Micro- and Mesopores Occurring in Transitional Shales: An Examination Based on Pore Radius and Origin

The microscopic pores in shales can be characterized based on different features, including pore morphology, pore size distribution and pore origin. In this work, shale samples from the Lower Permian Shanxi Formation and Upper Carboniferous Taiyuan Formation in the Linxing Block of the Ordos Basin were examined via experiments including X-ray diffraction (XRD), total organic carbon (TOC) analysis, field emission scanning electron microscopy (FE-SEM), automatic acquisition of large-image technology (MAPS), and low-temperature gas (CO₂ and N₂) adsorption/desorption experiments. The results show that clay minerals (average value of 53.9 wt.%) and quartz (average value of 39.0 wt.%) dominate the sample composition, with some K-feldspar, plagioclase, siderite, etc. The TOC values are between 1.15% and 8.46%, and all the shales are of middle-high maturity (the vitrinite reflectance (R_o) ranges from 1.23% to 1.75%). Based on FE-SEM images, the observed pore types include interparticle (interP) pores, intraparticle (intraP) pores, and organicmatter (OM) pores, and the pores and fractures at different scales are examined by MAPS. The distributions of micro- and mesopores and the proportions of the different pore sizes were determined from the N₂ and CO₂ adsorption results. The pore size distribution results suggest that N₂ mainly penetratesmesopores (2-50 nm), while CO₂ mainly penetrates large micropores (0.7-2 nm). Micropores are the largest contributors to the total pores. The mesopores were further classified as small, medium and large mesopores, and the proportion of mesopores decreased with increasing pore size.