The component of the SP comprised of the sum of the liquid-junction potential and the shale-membrane potential, both of which are related to the ratio of the activity of the formation water to that of the mud filtrate. The liquid junction potential is produced in the formation at the interface between the invading filtrate and the formation water, as a result of the differences in ion diffusion rates from the more concentrated to the more dilute solution. The negatively charged chloride ions have greater mobility than the positive sodium ions and an excess negative charge tends to cross the boundary, resulting in an emf. The shale membrane potential results because the shale bed acts as a cationic membrane, permitting the sodium cations to flow through it but not the chloride anions. That results in an excess of positive charges in the dilute solution and an excess of negative charges in the concentrated solution. The liquid junction potential and shale potential are additive. See SP and SSP.
