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Some Ordovician and Triassic oils in Block 9 are characterized by light oils,which have distinctly differentiated from heavy oils in other blocks in the Tahe Oilfield,Tarim Basin,Based on the whole oil gas chromatograms,this paper estimates the effect of oil migration and fractionation and the amount of depletion (Q) in terms of the n-alkanes depletion model.The results showed that the amount of depletion in the Ordovician reservoir is highest in the east of this block,e.g.the depletion is 97％ in Well T904.The amount of Q gets lower to the west,e.g,the depletion is 53.4％ in Well T115 and there is no sign of depletion in Well S69.It is suggested that the direction of gas washing is from the east to the west.The compositions and isotopic characteristics of associated gas in Ordovician oils indicated that the gas might be derived from Cambrian source rocks of the Caohu Depression which lies to the east of Block 9.In contrast,no obvious depletion of n-alkanes in Triassic oils was found,suggesting that the migration pathway of natural gas has been limited to the Ordovician karst fracture system formed in the Early Hercynian Orogeny.Different depletions of the Ordovician and Triassic oils can reveal fault activities in this region.