Study on the Development, Evolution, and Main Controlling Factors of Gypsum Moldic Vugs in Ma541, Central Ordos Basin

This study focuses on Member 5₄¹ of the Majiagou Formation in the central Ordos Basin. By comprehensively applying analytical techniques such as core observation, X-ray diffraction (XRD), and carbon and oxygen isotope analysis, it systematically reveals the development and evolution patterns of gypsum moldic pores. The study finds that gypsum moldic pores are mainly hosted in gypsum-bearing dolomites, and their development process is superimposed and modified by multiple diagenetic stages, including a sedimentary environment, epigenetic meteoric freshwater dissolution with differential filling, and burial dissolution enlargement. During the syngenetic stage, anhydrite commonly occurs as small nodules or euhedral prismatic crystals, mostly hosted in the micritic dolomite matrix, with significant stratigraphically controlled spatiotemporal distribution. Vertically, it shows enrichment during regression periods; horizontally, it distributes in a zonal pattern along the shallow-water basin-margin flat subenvironment between the Shaanxi Depression and the Central PaleoUplift structural belt of the Majiagou period. In the epigenetic diagenetic stage, meteoric freshwater intruded along fractures and the dissolution of anhydrite nodules, accompanied by a network of fractured and fragmented seams, jointly formed the early gypsum mold pore network. These pores were often filled to varying degrees with finely microscopic dolomite, vadose silt, or calcite. Controlled by differential karstification in paleokarst geomorphic units, the filling degree of gypsum mold pores shows horizontal differentiation, increasing gradientally from the western paleogeomorphic highlands to the eastern paleogeomorphic trenches. The burial diagnostic stage was dominated by secondary dissolution, mainly involving further enlargement of gypsum moldic pores, dissolution of intergranular pores in vadose silt, and dissolution of the dolomite bedrock. Taking Submember 513 as an example, the analysis shows that major producing areas, such as Hengshan-Jingbian-Zhangqu, were distributed in a gypsum–dolomite flat environment during the sedimentary period, located in the paleogeomorphic unit of the weathering crust karst platform in the epigenetic stage, with moderate-to-weak karst intensity. A large number of gypsum mold pore networks developed here, without undergoing intense erosion or calcite filling, and were further improved by superimposed burial dissolution, leading to the large-scale development of high-quality reservoirs. This reveals the development pattern of gypsum mold pores and the three-factor reservoir-controlling mechanism characterized by “material basis of gypsum–dolomite flat─effective epigenetic dissolution with weak filling─superimposed burial modification.” The results provide a theoretical basis for the evaluation of carbonate reservoirs in similar basins.