Investigating Pore Characteristics and Their Dependence on Shale Composition

Abstract

Shale reservoirs, often acting as caprocks for conventional hydrocarbon reservoirs, exhibit moderate to high porosity and remarkably low permeability. Organic-rich shales serve as reservoirs for unconventional hydrocarbons. This study focused on evaluating the characteristics of the source rocks and the factors influencing pore parameters in organic-rich shale from a Permian Basin in India, exploring its feasibility as both a CO2 sink and a natural gas source. Experimental techniques were employed to explore the mineral and the organic matter characteristics along with attributes of the pores hosted within them. The investigated shales displayed diverse thermal maturity levels, spanning from that in oil-prone to gas-prone zones, with the total organic carbon content varying from 0.72 to 24.98 wt %, indicating substantial organic richness. Rock-Eval pyrolysis results revealed a range of thermal maturity (Tmax) values between 426 and 474 °C, while X-ray diffraction analysis indicated significant quantities of illite and kaolinite, along with trace amounts of pyrite in certain samples. Field-emission scanning electron microscopy imaging and its detailed interpretation provided valuable insights into the pore structure and arrangement. In our study, we found that both the clay content and the organic matter significantly contribute to gas adsorption. While clay content strongly influences mesopore attributes, the organic matter predominantly affects micropore attributes. Furthermore, a direct relationship among fractal dimension, surface area, and pore volume, indicating increased complexities with these variables. Our examination of mesopore fractal attributes revealed that smaller mesopores exhibit a more convoluted and irregular configuration in comparison to the larger ones. These findings provide significant insights into the pore morphology of the analyzed shale sample.