erlian Basin, offering a foundation for resource assessment and development strategies in similar shale oil basins. • A combined Rock-Eval pyrolysis and NMR methodology is introduced to assess in situ fluid content and hydrocarbon generation potential. • The shale “ pore fluid - pore structure-mineral” coupling relationship and occurrence pattern were systematically elucidated. • Felsic-rich shales with low clay content and high-TOC shales with quartz laminae arepresent key targets for efficient shale oil development.