Abstract: Characteristics and genesis of Ma5¹⁺² sub-member reservoir (the 1^st and 2^nd sub-member of the 5^th member of Ordovician Majiagou Formation) in the northeastern Ordos Basin are studied based on core,thin section and physical property analysis.The result shows that diverse types of reservoir rocks and reservoir spaces occur in Ma5¹⁺² sub-member in the study area.The reservoir rock types include gypsiferous micritic to very fine crystalline dolomite,grain dolomite,saccharoidal medium-coarse crystalline dolomite,microbial carbonate and sandy rubble dolomite with caverns filled.And the corresponding reservoir space types are gypsum mould pore,micropore between pore fillings,intergranular (dissolved) pore,intragranular dissolved pore,intercrystalline (dissolved) pore,fenestral pores,inter-breccia residual vuggs.The reservoirs in Ma5¹⁺² sub-member are typical porous reservoirs with low porosity and low permeability as a whole.The micritic and very fine crystalline dolomite with gypsum mold pores have the best reservoir properties but fair frequency and scale of occurrence.In contrast,the grain dolomite and sandy breccia dolomite have poorer reservoir properties but larger frequency and scale of occurrence than the former.Reservoir genesis analysis indicates that the formation and distribution of the Ma5¹⁺² sub-member reservoirs in this region are mainly controlled by sedimentary microfacies and multi-stage superimposed karstification,thus they are typical facies-controlling karst reservoirs.The psammitic banks and the gypsiferous dolomite flat provided the material foundation for the reservoir development and later karst reformation.The dissolution of gypsum clumps via epigenic karstification in the immature stage,the dissolution caused by layer-parallel flowing of karst water along the porous beds of early psammitic banks played a key role in the formation of the reservoirs.However,the overprint of epigenic karstification in the mature stage on that in the immature stage and the hybrid filling destroyed the earlier reservoirs or made them disappear.

Key words: epigenic karstification, reservoir characteristics, reservoir genesis, Majiagou Formation, Ordos Basin