Organic and Inorganic Pore-Fracture Networks in Low-Maturity Lacustrine Shale: Insights from SEM Analysis in the Dongying Depression

The spatial distribution, scale, and structural properties of organic and inorganic pore-fracture networks are critical to evaluating shale petrophysical properties. This study conducted a quantitative analysis of the Shahejie Formation shale in the Dongying Sag based on field emission scanning electron microscopy (FE-SEM), revealing the organic-inorganic pore-fracture network characteristics, fractal characteristics and geological controlling factors of low-maturity shale. , and the influence of magnification on the observation results of pore structure was discussed. Results show that shale develops three types of pores: nanometer-sized intragranular pores (such as intercrystalline pores of clay minerals, calcareous mineral dissolution pores, and pyrite intercrystalline pores), and nanometer- to micron-sized intergranular pores (such as brittle minerals). intergranular pores and shrinkage fractures in organic matter) and micron-to-millimeter-scale fractures (such as feldspar fractures and organic-inorganic interface pores/fractures). Among these pores, inorganic pores account for 91.45% of the total pores and contribute 85.1% of the porosity; organic pores account for 8.54% and contribute 14.9% of the porosity; the surface porosity provided by organic pores is 2.2~2.6 times that of inorganic minerals. Among inorganic mineral pores, quartz and clay pores contribute about 49.8% of the storage space, followed by calcite (23.7%). Fractal analysis shows that inorganic pores have higher structural complexity, while organic pores dominated by organic-inorganic interface pores have lower fractal dimensions and relatively weak structural complexity and heterogeneity. The pore size distribution is unimodal, ranging from 10 nm to 4 μm, mainly concentrated in the 200 nm to 1 μm range. As the pore size increases, the contribution of pores of different scales to surface porosity gradually increases, with the micron-scale pore network accounting for 44.8% of the pore volume. The development of nanoscale pores is closely related to the proportion of clay mineral pores, and the degree of crack development is jointly controlled by the main diagenetic minerals (such as clay, quartz, and calcite). Magnification has a significant effect on surface porosity and pore complexity. At magnifications from 5000× to 20000×, the surface porosity of micropores (<200 nm) increased by 29.03 times, and the surface porosity of submicron pores (200-1000 nm) increased by 16.07 times. Fractal analysis further shows that the morphological complexity of inorganic pores is higher than that of organic pores. The number and surface porosity of mineral pores are closely related to mineral content. Pyrite has the largest porosity increment per unit area (2.19), while calcite has the smallest (0.732). These research results provide important data support for sustainable exploration of low-maturity shale and assessment of geological carbon sequestration potential.