The Ma 6 member of the Early Ordovician Majiagou formation distributes broadly along the southern margin of Ordos basin, its thickness increases gradually from north to south, and its rocks mainly of algal limestone, patchy limestone, powder crystal dolomite and grained dolomite. Based on the tectonic setting of the southern margin of the Early Ordovician North China platform, the sedimentary environments and sedimentary patterns of the Majiagou Formation are analyzed. The reservoir-forming factors and their space arrangement are researched using the outcrop data, downhole core data and lithologic combination features. The result shows that reef flat facies deposition is developed in some parts of the Majiagou Formation and they form dolomite reservoirs through later diagenesis and dolomitisation. These dolomite reservoirs and the neighboring muddy intervals form reservoir-forming combination of up generation and down storage, and have a exploration potential.