Abstract: Combining the data of geophysics, geology and drilling, the paper systematically summarizes the discovery process of large oil provinces in Mahu Sag. Based on reservoir dissection and the study of reservoir?cap assemblages in the upper Wuerhe formation and Baikouquan formation, the reservoir forming model has been perfected, which could guide the overall breakthrough of Baikouquan formation and upper Wuerhe formation in sedimentary fans of Xiazijie, Karamay, Zhongguai and Xiayan, and 2 large oil provinces such as Baikouquan formation in the northern Mahu sag and the upper Wuerhe formation in the southern Mahu sag have been discovered. It is considered that the favorable conditions of large oil province forming in Mahu sag consist of the large?scaled slope tectonic setting during the deposition of the upper Wuerhe formation and Baikouquan formation, the high?quality alkalin?lacustrine source rocks in the Lower Permian Fengcheng formation, the continuous superimposed conglomerate bodies distributed on the 2 unconformity surfaces and the high?steep faults and so on. Based on which, a new hydrocarbon accumulation model of lithological reservoir groups which is made up of multistage retrogradational conglomerate bodies in large and continuous distribution is established. The discovery of the large oil provinces and the establishment of the hydrocarbon accumulation model in Mahu sag can provide guidance and references for the exploration in the strata of Middle Permian and deep Carboniferous?Lower Permian in this area