The Lower Ordovician Majiagou Formation can be divided into the Lower, the Middle and the Upper assemblages from the lower to the upper and the Middle Assemblage can be further divided into three sections. The Middle and Lower Sections of the Middle Assemblage are the good gas reservoirs that are discovered recently in Ordos Basin. The dolostone reservoirs with gypsum-dissolution pores and intercrystal pores are common and the spacial distribution of these dolostone reservoirs is controlled by sedimentary environment. Comprehensive analysis shows that the gas in the Middle and Lower section reservoirs is the mixed gas that derives mostly from the coal-related gas in the Upper Paleozoic source rock and less from the marine-facies gas in the Lower Paleozoic source rock. Due to uplifting of the whole Ordos Basin at the end of Caledonian movement, Majiagou Member-5 strata were subjected to intensive denudation, which led to direct contact of the Middle and Lower section reservoirs of Middle Assemblage of Majiagou Member-5 with the overlying Upper Paleozoic coal-series source rock and so a good source-reservoir relationship was set up. Owing to development of gypsum-dissolved and intercrystal pores, these dolostone reservoirs obviously are of better physical property than the surrounding dolostone bodies so that they are favorable to form lithologic traps and hydrocarbon accumulation in them as the mixed coal-related and marine-facies gases migrated into the Middle and Lower section traps from the Upper Paleozoic and Lower Paleozoic source rocks. These gas reservoirs are characteristic of multi-layer superimposition and mixed source accumulation.