Divergent Thermal Damage Pathways in Coal-Measure Sandstone and Shale: Implications for High-Temperature Energy Subsurface Systems

High-temperature conditions significantly influence the physicochemical and mechanical characteristics of rocks, directly affecting deep geological applications, such as UCG, CCUS, and nuclear waste management. This research investigates the divergent thermomechanical response of sandstone and shale from the Barakar Formation, Jharia Basin, India, exposed from room temperature to 700 °C. A comprehensive multitechnique approach utilizing UCS, BTS, ultrasonic velocity, LPGA, He Pycnometer, and SEM was employed to analyze the interplay of porosity, microstructure, and mechanical integrity with temperature. We propose a three-zone paradigm for thermal degradation using the integrated data set, which mechanistically describes the nonlinear evolution of both rock types. Zone 1 is a conditioning stage characterized by minimal microcracking, mineral stability, and the loss of absorbed water. Due to intergranular tightness, sandstone temporarily gains strength, whereas shale loses strength due to clay mineral dehydration. The key damage-acceleration window, Zone 2, is characterized by organic matter pyrolysis, clay dehydroxylation, the α–β quartz transition, and the rapid formation of interconnected fracture networks. Both rocks experience significant strength loss, while shale disintegrates catastrophically. In Zone 3, high-temperature reorganization occurs due to partial sintering or recrystallization of certain minerals, expansion of fracture networks, and material degradation. Due to bond reprecipitation, sandstone exhibits a slight strength recovery, although shale’s structural integrity is still impaired. The zone-based paradigm offers a mechanistic understanding of the temperature thresholds governing bulk instability and pore–mineral interactions. The thermal design and risk assessment in UCG, geothermal operations, nuclear waste repositories, and fire-damaged rock engineering are directly impacted by these findings.