Abstract: The resistivity of formation water is an important parameter for calculation of the reservoir water saturation. However, there are few formation water samples at the early time of the development of an oilfield. Therefore, it is very difficult to determinate the resistivity of formation water. The logging data of exploratory wells in X exploration area are collected in this paper. Starting from searching for typical pure water layers, classification and statistical analysis were conducted for each geologic horizon. Finally, the PICKETT cross plot method is used to determine the resistivity of the formation water. The application results are very good in later fluid property identification.