Pore Accessibility and Retained Hydrocarbon Occurrence in Lacustrine Shale Oil Reservoirs: Implications for Relative Oil Mobility in the Second Member of the Funing Formation, North Subei Basin

ABSTRACT

Lacustrine shale reservoirs are characterized by complex pore systems and strong heterogeneity. The occurrence state and relative mobility potential of shale oil are closely related to pore size, pore accessibility, and retained hydrocarbon occupation. In this study, seven shale samples from the second member of the Funing Formation in Well Huaye 7, Huazhuang Sag, North Subei Basin, were analyzed using an integrated approach, including x‐ray diffraction, total organic carbon analysis, field‐emission scanning electron microscopy, low‐pressure N ₂ physisorption (NP) before and after oil extraction, Frenkel–Halsey–Hill fractal analysis, and ultrasmall‐ and small‐angle x‐ray scattering [(U)SAXS]. The results show that the studied shale is mainly composed of felsic and carbonate minerals, with mixed shales being dominant. The main storage spaces include intragranular pores, intergranular pores, and shrinkage microfractures. After oil extraction, pore volume increases in all samples, with significant increases commonly occurring in small pores and fine pore throats, especially in the <5 nm range for several samples. Fractal analysis further shows that the surface fractal dimensions of both <5 and >5 nm pores increase after oil extraction, with a more pronounced response in the <5 nm range. These results suggest a pore size dependent occurrence pattern of retained hydrocarbons: Pore filling is interpreted to dominate in pores <5 nm, whereas pore wall spreading is more likely in pores >5 nm. Integrated (U)SAXS and NP analyses indicate that poorly accessible pore fractions range from 5.9% to 52.0%, demonstrating that a substantial proportion of nanopore space cannot be effectively detected by NP alone. Massive mixed shales generally show high poorly accessible pore fractions, whereas layered to laminated shales tend to show lower values. Overall, shale oil occurrence and relative mobility potential in the second member of the Funing Formation are jointly controlled by pore size, occurrence mode, and pore accessibility. These results provide useful insights for pore‐system evaluation and sweet‐spot prediction in lacustrine shale oil reservoirs.