Epigenetic karst in carbonate buried hills and its influence on reservoir development: A case study of the Carboniferous Weixi’nan Sag, Beibuwan Basin, China

Karst is widely researched in the field of carbonate reservoirs. This study proposes a new formation mechanism and development model, specifically epigenetic karst, and asserts that high-quality reservoirs can be developed in the karst highlands and karst slopes of ancient landforms under the epigenetic karst with facies-controlled characteristics. The results indicate that there are three stages of karst in the study area. The first period is syngenetic karst, characterized by molds, intragranular dissolution pores, and micritization. Isotopic data reveal that the range of variation of carbonate rock 13C and 18O is similar to that of cement. The second period is epigenetic karst, the most significant karst in the study area. It undergoes a prolonged development time and features non-selective fabric dissolution. Oxygen and carbon isotopes exhibit negative values (18O ranges from −10.033‰ to −13.767‰ and 13C ranges from −2.56‰ to −0.52‰), which are significantly altered by atmospheric precipitation. The third period is buried karst, characterized by suture structure and asphaltene filling. The negative value of 18O in calcite veins is significantly <−15‰. The dissolved pores and caves formed by the epigenetic karst become the primary reservoir space for oil and gas in carbonate rocks. These exhibit facies-control characteristics, indicating that bioclastic limestone karst is well-developed and micritic limestone karst is underdeveloped. The study establishes a development model for karst reservoirs in the study area, selecting the highlands and slopes of ancient landforms as favorable exploration sites.