Paleokarst Reservoir Characteristics Of Ordovician Majiagou Formation In Fuxian Region

The Fuxian region with an area of38000km2is located in the Southeast Ordos basin. Majiagou Formation in this area has deposited a set of carbonate sedimentation dominated by dolomite with evaporated tide flat phase and gypsum rock etc.Based on the numerous observations on thin sections and the rocks casting sections, combined with the application of various analytical techniques, including the conventional core analysis, capillary pressure curve, pore permeability distribution and pore structure pattern analysis, the reservoir porous structure in this area can be classified into four types:pore type、carvern type, fracture type and complex type.On the basis of the formation stage, the Ordovician palaeokarst is divided into syngenesis karst, hypergene karst and burial karst, and the most important is hypergene karst. Palaeokarst geomorphic units are divided into karst landform and karst slope. Residual hills with larger difference of relief height and minor distribution range are tower karst on the karst slope. which have in karst slope. The development of residual hill, which is the main exploration area in karst slopes, mainly controlled by mineral composition structural setting, diagenetic environment, The formation and filling of the Ordovician dissolution pores indicate the creation and evolution of reservoir space during karstification. Almost all of the carbonatite and carbonate karst breccia have higher87Sr/86Sr values than ordinary Ordivician carbonat in the world. Reservoir space of the Majiagou formation is mainly composed of dissilution bores, caves and fractures, and characterized with lower porosity and permeability and high anisotropism. Palaeokarst topography was affected by karstification and different geologic processes. The different topography forms had controlled karst reservoir development. Kast residual slope generally have high porosity, permeability, reseivoir thikness, bauxitite thickness and are main optimum reservoir district.