Diagenesis and diagenetic facies of crust-weathered ancient karst carbonate reservoirs

In the north of the Jingbian Gas Field in the Ordos Basin, a case study was made of the first section of the fourth sub-member of the fifth member of the Majiagou Formation (M5₄¹). Through detailed subsurface geological analysis, a realistic description of typical diagenetic features, systematic sampling and laboratory testing, with geological background and petrological characteristics as main clue, the diagenesis characteristics of crust-weathered ancient karst carbonate reservoirs were analyzed, and the diagenetic facies were recognized. The marine diagenetic environment of M5₄¹ was influenced by dolomitization and anhydritization, and was controlled by the palaeogeographic framework. As to the epidiagenetic environment controlled by ancient karst physiognomy, dissolution and packing effects of dissolved pore, fracture and cave types took place. For the buried diagenetic environment, extensive dissolution was common. Based on the coupling relationship between diagenesis and porosity-permeability network, a "diagenesis in marine diagenetic environment + diagenesis in epidiagenetic environment" combination principle was adopted to divide the diagenetic facies into four typical types: (Ⅰ) medium-weak filling facies of anhydrite small nodule dolomitization-dissolved pore and fracture type; (Ⅱ) medium-strong filling facies of anhydrite small nodule dolomitization-dissolved pore and fracture type; (Ⅲ) strong filling facies of dolomitization-dissolved cave type; and (Ⅳ) strong filling facies of anhydrite lithification-dissolved pore and fracture type, and strong filling facies of dissolved cave type. With well-developed premium reservoir rocks, moderate karst intensity and medium-weak (deposition) filling, the type-Ⅰ area was favorable for fracture-cave reservoir development.