<p>This study examines the impact of CO₂- and N₂-rich brine interactions on the anisotropic fracture properties of shale rocks, specifically targeting carbonate-rich Eagle Ford and silicate-rich Permian formations under high-pressure and high-temperature conditions. Utilizing batch-type reaction experiments and micro-scratch testing, the research investigates how exposure to reactive brines induces changes in the mechanical properties of shale, revealing distinct zones of ductility and brittleness within the rock structure. Key observations include the mapping of scratch and vertical hardness profiles along the scratch path. The vertical-to-scratch hardness ratio (Hv/Hs) and its rate of change serve as critical indicators of whether the shale exhibits ductile or brittle behavior during the test. A lower Hv/Hs ratio, particularly near the reacted surface, reflects increased compressibility and ductility, prominently observed in both CO₂- and N₂-reacted samples. Notably, the Permian CO₂-reacted sample in the arrester direction exhibited a higher Hv/Hs ratio further from the reacted surface, suggesting increased brittleness during testing deeper within the altered region. These patterns are interpreted within a spatially zoned chemo-mechanical framework: the heavily reacted zone (HRZ), where mineral dissolution and mechanical weakening dominate; the Sub-Reacted Zone (SRZ), showing interfacial alterations and moderate damage; and the Unaltered Zone (UZ), where the rock retains its original properties. The study also explores potential mechanisms contributing to increased brittleness, particularly within the SRZ, focusing on the role of clay minerals in the context of CO₂–brine–rock interactions, and highlights the need for further investigation to fully elucidate the governing chemical processes. These findings offer critical insights into the chemo-mechanical evolution of shale under reactive conditions and inform applications such as geo-structure stability, CO₂ sequestration, and shale gas development.</p>