Effects of Rapid Thermal Exposure from Igneous Intrusions on the Pore Structure, Thermal Maturity, and Nanomechanical Properties of Coal and Shale

Rising energy demands and concerns about global warming have made coal and shale increasingly important not only as unconventional energy sources but also as potential storage sites for CO2 sequestration. However, selecting these formations for either energy extraction or carbon storage requires comprehensive geological and petrophysical assessments. This is particularly important because, in some cases, localized thermal metamorphism from igneous intrusions can significantly alter their organic and structural properties, beyond natural thermal gradients, potentially making them unsuitable. Despite this, studies investigating the effects of igneous intrusions on the pore structure, geochemical attributes, and mechanical behavior at the micro- and nanoscale remain limited, especially in the Indian geological context. To address this gap, the present study offers the first integrated comparison of thermally altered and unaltered Indian coals and shales from the Raniganj Basin, with a focus on evaluating their suitability for CO2 sequestration. A range of analytical techniques was applied, including Rock–Eval pyrolysis, petrography, X-ray diffraction (XRD), N2 and CO2 low-pressure gas adsorption, and nanoindentation, to assess the influence of igneous intrusion on the chemical, mineralogical, pore structural, and nanomechanical characteristics of both shale and coal. The results indicate a reduction in the hydrogen index, increased thermal maturity, higher quartz content, finer pore structures, elevated pore volumes and adsorption capacities, and significantly greater Young’s moduli in the heat-altered samples compared to unaltered ones. The increased stiffness observed in the thermally altered samples is attributed to the elevated thermal maturity of organic matter, marked by a higher proportion of aromatic carbon. This transformation, driven by aliphatic chain cracking and hydrocarbon expulsion, leads to the formation of devolatilization vacuoles and the development of finer micropores. Additionally, mineral transformations documented by XRD analysis, particularly the reduction of kaolinite and the enrichment of quartz, contribute to the enhanced mechanical stiffness. These changes result from abrupt, short-duration thermal exposure at high temperatures associated with igneous activity, which contrasts with gradual burial-related maturation processes. Overall, the findings provide new insights into the transformative effects of igneous intrusions on coal and shale properties, extending beyond conventional burial histories. They highlight the implications for hydrocarbon recovery, CO2 sequestration potential, and geomechanical stability in thermally altered basins and support the need for further targeted studies.