The Mahu sag in the northwestern margin of the Junggar Basin, NW China, is a wellknown petroliferous sag globally. Recent exploration results indicate the oilbearing characteristic in this area. Therefore, in order to summarize the geological theory and exploration methods in superimposed basin, this study performed detailed oilforming condition and preferential areas for this hydrocarbon province. The results show that the formation of the giant oil and gas field in the Mahu sag was based on the three preferential conditions: highquality alkaline lacustrine source rocks formed under foreland settings, welldeveloped multiple types of migration pathways, and three types of effective reservoirs and reservoircap rock assemblages. As a consequence, large amounts of oil and gas migrated along multiple types of pathways and are accumulated in the three types of reservoirs and favorable traps under the effective sealing of three sets of caprock sequences. Oil and gas at the condition of sufficient hydrocarbon migrates along the transport system, are effectively covered by three regional caprocks, and finally form three kinds of reservoirs in the favorable traps. Therefore, the enrichment rule of oil and gas can be summarized as sourcemigration controlling the primary hydrocarbon accumulation and reservoircaprock controlling the hydrocarbon pool formation. Accordingly, three hydrocarbon accumulation fields formed from top to bottom are stratigraphic oil (gas) plays in the lower Triassic Baikouquan Formation, tight oils in the lower Permian Fengcheng Formation, and structural oil and gas plays in the Carboniferous to lower Permian. The exploration breakthrough in the Mahu giant oil (gas) province was initiated by theoretical innovations, which will have huge exploration potential and fundamental scientific significance.