USE OF ELECTRIC LOGS TO ESTIMATE WATER-QUALITY OF PRE-TERTIARY AQUIFERS

Electric logs provide a means of estimating ground-water quality in ar eas where water analyses are not available. Most of the methods for in terpreting these logs have been developed for the petroleum industry a nd are most reliable in saline aquifers (concentration of dissolved so lids as sodium chloride greater than about 50,000 mg/l). The resistivi ty-porosity and spontaneous-potential methods were evaluated to determ ine if they could be applied to identify zones of fresh water (concent ration of dissolved solids as sodium chloride less than 1,000 mg/l) in three potential aquifers in central Wyoming. The potential aquifers h ave different lithologies-sandstone, clayey sandstone, and carbonate. The two methods generally were reliable predictors of water quality in the sandstone and carbonate potential aquifers. In the clayey sandsto ne potential aquifer, predictions of the dissolved-solids concentratio n using the two methods differed by more than an order of magnitude in several cases. When the resistivity values are corrected for the pres ence of clay and shale as identified on a natural gamma log, the agree ment between the results of the two methods improved by an average of 58 percent.