Knowledge of the way pore geometry affects the conductivity of rocks is important for the evaluation of sedimentary rocks for hydrocarbon potential. Virtually all formation evaluation from logs, however, uses the empirical Archie equation that has never been given a satisfactory physical basis. As an alternative, we have formulated a new model based on the efficiency with which water-bearing rocks conduct electric currents as determined by the conducting phase geometry. Electric current densities are not uniform throughout the conducting phase in the pore system of a rock. Numerical model studies show high current density in pore throats and low current density in nearly stagnant volumes in isolated parts of the pore system. The electrical efficiency of a rock is the ratio of the average power developed in all water-bearing parts of the pore system to the power developed in a straight tube with the same length and water volume as the rock. It is inherently independent of the bulk volume of water in the rock and determined only by the nonuniform current distribution caused by pore geometry and hydrocarbons in the pore system. Electrical efficiency can be calculated a priori given the geometrical distribution of the conducting phase. Empirical relationships between electrical efficiency and water content can be used to calculate hydrocarbon saturations from log data.