A method is disclosed for the in situ examination of each subsurface formation penetrated by a borehole to ascertain the cation exchange capacity of such formations within a geological region. Natural gamma ray logging is used to develop signals functionally related to the total gamma radiation and to the potassium -40, uranium and thorium energy-band radiations. A first borehole is traversed by a potential gamma ray spectrometer to provide selected measurements of natural gamma radiation. Core samples are taken from the logged formation and laboratory tests performed to determine the cation exchange capacity thereof. The cation exchange capacities thus are developed then correlated with selected parameters provided by the gamma ray spectrometer to establish functional relationships. Cation exchange capacities of formations in subsequent boreholes within the region are then determined in situ by use of the natural gamma ray spectrometer and these established relationships.