Mineral Alteration and Pore-Fracture Structure Evolution of Different Lithologies in the Liujiagou Formation of the Ordos Basin during CO ₂ Geological Storage

To clarify the evolution of mineral composition and pore-fracture structure in different lithologies of the Liujiagou Formation, Ordos Basin, during CO2 geological storage, three representative lithologies were investigated: the fine-grained sandstone–mudstone interface, sandy mudstone, and fine-grained sandstone. These samples were collected from depths of 1900–2200 m. They were immersed in CO2-saturated brine for 30 days under simulated formation conditions of 80 °C and 20 MPa. Mineral, fracture, and pore changes were then characterized using scanning electron microscopy (SEM), computed tomography (CT), and high-pressure mercury intrusion porosimetry (MIP). Calcite dissolution dominated in the interface samples. In sandy mudstone, extensive clay mineral dissolution generated numerous dissolution pores. In fine-grained sandstone, partial K-feldspar dissolution occurred together with slight quartz precipitation. Fracture evolution also differed markedly among the three lithologies. The fracture volume fraction increased 2.85-fold in the interface samples and showed enhanced connectivity. In sandy mudstone, it increased 7.12-fold, but most of the newly formed fractures remained isolated microfractures. In fine-grained sandstone, the main change was fracture widening from 0.02–0.03 to 0.03–0.04 mm. Pore structure evolution was also lithology dependent. The average pore diameter of fine-grained sandstone increased by more than 170%, accompanied by an increase in pore volume. Sandy mudstone showed the highest total pore volume after immersion, reaching 0.0192 cm3/g. In contrast, both pore volume and average pore diameter decreased in the interface samples. Fractal dimension analysis showed that sandy mudstone had the highest pore complexity, whereas fine-grained sandstone exhibited reduced heterogeneity after immersion. Overall, fine-grained sandstone is the most favorable storage interval. Sandy mudstone may serve as a potential auxiliary sealing layer. In contrast, the sandstone–mudstone interface requires targeted leakage risk control. These findings provide a basis for strata selection and safety assessment for CO2 geological storage in the Ordos Basin.