Fractal disposition, porosity characterization and relationships to thermal maturity for the Lower Permian Raniganj basin shales, India

Pore structural parameters of organic-rich shales control their gas storage properties and gas transport behaviors. In this work, pore structural parameters of selected organic-rich shale samples belonging to the Lower Permian Barakar Formation from Raniganj basin (India) are studied using low-pressure nitrogen adsorption combined with field emission scanning electron microscopy (FE-SEM), Rock-Eval pyrolysis and energy-dispersive X-ray spectroscopy (EDX). The samples are predominantly mesoporous and in the early to peak range of thermal maturity, with one late mature sample affected by a nearby igneous intrusion. The Brunauer, Emmett, and Teller specific surface area (BET SSA) of the samples display no correlation with total organic carbon (TOC), and a positive correlation with Rock-Eval Tmax. The average pore radii of the shale samples display a negative relationship with the BET SSA and Rock-Eval Tmax values. As thermal maturity advances finer pores are formed, which progressively increases the SSA of shales. A novel fractal-discriminating parameter, ΔS [(slope of the linear segment at P/P0 of 0.5-1.0) - (slope of the linear segment at P/P0 of 0-0.5), where S = D-3] displays a minimum value for a silty-shale sample, and a maximum value for the intrusion-affected shales. The less distinct fractal dimensions for the silty-shale, and the most distinct fractal dimension for the intrusion-affected shales shows influence of both the mineral composition and thermal maturity on fractal dimensions of shales. Among the samples studied, the intrusion-affected shales exhibited the highest BET SSA, pore volume, least average pore radius, larger ΔS and fractal dimensions (D1 and D2), all signifying the impact of high thermal maturity (thermal stress) on pore structural parameters.