Microscopic pore structure characteristics and mobility of shale oil reservoirs in Liushagang Formation, Weixinan Sag, Beibu Gulf Basin

Shale oil reservoirs are characterized by tightness and strong heterogeneity, and the microscopic pore structures affect the storage and flow of shale oil in reservoirs. However, conventional single analytical methods often fail to accurately characterize these microscopic pore structures. This study aims to reveal the microscopic pore structure and mobility characteristics of shale oil reservoirs, thereby guiding efficient exploration and development of offshore shale oil. Three types of shale oil reservoirs—matrix-type, laminated-type and interbedded-type—in the Liushagang Formation, Weixinan Sag, Beibu Gulf Basin were selected as the research objects. Integrated analytical and testing methods were employed, including cast thin-sections, scanning electron microscopy, high-pressure mercury intrusion, nitrogen adsorption, and nuclear magnetic resonance, to analyze pore structure parameters, mercury intrusion morphology, and adsorption curve characteristics. The results showed that the matrix-type and laminated-type reservoirs exhibited finer grain sizes and relatively underdeveloped pores, dominated by slit-shaped pore morphologies. These reservoirs commonly featured bedding fractures, organic pores, interlayer pores within clay minerals, and intercrystalline pores within pyrite. The interbedded-type reservoirs mainly had ink-bottle-shaped pore morphologies, along with intergranular pores within mineral particles, dissolution pores, and fracture networks, showing good pore size distribution and reservoir connectivity. Analysis of shale oil mobility through fluorescence thin sections and nuclear magnetic resonance revealed that the matrix-type and laminated-type reservoirs exhibited relatively poorer mobility, with movable porosities of 0.72% and 4.62%, respectively, along with lower movable oil content. The interbedded-type reservoir exhibited a movable porosity of 6.37%, with lighter hydrocarbon components, better mobility, and higher movable oil content, making it the most favorable reservoir type for shale oil exploration in the Weixinan Sag.