Abstract: Ordos basin was a stable kratogenic inner depressed ba- sin in the Ordovician period.Majiagou Formation in Ordos basin is a karst palaeogeomorphology,with abundant natural gas resources.Majiagou 51 sub-member belongs to re- stricted platform setting,its main rock types are mud-thin crystal powder dolomite and nodwlar anhydrite dolomite.In view of the reservoir as a weathering crust karst0 the micro- scopic and microcosmic types and the characteristics of reser- voir spaces are elaborated,and the physical properties of ho- rizons in Majiagou 51 sub-member are analyzed compara- bly.It is concluded that Majiagou 51 sub-member is char- acterized by low porosity and low permeability.The above studies on characteristics of sedimentation and reservoir sug- gest that the factors controlling reservoir's storage and pres- ervation ability are lithologyo sedimentary microfacies,karst palaeogeomorphology and diagenesis.