| This paper investigated the Cambrian-Ordovician stratum of middle Tatime-basin, collected the drilling data, logging data, seismic data, geochemistry data etc. Using theories as sedimentology, petroleum geology, stratigraphic sequence, geophysics etc, and combined with former achievement, This paper systemly investigated the character of deposition system and sedimentary facies distributing in the middle tarim-basin, and detailed analysed the character and controlling factor of the petroleum accumulation in middle of Tarim-basin. On the base of former research, this paper predicted the favorable area. and indicated the background of the petroleum accumulation.According to the studying of Cambrian-Ordovician stratum deposition system and sedimentary facies in middle of Tarim-basin, Cambrian-Ordovician stratum deposition system are main composed of carbonate platform sedimentary system in middle of Tarim-basin. This carbonate platform has undergone four phases, the gentle incline carbonate platform in Early-middle Cambrian. The weak trimmed carbonate platform in Late Cambrian-Early Ordovician, Isolated carbonate platform in Middle-Late Ordovician, Submerged carbonate platform in the intermediate stage of late Ordovician. Sedimentation is a process of water body gone through from shallow to deep. In lithology it is composed of dolomite in Cambrian-under Ordovician, in middle Ordovician-upper Ordovician below part it is mainly composed of limestone ,and in upper Ordovician top part it mainly sediment mudstone.According to research of the controlling factor of the petroleum accumulation, the base of the petroleum accumulation are two hydrocarbons, one is Cambrian-under Ordovician hydrocarbon, the other is middle-upper Ordovician hydrocarbon. The controlling factor of the accumulation are two container rocks and good cover rocks, one is Cambrian-under Ordovician dolomite container rocks, the other is Ordovician karst...
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