It is porous but heterogeneous in the Lower Ordovician Majiagou Submember-51 carbonate reservoir in Zhidan area,Ordos Basin.According to pore evolution and associated lithologic structures,two types of different pores can be divided.One is the fabric selective pores that include intercrystal pores,intercrystal solution pores and moldic pores,and the other is the non-fabric selective pores that mean fissures,interbreccia solution pores and fissu re-corrosion enlarged pores.It is indicated that sedimentary environments and late diagenesis are the factors influencing and controlling pore development in the carbonate reservoir.Diagenesis plays the key roles,such as solution and dolomitization resulting in secondary pores,compaction and pressolution decreasing pore volumes,and recrystallization influencing reformation of reservoir space.Fissures play a role of enlarging reservoir space as forming seepage ways.The evolution of pores experienced four stages in this area,the format ion-reduction stage of primary pores,the development to filling stage of secondary pores,and the reburial-filling stage of secondary pores.It is shown that the pores are different in type with changing diagenetic environment during these stages.