|Preview||An investigation has been made of the nature of electrical conductivity in shales to establish the feasibility of determining from logs the types of clays present in shales. Conductivity data from well logs have been compared with both cation exchange capacities and surface areas measured from cores of Tertiary, Cretaceous, Pennsylvanian, Mississippipain, and Ordovician shales from California, Louisiana, Oklahoma, and Wyoming. This investigation has established that electrical conductivity in shales is principally a function (1) of the cation-exchange capacity and distribution of clay minerals in the shales and (2) of the temperature of the formation. Electrical conductivity in shales is influenced to a lesser and usually insignificant degree by the salinity of the shale pore water. The conductivity of shales can be expressed with knowledge of their depth, formation travel time, temperature, and cation-exchange capacity (CEC). Surface area can be substituted for CEC.