Diagenesis and Porosity Evolution of Deeply Buried Carbonate Reservoirs: Insights from the Permian Maokou Formation, Sichuan Basin

Abstract

Deeply buried carbonate reservoirs represent a critical frontier and high-priority target for global hydrocarbon exploration. However, the genetic mechanisms and spatial distribution of high-quality deeply buried carbonate reservoirs remain highly contentious. This study focuses on the Middle Permian Maokou Formation in the central Sichuan Basin, integrating multidisciplinary analytical methods (petrology, geochemistry, and geochronology) to systematically investigate the diagenetic evolution sequence and pore evolution processes of deeply buried carbonate reservoirs. The lithology of Maokou Formation reservoirs is dominated by porous fine- to medium-crystalline dolostone (FMD) and tight bioclastic limestone (BL). Four stages of calcite cementation (Cal-1 to Cal-4) and two stages of dolomite cementation (Dol-1 to Dol-2) were identified in these reservoirs. The δ18O, δ13C, and 87Sr/86Sr data indicate that the genesis of Cal-1 and Cal-2, which are closest to the edges of pores and vugs within BL, was related to coeval seawater and meteoric water during the eogenetic stage, respectively. However, the primary and eogenetic porosity in the BL intervals was almost completely lost due to calcite cementation (particularly Cal-3) and compaction. FMD was derived from hydrothermal dolomitization of porous BL during shallow burial. The U–Pb age of FMD indicates that it formed after pore formation but before Cal-3 precipitation. Notably, Cal-3 is poorly preserved in FMD, indicating that FMD exhibits strong resistance to compaction and pressure solution. Dol-1 is a hydrothermal dolomite cement that precipitated following hydrothermal dissolution. Hydrothermal dissolution predominantly occurred in FMD and increased porosity by 8%–12% along fractures. In contrast, Dol-1 precipitation reduced porosity (approximately 10%) with increasing distance from major faults. Late-stage cementation (Cal-4, Dol-2, and other noncarbonate minerals) further reduced porosity by approximately 5%. Collectively, eogenetic dissolution-modified bioclastic shoals provided the foundational porosity framework for Maokou Formation reservoir development, while superimposed hydrothermal dolomitization and dissolution represented the key diagenetic processes driving high-quality reservoir formation.