Abstract: Based on the core data from 8 core hole,the water top (oil-water transitional belt)in the two oilfields is redivided.It is concluded that its thickness range reaches 36.6-68.6m more than that acquired by the data of electric loggings and oil testings.Therefore,it means the oil saturation being 30～40%,an increase in oil/gas reserves for the oilfields.The study also discusses about the cause for the larger thickness of this transitional belt,and the inadequacy and affects of the determination with the data of electric logging andpotential tests.