1. College of Geo- exploration Science and Technology��Jilin University��Changchun 130026��China; 2. Wireline Logging Company of Daqing Drilling Engineering Company��Daqing Oilfield��Daqing 163412��Heilongjiang��China

The formation water of Jurassic and Cretaceous in Chepaizi area of Junggar Basin is low TDS whole- ly and mainly is of the NaHCO 3 type�� Based on the chemical composition characteristics of the formation water in the Chepaizi area��the authors calculate the ion ratios to analyze the relationship with the oil and gas accumulation�� The research shows that the ion ratios have obviously differentiable characteristics in the north and south�� The south is on the edge of uplift which has adverse closure property while the north is on the palaeohigh which has good clo- sure property�� The oil and gas from Sikeshu sag of southern adjacent area were migrated to the north by petroleum migration pathways such as south faults�� The Jurassic formation water had suffered from more strong infiltration of atmospheric precipitation��which led to oxidation of early oil and formed the current heavy oil reservoir�� The closure property of the Jurassic formation water is better than that of Cretaceous��which favors oil and gas in accumulation and reservation��