Fractal Characteristics and Controlling Factors of Pore Structure in Marine-Continental Transitional Shale in the Shanxi Formation, Central Ordos Basin

Abstract

To examine the fractal characteristics and pore architectures within marine-continental transitional shale, experimental analyses were carried out for Shanxi shale samples collected from the central Ordos Basin. Multiple test approaches including scanning electron microscopy (SEM), X-ray diffraction analysis, total organic carbon (TOC) analysis, and gas adsorption experiments were applied in this work. According to experimental measurements, the mineral composition of collected Shanxi shale samples exhibits dominant quartz and clay minerals, averaging 38.2% and 54.36%, respectively, with an average TOC content of 1.635%. CO2-accessible micropores make an important contribution to specific surface area, whereas N2-accessible mesopore range pores contribute appreciably to pore volume. The pore size distribution is characterized by two dominant peaks within the ranges of 8–20 nm and 80–85 nm. Mineral pores constitute the primary pore type within Shanxi shale, while pores associated with organic matter are infrequently observed. Micropore-specific surface area parameters show positive correlative trends relative to TOC, yet only weak correlations exist between this parameter and quartz and clay minerals. Meso-macopore-specific surface area exhibits a negative correlative tendency versus TOC content, weakly correlated with clay minerals, and no relationship with quartz content. Pore volume magnitudes are predominantly controlled by organic matter abundance and clay mineral content, with quartz content having no significant impact. Fractal dimensions computed through the Frenkel–Halsey–Hill (FHH) model indicate that D1 values for the low-pressure interval vary between 2.654 and 2.834, and D2 corresponding to the high-pressure interval are distributed from 2.638 to 2.867. The internal pore structure’s complexity and heterogeneity are more pronounced than the surface roughness of the pores. Compared with typical marine and continental shales, marine-continental transitional shale displays intermediate characteristics in both pore morphology and fractal features. The key results demonstrate that TOC content has strong positive linkages with fractal dimensions D1 and D2. By contrast, clay minerals exhibit a marked negative association against D2. Quartz content does not show a significant correlation with D1, yet faint positive linkages are detected between this indicator and D2. The present work offers quantitative constraints to assess pore structure heterogeneity and gain insights into reservoir properties of marine-continental transitional shale.