Interpretations of the Gamma Ray and Spontaneous Potential curves generally assume sodium chloride to be the dominant salt in both the formation water and the mud filtrate. However, potassium-salt muds are increasingly be- ing used by the oil industry. The potassium cation is significantly different CWLS Journal Vol. 9, No. 1, December 1976 35 J. W. COX & L. L. RA YMER from the sodium cation in its radioactive and electrochemical properties.