Pore System Characterization of Untapped Himalayan Shale Using an Integrated Multianalytical Approach: Bridging Nano- to Macro-Scale Investigation

The pore network system, defined by Pore Size Distribution (PSD), geometry, and connectivity, plays a crucial role in controlling the storage and transport behavior of shale gas. Despite extensive investigations of shale formations across various Indian basins, pore characterization studies in the Lesser Himalaya remain largely unexplored. In particular, the Neoproterozoic Krol shale from the northwestern Lesser Himalaya represents one of the untapped units with promising gas-bearing characteristics. In this study, shale samples from the Krol Formation were systematically analyzed for pore characteristics across nano- to macro-scales using a combination of Low-Pressure Gas Adsorption (LPGA) with N2 and CO2, Small-Angle X-ray Scattering (SAXS), Field Emission Scanning Electron Microscopy (FE-SEM), and Micro-Computed Tomography (Micro-CT). LPGA N2 adsorption exhibited Type IIB isotherms with H3 hysteresis loops, indicating slit- and wedge-shaped mesopores, while CO2 adsorption revealed Type I isotherms, reflecting micropore filling. PSD derived from LPGA showed bimodal and multimodal patterns using Barrett–Joyner–Halenda (BJH) and Density Functional Theory (DFT) models, respectively. SAXS, sensitive to both open and closed pores, effectively captured the PSD, and the Porod–Debye Scattering Profile (PDSP) model confirmed the dominance of mesopores. FE-SEM observations revealed irregular to elongated nanopores (37–54 nm) and a moderately complex pore structure, as supported by fractal dimensions derived from LPGA, SAXS, and FE-SEM. Micro-CT analysis further indicated the presence of macropores and fractures (≥103 nm), with a mean Feret diameter of 0.07 mm and a pore volume of 0.02 mm3. Porosity values estimated from SAXS, FE-SEM, and micro-CT were 4.98%, 5.64%, and 4.55%, respectively. This integrated multiscale characterization, bridging nano to macroscale pore systems, provides new insights into the storage and transport behavior of gas and emphasizes the Krol shale as a future shale gas formation in the Lesser Himalaya.