Abstract: Lower Ordo vician Majiagou formation Ma-6 section distributes abroadly along the southern margin of Ordos Basin,its thichness increases gradually from north to south,mainly consists of algal limest one,patchy limest one,powder crystal dolomite,and grained dolomite.Based on the geotectonic background of the southern margin of the North China plat form of the lower Ordovician,the sedimentary environments and sedimentary patterns of Majiagou Formation are analyzed and the reservoir-forming factors and their space arrangement researched according to out crop section,downhole core data,and lithologic combination features.The result shows that the reef flat facies deposition is developed in some parts of the Majiagou Formation and form dolomite reservoirs through later diagenesis and dolomitisation. These dolomite reservoirs and neighboring muddy intervals form reservoir-forming combination of up generation and down reservoirs,and have a good exploration potential.