The Characteristics Of Palaeokarstic Reservoir Majiagou Formation,Ordovician, Daniudi Gas Field, Yulin Area,Ordos Basin

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, geochemistry characteristics, pore structure, the type of karst,zone of karst, diagenesis of palaeokarstic reservoir Majiagou Formation, Ordovician, Daniudi gas field.The auther reconstruct the type of karstic palaeogeomorphology according to residual thickness method and impression method.The auther also analyse control factors about palaeokarst and control factors about palaeokarstic reservoir. Comprehensive consideration karstic palaeogeomorphology and the distribution of dolomite,the auther forecast the best area.Results showed:Daniudi gas field, mainly with karst breccias and micritic dolomite, developing caves and fractures and intergranular pores, carbon and oxygen isotope has obviously negative bias, the87Sr/86Sr increased; the palaeokarst in study area is divided into syngenesis karst, hypergene karst and burial karst, and the most important is hypergene karst; karst zone is divided into vertical vadose and level runoff zone,the karst more development in level runoff zone;the palaeokarstic reservoir experienced sea water, fresh water and buried diagenetic environment, dissolution and reflux dolomitization are constructive to the reservoir pore development, while dedolomitization and compaction-pressure solution are destructive;the internal factors,such as properties of rocks, sedimentary facies, geological structure,etc.,are control factors for the development of karst,the external factor,such as climate, hydrogeological conditions,etec.,are the basic conditions for the development of karst;there are3types of the second level,in which, the karstic ramp received strong karstification; palaeokarstic reservoir mainly controlled by properties of rocks, geological structure and hydrodynamic factors;there three best areas based on the analyses of karstic palaeogeomorphology and thickness of dolomite,which are D8-D30,D23-D79and Tong22-Tong20well field..