Multiscale Pore Characterization of the New Albany Shale: Insights from Complementary Analytical Techniques

The extraction of hydrocarbons from shale formations has become increasingly important, necessitating a deeper understanding of their morphological and structural characteristics, particularly pore types and pore structure parameters, which are essential for determining the storage and productive potential of shale oil reservoirs. This research addresses this need by investigating the distribution, formation, and mineralogical relationships of pores in the three lithofacies of the New Albany Shale (NAS) in the Illinois Basin. Utilizing scanning electron microscopy (SEM), nitrogen (N₂) physisorption, small-angle X-ray scattering (SAXS), X-ray diffraction (XRD), polarizing microscopy, and TOC analyses, the study comprehensively analyzes the pore structure and morphology across these lithofacies. The Brunauer–Emmett–Teller (BET) specific surface area (SSA) ranges from 0.3 to 24.6 m²/g (average of 5.5 m²/g), with the total pore volume (TPV) ranging from 0.002 to 0.040 cm³/g (average 0.012 cm³/g). The results reveal that NAS exhibits a heterogeneous pore structure, characterized by various pore sizes and shapes, varying from ink-bottle-shaped in lithofacies NAS-1 and NAS-2 to predominantly wedge-shaped in lithofacies NAS-3. Inorganic pores, including mesopores and macropores, play a significant role in the pore system, and their presence is influenced by the specific lithofacies types within NAS. Organic matter (OM) pores exhibit limited development across all lithofacies, which could be attributed to the lower thermal maturity. In contrast, microfractures associated with organic matter and brittle minerals are comparatively well-developed. The NAS-2 lithofacies is particularly important for fluid migration due to its favorable pore structure parameters. A detailed understanding of the mechanisms of pore formation and their structural attributes across different minerals and lithofacies is crucial for advancing the exploration and development of shale oil and gas deposits.