Study On The Reservoir Characteristics And Control Factors Of Ordovician Dolomitereservoirs In The East Of Central Paleouplift In Ordos Basin

Based on core observation，thin-section analysis and geochemical analysis of dolomite inthe fifth member of Majiagou Formation in the east side of Central Paleouplift in OrdosBasin，the formation mode of dolomite in study area was discussed. The carbon and oxygenisotopes analysis indicate that the isotope of δ13C is around null，and the values of δ18O displaynegative distribution. Lanthanon Ce anomalies is not obvious but the negative Eu anomalies isstrong.Meanwhile，the molar ratio of major element Mg/Ca has large range. All theseevidences indicate that the formation of dolomite in study area has three stages: evaporation-submergence-Burial dolomitization.There are3types of dolomite reservoir space in the upper group of the fifth member ofMajiagou Formation including cavity，pore and fissure.The main reservoir space is anhydritenodules dissolved pore and crevice, followed by cave，intergranular pore and dissolved pore.There are4types of dolomite reservoir space in the middle group including dolomiteintercrystal pores， intergranular micropores，Intergranular dissolved pore and dissolvedpores，The development of fissure is beneficial to improve the porosity and permeability ofreservoir.In the seawater，intergranular pore was filled with cement，fenestra is destroyed. Gypsumand halite with dolomite formation interbed and then bounded the reservoir，or sometimesementation destroyed the pore.The meteoric water has great contributed to dolomite reservoirpore in the fifth member of Majiagou Formation. The dissolution of unstable mineral formedpart of moldic pore and intercrystal microchannel,next, paleokarstification formed lots ofmoldic pore, cave and solution fractures. In burial environment, dolomitization, rejuvenationof crystals, denudation,buried karstification and the fracturing effect were beneficial to theformation of pores, filling and cementation took moreserious destruction of the reservoir pore.The distribution of the fifth member of Majiagou Formation dolomite reservoir iscontrolled by sedimentary facies belt and ancient karst landform: gypsum-dolomite flat andgypsodolomite flat are the best reservoir belts for development of Paleokarst reservoir in uppergroup，the best belts for dolomite reservoir in upper group are grain beach and dolomiticflat.The study area is located in the slope area of ancient weathering crust karst，Erdos Basin， it is beneficial for reservoir development.