Abstract: When an oil reservoir is watered-out by freshwater and sewage miscible phase in a water flooding oilfield, formation water resistivity of each water-out zone can' t be accurately determined, which resulted in bigger error in log interpretation of water-out zone.An interpretation chart of water saturation (the relationship curve of formula resistivity and water saturation)was presented by applying mathematical statistics, performance testing and establishment of geologic model etc., combined with all production data from development of an oilfield.Verification of the chart was made by using core data from four sealed coring wells in three blocks of Karamay oilfield.It is concluded that the conventional log interpretation of water-out zone can be improved greatly by this method.

Key words: Karamay oilfield, Porous reservoir, Water injection, Development, Water-out, Log interpretation, Formation water resistivity, Remaining oil saturation