Multiscale Characterization of Pore Evolution in Lesser Himalayan Black Shale from Krol Formation under Oxic Thermal Heating: Relevance to Untapped Shale Gas Extraction

Thermal stimulation under oxic heating has emerged as a transformative approach for enhancing gas recovery from tight shale reservoirs, which is typically challenging due to their complex pore structures, low permeability, and inherent anisotropy. Considering this, an untapped Neoproterozoic shale from the Lesser Himalayan region was heated up to 400 °C to investigate pore structure evolution using a combination of small-angle X-ray scattering (SAXS), low-pressure N₂ gas adsorption (LPGA), and field emission scanning electron microscopy (FE-SEM). Given the shale’s fair hydrocarbon generation potential, as indicated by total organic carbon (TOC) and vitrinite reflectance (%VRo) data, understanding its thermal-induced pore structure evolution is critical for enhancing gas extraction efficiency. The outcomes of this study demonstrate that oxic heating significantly alters the pore structure of shale, marked by a progressive shift from micropores to meso- and macropores. SAXS and LPGA analyses reveal a strong positive correlation between pore size distribution (PSD) and thermal treatment. The increased steepness in LPGA-derived BET isotherms and hysteresis loop transition from H4 to H3 confirms the development of larger pores. These substantial changes result from the coalescence and collapse of the adjacent pores during combustion. Along with the expansion of pre-existing pores, new pores were also developed during combustion, which was obvious with the findings of pore area and specific surface area (SSA) derived from SAXS and LPGA analysis. Furthermore, a notable rise in the pore area at 400 °C was also observed, suggesting the breakdown of organic matter and the formation of numerous organic matter pores. This organic matter breakdown was evident with the thermogravimetric analysis (TGA), where a rapid mass loss was observed at 400 °C. The SEM photomicrographs with widened pores, fractures, and numerous finer pores at higher temperatures further supported the above findings. This study highlights the potential of combustion-induced thermal stimulation and suggests that oxic thermal treatment can effectively alter shale microstructure, thereby offering a viable enhancement technique for shale gas recovery.