Quantitative Analysis of Pore Complexity in Lacustrine Organic-Rich Shale and Comparison to Marine Shale: Insights from Experimental Tests and Fractal Theory

Continental shale gas enrichment is significantly influenced by pore structure, yet research on this topic remains limited compared to marine shale. This study investigates the complex pore structure of lacustrine Da'anzhai (DAZ) shale from the YB area, employing X-ray diffraction (XRD), scanning electron microscope (SEM), and low-temperature N2 adsorption. The fractal dimension (FD) of the pore structure, calculated using the Frenkel–Halsey–Hill (FHH) model, revealed two distinct dimensions: D1 (2.3438–2.7933) and D2 (2.5235–2.82). Both D1 and D2 exhibited positive correlations with total organic matter (TOC) content, thermal maturity, and other pore structure parameters, with micropores exhibiting the strongest influence. Results indicate that the DAZ shale comprises quartz (28.1%–42%), clay (41.5%–60%), and carbonate minerals (0.7%–11.2%), with TOC content ranging from 0.32% to 1.74%, indicating low to high thermal maturity. Total pore volume (PV) varies from 0.0105 to 0.0193 cm3/g, and specific surface area (SSA) from 3.14 to 11.33 m2/g. While micropores and mesopores dominate the pore size distribution, with micropores contributing most to SSA, mesopores and macropores significantly influence overall PV. Interestingly, regular pore morphology, primarily associated with well-sorted and rounded terrigenous quartz, negatively correlated with D1 and D2. Conversely, feldspar primarily contributed to macropore formation, exhibiting a negative correlation with both FD. Clay minerals, compacted over time, contribute to reduced pore size and increased micropore abundance, resulting in a positive correlation between intricate pore morphology and both D1 and D2. Compared to the marine Longmaxi (LMX) shale, the lacustrine DAZ shale exhibits lower organic matter (OM) and quartz but higher clay mineral content. Furthermore, pore types in the DAZ shale are dominated by intergranular pores within clay minerals, and the development of OM pores is significantly lower than in the LMX shale. These key differences result in higher pore structure complexity and FD in marine shale compared to lacustrine shale. This comparative analysis of DAZ shale provides valuable insights into the distinct pore structures of lacustrine shale and offers a framework for comparative analysis of pore structures in different sedimentary facies.