Micro-nanoscale pore structure characteristics and enrichment mechanisms of shale oil in mixed sedimentary reservoirs of the Jimusar Sag, Junggar Basin, northwestern China

The successful development of shale oil in the United States has completely changed the global energy landscape. In recent years, shale oil in the Junggar Basin in China has attracted widespread attention. The Permian Lucaogou Formation in the Junggar Basin features a mixed sedimentary reservoir with a complex pore structure, which hinders the understanding of the micro- and nano-scale enrichment process of shale oil. In this paper, high-pressure mercury injection (HPMI), low-temperature nitrogen adsorption (LTNA), and laser scanning confocal microscopy (LSCM) are conducted to quantitatively characterize the pore structure of the reservoir. The results show that the pore size distribution is bimodal, with the main peak appearing near 1.89–3.15 nm and the secondary peak appearing near 30.75–84.58 nm. The specific surface area (SSA) ranges from 3.155 m²/g to 20.681 m²/g, and the mesopores are the main contributor. The pore space is characterized by multiple fractals, with D_M ranging from 2.0366 to 2.9872 derived from high-pressure mercury injection. The D_N1 and D_N2 obtained from low-temperature nitrogen adsorption are 2.1662–2.6254 and 2.0768–2.7283. Oil content is strongly influenced by reservoir porosity. The fractal characteristics of pores (2.32–36.90 nm) have a more obvious effect on oil content. The maturity stage of organic matter determines the generation of light and heavy components in shale oil, while the coupling of charging force and pore capillary resistance controls the enrichment pattern of light components in the pore center and heavy components in the pore edge. The results are helpful for the optimization of favorable shale oil blocks and provide theoretical guidance for the exploration of oil and gas resources and the development of oil recovery technology.