Pore Structure Characteristics and Main Controlling Factors of Lacustrine Hybrid Shale of China: A Case Study of the Fengcheng Formation in the Mahu Sag

In recent years, significant breakthroughs have been achieved in the exploration of lacustrine shale oil in China, with mixed sedimentary shale oil in saline–alkaline lakes demonstrating substantial resource potential. This study focuses on the shale of the Fengcheng Formation in the Mahu Sag, Junggar Basin. By integrating field-emission scanning electron microscopy (FE-SEM), low-temperature nitrogen adsorption (LTNA), mercury intrusion capillary pressure (MICP), and low-field nuclear magnetic resonance (NMR), the pore structure and fractal characteristics of the shale were systematically characterized both qualitatively and quantitatively. The results indicate that inorganic pores dominate in lacustrine shale, including interparticle pores, intraparticle pores, intercrystalline pores, and microfractures, with pores in the II-micropore range (10–100 nm) contributing most significantly. The fractal dimension D<sub>max</sub> (2.9204) exceeds D<sub>min</sub> (2.2572), suggesting that the structural complexity of the pore system is greater than its surface roughness. Quartz exerts a dual influence on pore development, while higher calcite content is conducive to pore formation. In contrast, the cementation effect of dolomite may limit its positive contribution to porosity. Additionally, the relatively well-developed fractures in alkaline lacustrine shale facilitate the formation of complex fracture network systems, which are of critical importance for shale oil enrichment and reservoir stimulation.