Thermal and Fluid Evolution of Paleozoic Rocks in the Sakmara Zone and Eastern Pre-Caspian Basin, Western Kazakhstan

Understanding the thermal evolution of foreland basins is essential for reconstructing hydrocarbon generation, migration, and accumulation, and for improving petroleum-system models in thrust-influenced foreland basins worldwide, including the Pre-Uralian Foredeep. However, the thermal regime of Paleozoic formations along the eastern margin of the Pre-Caspian Basin remains poorly constrained, particularly with respect to the relative roles of burial heating and tectonically driven hydrothermal circulation. This study investigates the thermal history and petroleum implications of calcite-hosted fluid inclusions in fractures and veins within Upper Silurian to Lower Permian sedimentary rocks of the Sakmara accretionary zone (Mugodzhar, Southern Urals), the Pre-Uralian Foredeep, and the eastern Pre-Caspian Basin, western Kazakhstan. Aqueous fluid inclusions yield homogenization temperatures ranging from ~50 to 280 °C and define several fluid populations. The dominant population (Th ≈ 87–146 °C) is interpreted as burial-related, whereas low-temperature inclusions (50–78 °C) reflect early diagenetic and/or late-stage fluid circulation. Higher-temperature inclusions (>180 °C) record transient hydrothermal events associated with fault-controlled fluid flow. These results indicate that the basin’s thermal evolution was governed by the combined effects of burial heating and episodic hydrothermal activity. Comparison with available Rock–Eval, vitrinite reflectance, molecular maturity, and burial-history data suggests that the sampled Paleozoic formations were generally immature to marginally mature, whereas hydrocarbons were likely generated in deeper or adjacent, more mature zones (>4 km) and subsequently migrated into the studied units. The results provide a regional framework for understanding fluid-flow and thermal processes in the Uralian foreland system and demonstrate the broader value of integrating fluid-inclusion microthermometry with structural and burial-history analyses to reconstruct basin evolution and hydrocarbon migration in foreland basins worldwide.