Karstification and Its Role in the Formation of Carbonate Reservoirs: A Case Study from the Ordovician Majiagou Formation in Jingbian, Ordos Basin, North China

The discovery of natural gas reserves in Lower Paleozoic carbonate formations has generated significant enthusiasm regarding the potential for oil and gas exploration in the deeper carbonate reservoirs of the Ordos Basin. Significant progress has been made in the field of natural gas exploration, especially within the carbonate reservoirs of the Ordovician Majiagou Formation. In this study, we conducted a comprehensive analysis, including microscopic examination of thin sections and the inspection of 60-m cores from five wells, to classify pore types and investigate various forms of karstification in the fifth member of the Majiagou Formation. Our findings have identified distinct pore types, including interparticle pores, separate-vuggy pores, and touching-vuggy pores, in this formation. The dominant forms of karstification in this area were identified as syngenetic karstification, weathering crust karstification, and burial karstification. Importantly, our investigation emphasizes the significant influence of factors such as sea level fluctuations, exposure duration, and the presence of acidic formation water in shaping the observed patterns of karstification. Syngenetic karstification and shallow-buried karstification have emerged as key stages in the formation of natural gas reservoirs. Moreover, our research emphasizes the importance of structural and composite fractures observed in the fifth member of the Majiagou Formation. These unfilled fractures serve as crucial microfracture networks, facilitating oil and gas migration and contributing to the overall development of reservoirs. The research results are of great significance for understanding the formation process of carbonate reservoirs in the Majiagou Formation, Ordos Basin.