More than 2,000 chloride-concentration measurements made on extracted water from oil-base-mud cores show areal and vertical variations in the salinity of Ivishak and Sag River reservoir brines. These data compare favorably with compositional analyses of produced and extracted water, spontaneous-potential (SP) analyses, and resistivity-log calculations. In calculating water saturations with the Archie equation, it was critical that variations in resident water resistivity be recognized. This paper describes the variations observed and the work done to validate the extent and magnitude of the salinity variations.