| Based on the analysis of regional of geologic background and core,slice and logging,as well as geochemistry,the author take a system research for petrological features, the type of karst, diagenesis of palaeokarstic reservoir Majiagou Formation, Ordovician, South area of Ordos Basin. The auther also analyse control factors about palaeokarst and control factors about palaeokarstic reservoir.Results showed:The study area, mainly with karst breccias and micritic dolomite,the m5of Majiagou formation in southern ordos is deposited in limited carbonate platform under the condition of epicontinental sea,which consist of gyprock lake,gypsum dolomite and lime dolostone.According to the diagenetic environment and diagenetic stage,the palaeokarst in study area is divided into syngenesis karst, hypergene karst and burial karst,karst controlled by rock composition,diagenetic environment and construction conditions; according to residual thickness method and impression method, the karst topography is classified into karst platform,karst slope and karst basin,the karstification developed well on karst slope; dissolution and reflux dolomitization are constructive to the reservoir pore development, while dedolomitization, cementation and compaction are destructive. Reservoir space is given priority to with secondary dissolution pores and seam.The porosity ranges from0.01%to12.51%,averaging1.89%,and the permeability ranges from0.01×10-3μm2to44.79×10-3μm2,averaging0.83×10-3μm2.The reservoir is characterized by low porosity and permeability. The palaeokarstic reservoir mainly controlled by sedimentary facies, diagenesis and karst topography. Yi5-Yi6-Yi15-Yi12-Yi13area and Daning-Yi3-Hong3area were distributed transational zone between gypsum dolomitic flat and lime dolomitic flat,which favorable for anhydrock,formed anhydrock dissolve hole under the dissolution. they were also located on karst slope.Therefore,they were favorable reservoir area. |
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