Petrological Controls on Nanoscale Pore Structure and Surface Morphology in Coal-Measure Shale

Abstract

Coal-measure shale (CMS) in marine–continental transitional successions is compositionally heterogeneous, but the relationship between local nanoscale surface morphology and bulk adsorbate-accessible pore space remains uncertain. In this study, atomic force microscopy (AFM), low-pressure N2 adsorption (LPNA), and low-pressure CO2 adsorption (LPCA) were applied to characterize the nanoscale surface morphology, roughness parameters, and pore structure of nine high-maturity CMS core samples from the Pingdingshan mining district at the southern margin of the North China Basin. A multiparameter correlation-analysis framework was subsequently established. The results reveal pronounced surface heterogeneity among the samples. The arithmetic mean roughness (Ra) ranges from 5.67 to 47.21 nm, the fractal dimension ranges from 2.205 to 2.900, and the AFM-derived mean surface pore diameter ranges from 8.23 to 15.88 nm. The characteristic pore-size ranges derived from AFM and LPNA broadly overlap, indicating that AFM and LPNA provide complementary information on different pore domains. TOC content is positively correlated with micropore volume, while clay-mineral content is positively correlated with Ra, suggesting that organic-matter abundance and clay-mineral content are associated with micropore development and local surface roughness, respectively. Therefore, integrating AFM, gas adsorption, and permeability measurements provides a multiscale framework for jointly characterizing local surface heterogeneity, adsorbate-accessible pore structure, and gas-transport behavior, thereby supporting a preliminary assessment of reservoir storage and transport properties.