Spontaneous potential corrections for groundwater salinity calculations — Carter County, Oklahoma, U.S.A.

Abstract

The spontaneous potential log was used to compute the thickness and distribution of groundwater with a total dissolved solids (TDS) of less than 1000 mg l −1 for Carter County, Oklahoma. To compute accurate salinities from the spontaneous potential in high-resistivity zones, empirical correction factors had to be developed and applied. For the formations present equivalent water resistivity vs. water resistivity curves were developed from chemical analyses and digitized. Water resistivity vs. TDS curves were also developed and digitized. The existing empirical thin-bed corrections were curve-fit with equations and applied to each zone. Except in beds less than 3.65 m thick they had little effect. It was found that after applying the thin-bed corrections used by logging companies, the computed TDS was a function of resistivity of the mud filtrate at 24°C. An empirical relationship was developed between spontaneous potential and resistivity of the mud filtrate at 24°C, using well logs of beds with known water chemistry. This relationship was entered as a correction equation into a computer program developed by the author. The correction is necessary since the voltage drop in a freshwater formation is greater than predicted by earlier electric analog modeling, and the spontaneous potential is less than the static spontaneous potential. Testing of the correction factors shows that in a formation with TDS of ∼750 mg l −1 the average error decreases from ∼37% to 10%. A data set of 704 well logs was built with data from each freshwater zone from the base of the regolith to a depth where the TDS was greater than 10,000 mg l −1. The TDS was computed for each zone. Two maps were prepared from the results; an isopach of the formations with TDS of less than 1000 mg l −1, and a depth to the base of the water with TDS of 1000 mg l −1.