Abstract: In view of formation water and crude oil physical properties from all sequences in Chepaizi oilfield of Junggar basin, it is considered that the enclosed environments from Carboniferous to Cretaceous are worse and worse, among which Jurassic is the most perfect oil-bearing reservoir as a transitional zone from enclosed environment to open environment. It is concluded that the formation water type can be used to judge the enclosed quality of fault or fracture. The Jurassic in A zone of Chepaizi oilfield is enclosed and that in B zone is open, so the former (A zone) could be the main target for petroleum exploration in this field.

Key words: Junggar basin, Chepaizi oilfield, Jurassic, formation water type, hydrocarbon accumulation