The paper describes the plane distribution and vertical variation of formation water of the Hongshangzui oilfield in the northwestern margin of Junggar Basin for the purpose of investigating the hydrocarbon migration and accumulation patterns.Crude oil in Hongshanzui oil field was sourced from the Permian Fengcheng Formation and Wuerhe Formation and migrated along the unconformity and of the nearly S-N trending fault zone that was formed during the Yanshanian movement.Correlation of reservoir distribution with chemical feature of formation water indicates that the environment formation water preservation is an direct indicator of the setting of hydrocarbon accumulation and all of the sealed environments indicated by formation water have oil/gas discoveries.The strata from the Carboniferous to the Cretaceous were faulted by the Yanshanian Movement.The comparison of formation water properties between the hanging wall and foot wall shows that the N-S trending faulted zone formed after the Yanshanian movement is open and the northern segment of the Hongche fault belt has poor sealing condition resulting in loss of hydrocarbon and local light oil reservoirs.Analysis of relationship between crude oil density distribution and saturated hydrocarbon gas chromatography shows that a heavy oil zone was formed in the northern segment of the Hongche fault belt by washing and biodegradation.In contrast

the sou-thern segment of the fault belt has good reservoir preservation conditions due to the sealing of the huge thick mudstone in the Baijiantan Formation.Good fault block reservoir can be formed under the sealing of the E-W trending fault.