Abstract: Four types of flooded formations have been noticed in the Shengli oil field on the introduction of water flooding,each with its own electric properties and different way of differentiation. Based on the changes in the salinity of water,oil saturation and physical properties of reservcirs,six methods are proposed for differentiation.For formations flooded with fresh water,observation of the shifting of S.P.curve base line and application of induced potential may help discern whether the reservoir is partially or completely and uniformly flooded and to determine the thickness of the flooded parts.In case of formations flooded by edge water,with medium and high salinity,a qualitative analysis of the correspondence between the S.P.curve and the resistivity curve may generally achieve satisfactory results.As to the formations flooded with recycle water and the flooding of formations with reverse rhythmic succession,movable fluids analysis and comparative study of lateral resistivity logs may give better differentiation. Experiments in 34 new producers on 130 pay zones with these methods claim a credibility up to 90%.