Abstract: The potential for gas exploration of the non-karst reservoir,Majiagou formation of the Ordovician in Ordos basin,is a hot issues currently.The petrology properties,porosity types,physical properties,diagenesis are studied in this paper.The results show that the reservoir rocks of the middle assemblages of the Ordovician in the well Su217 area are grain dolomite and residual dolarenite.The dolomite reservoir is characterized by low porosity and low permeability,whose reservoir space mainly include intercrystal pores,remaining intergranular pores,dissolved pores and dissolved fractures.The densifying factors for the tight reservoir are the compaction&pressolution,cementation,packing action and rejuvenation of crystals.The atmospheric water dissolution and the selective dolomitisation in the bank microfacies are the key factors to the formation of the relatively high-quality dolomite reservoir.