The Significance of Diagenetic System Evolution to the Formation and Preservation of Hydrothermal Dolomite Reservoirs: A Case Study of the Permian Qixia Formation, Southwestern Sichuan, China

Understanding the evolution of diagenetic systems is crucial for elucidating the formation mechanisms of high-quality carbonate reservoirs. This study focuses on hydrothermal dolomite reservoirs developed in the platform margin zone of the Qixia Formation in the southwestern Sichuan Basin. Based on core observations, thin sections, porosity, and permeability data, carbon–oxygen isotopes, trace element geochemistry, fluid inclusions, and geophysical data, the genesis and evolution of these reservoirs were systematically analyzed. The diagenetic evolution of the reservoir can be described within the framework of penecontemporaneous (near-surface), shallow-burial, and intermediate- to deep-burial diagenetic settings. During the near-surface stage, the system was predominantly open, and the Qixia Formation experienced early dissolution, which significantly enhanced reservoir quality. During the shallow-burial stage, hydrothermal dolomitization driven by thermal convection associated with the Emeishan Large Igneous Province contributed to the preservation of primary porosity. By the Middle–Late Permian transition, intensified volcanic activity reactivated deep faults, facilitating the influx of hydrothermal fluids into the Qixia Formation. With increasing burial depth, the system evolved toward intermediate- to deep-burial conditions, becoming progressively more closed. This led to the precipitation of abundant saddle dolomite, which occluded pore space and reduced reservoir quality. These results indicate that open-system conditions in near-surface settings are more favorable for pore development, whereas reservoir formation and preservation depend on the dynamic coupling between geological processes and the evolution of diagenetic systems.