Coupled Evolution and Mechanism of Pore Structure and Thermal Maturity in Alkaline Lacustrine Dolomitic Shale

Abstract Alkaline lacustrine shale oil is a critical type of unconventional hydrocarbon resource. However, systematic insights into the coupled evolution between multiscale pore structure and thermal maturity in alkaline lacustrine dolomitic shale are still inadequate, limiting effective exploitation of this resource. Therefore, this study investigates the coupled evolution of thermal maturity and multiscale pore structure in alkaline lacustrine dolomitic shale from the Lower Ganchaigou Formation in the Qaidam Basin. Combining semiclosed pyrolysis simulation with multiple pore characterization techniques, we elucidate the evolutionary patterns and controlling factors of pore structure during thermal maturation. Results reveal that pore volume and specific surface area increase significantly with increasing Ro, accompanied by an overall shift toward larger pore sizes. The volumes of all pore types increase progressively, and differences among their volumes dominate pore heterogeneity. Pore heterogeneity and connectivity exhibit distinct stage-dependent variations. At low maturity (Ro < 0.63%), heavy hydrocarbon filling blocks pore throats and reduces pore space. Within the favorable mature stage (0.63%–1.32% Ro), organic acid dissolution effectively enlarges carbonate mineral pores and boosts macropore development. At the high- to overmaturity stage (Ro > 1.32%), hydrocarbon transformation improves pore connectivity, while mineral recrystallization and compaction homogenize pore distribution and weaken heterogeneity. The maturity interval of 0.63%–1.32% is confirmed as the optimal golden window for reservoir space and retained oil coenrichment. This finding offers quantitative references for pore evolution research and practical guidance for the prediction of favorable shale oil areas in alkaline lacustrine shale sequences.