Characteristics and genesis of Ma5

1+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 Ma5

1+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 Ma5

1+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 Ma5

1+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.