The microscopic occurrence characteristics primarily constrain the enrichment and mobility of shale oil. This study collected the lacustrine shales from the Palaeogene Funing Formation in the Gaoyou Sag, Subei Basin. Conventional and multistage Rock-Eval, scanning electron microscopy, and nuclear magnetic resonance (NMR) T₁–T₂ were performed to analyze the contents and occurrence characteristics of shale oil. Low-temperature nitrogen adsorption-desorption (LTNA/D) experiments were conducted on the shales before and after extraction. The relationships between shale oil occurrence with organic matter and pore structures were then discussed. Predominantly, the shale oil in the Funing Formation is found within fractures, with secondary occurrences in interparticle pores linked to brittle minerals and sizeable intraparticle pores associated with clay minerals. The selected shales can be categorized into two types based on the nitrogen isotherms. Type A shales are characterized by high contents of felsic and calcareous minerals but low clay minerals, with larger TOC and shale oil values. Conversely, Type B shales are marked by abundant clay minerals but diminished TOC and shale oil contents. The lower BET specific surface area (SSA), larger average pore diameter, and simpler pore surfaces and pore structures lead to the Type A shales being more conducive to shale oil enrichment and mobility. Shale oil content is predominantly governed by the abundance of organic matter, while an overabundance of organic matter typically equates to a reduced ratio of free oil and diminished fluidity. The BET SSA, volumes of pores less than 25 and 100 nm at extracted state all correlate negatively with total and adsorbed oil contents but display no correlation with free oil, while they have positive relationships with capillary-bound water. Consequently, pore water is mainly saturated in micropores (<25 nm) and minipores (25–100 nm), as well as adsorbed oil, while free oil, i.e., bound and movable oil, primarily exists in mesopores (100–1000 nm) and macropores (>1000 nm). These findings may enhance the understanding of the microscopic occurrence characteristics of shale oil and will contribute to guide resource estimation and shale oil sweet spot exploitation in the Gaoyou Sag, Subei Basin.

The occurrence characteristics of shale oil are of great significance to the movability of shale oil. In this study, the occurrence characteristics of oil in the shale matrix at Funing Formation shale in Subei Basin were quantitatively evaluated by organic geochemistry and microscopic pore structure characterization experiments. The Multiple Isothermal Stages Pyrolysis (MIS) experiment results show that the content of total oil, adsorbed oil, and free oil in the shales are 3.15–11.25 mg/g, 1.41–4.95 mg/g, and 1.74–6.51 mg/g, respectively. among which the silicon-rich shale has the best oil-bearing. The relative content of free oil shows an increasing trend in pores with pore diameters greater than 3 nm. When the relative content of free oil reaches 100%, the pore size of silicon-rich shale is about 200 nm, while that of calcium-rich shale, clay-rich shale, and siliceous mixed shale is about 10 nm. The occurrence law of adsorbed oil is opposite to that of free oil, which indicates that shale oil will occur in the pores and fractures in a free state in a more extensive pore size range (> 200 nm). This study also enables us to further understand the occurrence characteristics of shale oil under the interaction of occurrence state and occurrence space.