Abstract: The ordovician in Ordos basin is one of the key areas in marine carbonate exploration in China.In recent years,new discoveries have been attained on the southwest margin which shows the area has a better exploration potential.From studying the field outcrops,drilling cores,resin thin-sections and physical property measurements,the authors know that there are three types of reservoir on the southwest margin of the basin,which are the reef-bank,the dolostone and the ancient karstic,among them the dolostone type in Majiagou Formation is of importance.The authors delinate two favorable exploration regions through the research of reservoir rocks,sedimentary facies,karst landform,physical properties and space distribution,which is instructive and meaningful to the natural gas exploration of carbonate rocks on the southwest margin of the Ordos Basin.