Middle Ordovician Pingliang Formation is in the shape of “L” along the western and southern margin of Ordos Basin, and mainly consists of limestones and muddy limestones partly with thin mudstones and has bigger thickness. Previous research considered that local intervals of Pingliang Formation were rich in mud and could act as Lower Paleozoic hydrocarbon source rocks, but they were not exploration targets. Based on the geotectonic background of the southern margin of the North China platform of Middle Ordovician, the sedimentary environments and sedimentary patterns of Pingliang Formation are analyzed. The reservoir\|forming factors and their space arrangement are researched using outcrop section, downhole core data and lithologic combination feature. The result shows that reef flat facies deposition is developed in some parts of the Pingliang Formation and they form dolomite reservoirs through later diagenesis and dolomitisation. These dolomite reservoirs and neighboring muddy intervals form reservoir\|forming combination of self-generation and self-reserve, and have exploration potential.