To address the challenges of conflicting macroscopic mechanical tests and the inability to reveal complex nanoscale mechanisms during deep shale thermal modification, this study comprehensively investigates the microstructural and nanomechanical evolution of Longmaxi shale under heat treatment. This involves the innovative combination of gas adsorption, atomic force microscope high-resolution mapping, and multi-level spatial statistical analysis to systematically elucidate the spatial evolution of the pore structure of shale, surface morphology, and nanomechanical properties. The findings reveal a unique hardening-softening-rehardening three-stage pattern: From 25-400 ℃, the average reduced modulus increases due to dehydration; from 400-600 ℃, organic matter pyrolysis significantly decreases the modulus, with intense atomic force microscope topographic uplift; from 600-900 ℃, the modulus slightly increases again due to structural collapse and pore network regeneration. Local indicators of spatial association analysis shows that this macro evolution stems from synergistic microscopic phase space reshaping. Crucially, the mesopore volume increases most significantly in the 400-500 ℃ range, and it exhibits its most notable increase. Considering energy efficiency and feasibility, this study demonstrates for the first time that 400-500 ℃ is an ideal temperature window for effective organic matter pyrolysis and nanopore optimization from a nanoscale spatial distribution and geometric stability perspective. This work provides crucial micro-mechanical mechanism support and precise temperature guidance for deep shale thermal modification, significantly outperforming the temperature ranges from traditional macroscopic experiments and filling the gap in macro-nanoscale mechanical discrepancies and spatial feature analysis.

This study employs transformative nanoscale-spatial methodology, which reveals the complete thermal damage evolution mechanism of sandstone from 25 to 700 ℃, fundamentally challenging the conventional linear degradation paradigm. Spatial autocorrelation analysis is innovatively applied to atomic force microscopy data, establishing that thermal damage follows a non-monotonic pathway governed by spatial heterogeneity evolution rather than simple progressive weakening. Four critical temperature thresholds are identified: 200 ℃ marks the initiation of localized damage through expansion of weak zones with 61.4% stiffness reduction; 400 ℃ features unexpected transient hardening due to clay mineral dehydroxylation and pore restructuring, reaching the peak adhesion force; 500 ℃ represents not only increased damage but also the thermal decomposition of the 400 ℃ hardened structure, evidenced by maximum spatial heterogeneity and negative adhesion; and the 700 ℃ following quartz phase transition establishes a completely reconfigured structure with a novel "harder-in-lower-areas" pattern. Crucially, this research demonstrates that the spatial clustering of mechanical properties evolves from balanced high-low zones at 200 ℃ to a percolating low-stiffness network at 500 ℃, establishing spatial reorganization as the governing principle of thermal damage. The findings provide a quantitative framework that can accurately predict rock behavior in geothermal systems and underground energy storage applications.

Oil shale is characterized by a dense structure, low proportion of pores and fissures, and low permeability. Pore-fracture systems serve as crucial channels for shale oil migration, directly influencing the production efficiency of shale oil resources. Effectively stimulating oil shale reservoirs remains a challenging and active research topic. This investigation employed shale specimens obtained from the Longmaxi Formation. Scanning electron microscopy, fluid injection experiments, and fluid–structure interaction simulations were used to comprehensively analyze structural changes and fluid flow behavior under high temperatures from microscopic to macroscopic scales. Experimental results indicate that the temperature has little effect on the structure and permeability of shale before 300 ℃. However, there are two threshold temperatures within the range of 300 to 600 ℃ that have significant effects on the structure and permeability of oil shale. The first threshold temperature is between 300 and 400 ℃, which causes the oil shale porosity, pore-fracture ratio, and permeability begin to increase. This is manifested by the decrease in micropores and mesopores, the increase in macropores, and the formation of a large number of isolated pores and fissures within the shale. The permeability increases but not significantly. The second threshold temperature is between 500 and 600 ℃, which increases the permeability of oil shale significantly. During this stage, micropores and mesopores are further reduced, and macropores are significantly enlarged. A large number of connected and penetrated pores and fissures are formed. More numerous and thicker streamlines appear inside the oil shale. The experimental results demonstrate that high temperatures significantly alter the microstructure and permeability of oil shale. At the same time, the experimental results can provide a reference for the research of in-situ heating techniques in oil shale reservoir transformation.