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.%鄂尔多斯盆地奥陶系马家沟组五段中组合中下段为该盆地天然气勘探新发现层系。主要发育膏溶孔型和晶间孔型白云岩储层，其空间分布主要受沉积环境控制。综合分析表明，中组合中下段的气源为上古生界煤型气与下古生界海相烃源气的混源气，但以煤型气为主。加里东末期，由于盆地整体抬升造成古隆起地区马家沟组五段遭受强烈剥蚀，致使其东侧中组合中下段储层与上古生界煤系烃源岩直接接触，且海退沉积旋回中的局部洼陷也能提供一定的烃源，从而构成了良好的源-储配置关系。两类白云岩储层中，由于膏溶孔或晶间孔发育，因此物性明显好于周围白云岩体，空间上可形成岩性圈闭，当上古生界煤系烃源及下古生界海相烃源运移至中组合中下段圈闭内，即可聚集成藏。整体上这种气藏具有“多层系叠合，混源成藏”的特点。
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