Reservoir characteristics and distribution of the Majiagou Formation in the Fuxian area of Ordos Basin, China

The Majiagou Formation in the Fuxian area of the southeastern Ordos Basin has undergone a complex diagenetic evolution history under the influence of eustacy and the Caledonian karstification, resulting in several complex reservoir types. Through analyses of mineralogy, petrology, and reservoir geology, three major types of dolomite reservoirs with different genetic mechanisms, including anhydritic moldic-dissolved pore type, dolomitic intercrystalline-pore type, and fractured type were identified, and their formation mechanisms and distribution patterns were examined. The aphanocrystalline to very fine-crystalline anhydritic dolomite was resulted from Sabhak dolomitization, and is characterized by small size of crystals and high content of anhydrite. Dolomite reservoirs of anhydritic moldic-dissolved pore type were developed in multi-stage dissolution processes and mainly distributed at higher positions of the paleogeomorphology where the filling was weak. The very fine to fine-crystalline dolomite of shoal facies was formed under seepage-reflux dolomitization, and characterized by larger sizes of crystals and well-developed intercrystalline pores. Dolomite reservoirs of intercrystalline-pore type were mainly developed at the lower positions of the paleogeomorphology where bedding-parallel karst dissolution was strong. The fractured dolomite reservoirs, generated by the anhydrite swelling and karst cave collapse, occur in multiple horizons but within limited areas due to multi-stage fillings.