The electromagnetic force generated across an ion-selective membrane when solutions on either side of the membrane have different salinities. Shales and clays are cationic membranes, since they allow the passage of cations, such as Na⁺, but not anions, such as Cl^-. When the drilling mud in the borehole and the formation water have different salinities, a membrane potential is generated at the boundary between a shale and a permeable formation. This potential is one component of the electrochemical potential, from which the spontaneous potential (SP) log is derived. The other, much smaller component is the liquid-junction potential. The membrane potential is reduced if the shale is not a good cationic membrane, or in other words has a low cation-exchange capacity.

A membrane potential may also be generated across the mudcake if there is no flushed zone; for example if the mud filtrate has moved vertically since invasion took place, and by clay within a shaly sand, but with the opposite polarity to the normal SP potentials.

The membrane potential is also used in core analysis to determine the cation-exchange capacity of a sample. In this case, the clay within the sample is the ion-selective membrane, and the potential generated across it is related to the cation-exchange capacity per unit pore volume, Q_v. As a method of measuring Q_v, the technique is faster than the multiple salinity method, and more representative of the in-situ value than destructive methods such as conductometric titration. However, care is needed in making the measurement and deriving the appropriate Q_v.