A new method is presented to determine formation water resistivity (Rw) in sandstones. This procedure does not use the Spontaneous Potential (SP) log. Instead formation Rw is obtained from the overlying shale zones.
To understand the correlation between shale and sandstone waters, consider that during deposition their salinities are approximately equal. After compaction the waters in both shales and sandstones are in equilibrium. Because shales are impervious to drilling fluids a true value of water resistivity and salinity in the shale can be obtained from Archie's equations. The formation water resistivity is determined from the shale salinity and a relationship developed from core data.
The shale method is simple to use requiring only the shale resistivity and porosity. This makes it ideal for field use. It also is accurate for both normal and geopressured wells.
The presented technique is compared to other interpretation methods on several wells. The results compared well with other data.