| In recent years, exploration activities at Wuqi-Dingbian area of the western Ordos basin have obtained great improvements and displayed a giant or super giant oilfield. Therefore, To understand the depositional environment in the early period of Late Triassic in the researched area and to know the types and development mechanism of sedimentary facies of lower assemblage of Yanchang Formation are of great significance for exploration activity. On the basis of synthesizing regional geologic information and the earlier researches, guided by leading edging theories of Sedimentology, Lithofacies-paleogeography, Geochemistry of trace elements and Logging geology, and based on the cores, well logging, outcrops data, the sedimentary provenance direction of the research area is studied by means of clustering analysis of heavy minerals and palaeocurrent analysis. The aqueous medium environment is also studied by the method of trace elements geochemistry. The types and characteristics of sedimentary facies and paleogeographic evolution are studies by means of multidisciplinary analysis, including litho facies, palaeoecologic markers, logging facies, depositional sequence, sandbody thickness and facies diagram of the ratio of sand to land thickness, and the distribution regulations of sandbodies are defined,it is combined the above mentioned research with the exploration activities, the dominant factors that control hydrocarbon accumulation are discussed, and the hydrocarbon accumulation rules are revealed. The following conclusions have been obtained.There were three provenance systems from northwest, northeast and southwest in Wuqi-Dingbian area of the early period in the Late Triassic, while provenance of the northwest and the northeast dominated, and the sand deposit sourced from southwest does not developed well. It was limno-geotic of brackish freshwater in the research area during the early period of Late Triassic, and the salinity increases gradually during Chang 7 period. As the climate was getting warmer and damper from Chang 9 to Chang 7 period, it was in a deepwater and reductive depositional environment in the early stage of Chang 7 period, the same environment condition with the late Chang 9 period. And it was mainly shallow lake and deltaic deposit during Chang 10 period; deltaic deposit and locally semi-deep lacustrine sediments during Chang 9 period; deltaic deposit dominate during Chang 8 period; Chang 8 and Chang 9 periods were shallow water terrace type deltas, and the distributions of subaqueous distributary channels of deltas front in different directions have the characteristics of becoming width and sheet like overiding above river mouth; During Chang 7 period, it was semi-deep to deep lacustrine deposit in the western and southern of the research area and sublacustrine fan deposit in part. In the northeast, it was shallow lake sediments and small scope of delta sediments; From Chang 9 to Chang 7 periods, the depocenter removes from the east to the west. The deposition of the lower assemblage of Yanchang Formation recorded the geological evolution process of the Ordos basin from initial stage, enlargement, to the prosperous.The lower assemblage of Yanchang formation has the characteristics of source rocks in upper part and reservoirs in lower part. The semi-deep and deep lacustrine argillutite of the lower of Chang 7 oil-bearing formation and the upper of Chang 9 oil-bearing formation provided high quality source rocks and capping formation. The delta front sands of Chang 8 and Chang 9 oil-bearing formation provided the effective reservoir. The abnormal pressure generated from the semi-deep and deep lacustrine argillutite of the bottom of Chang 7 and the overpressure difference generated from Chang 8 sandbody offered the mainly energy for petroleum migration to the bottom of Chang 8 and Chang 9. Gas accumulation areas mainly locates higher pressure areas of overpressure background at vertical orientation of Chang 8 and Chang 7.
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