The Majiagou Middle Assemblage refers to the whole gas-bearing reservoir beds from Submember-5 to Submember-10 of Lower Ordovician Majiagou Member-5 in Ordos Basin. Intercrystalline pore, intercrystalline dissolved pores and dissolved vugs are predominating in the Middle Assemblage dolostone reservoirs that are widely distributed on the east side of the Ordovician Central Paleouplift, Ordos Basin. The Ordovician Middle Assemblage dolostone reservoirs experienced a long process of hydrocarbon accumulation and reservoir evolution. The good source-reservoir-cap assemblage consists of the Middle Assemblage karst dolostone as reservoirs and the overlying thick Carboniferous-Permian coal measure strata as cap and source rock. Attributed to the upper Paleozoic source rock that had reached to the peak of hydrocarbon generation and expulsion during late Jurassic to early Cretaceous period, the "source in the upper and reservoir in the lower" styled gas reservoirs formed widely in the Middle Assemblage. Because of modulation of the earlier gas reservoirs caused by structural evolution since early Cretaceous, nowaday gas reservoirs commonly formed in the laterally-sealling locations within Middle Assemblage. It is suggested that lithologic traps are the favorable exploration targets.