The component of the SP (self-potential) comprising the sum of the liquid-junction potential and the shale potential, both of which are determined by the ratio of the activity of the formation water to that of the mud filtrate. The liquid-junction potential is produced at the contact between the invasion filtrate and the formation water as a result of differences in ion diffusion rates from a more concentrated to a more dilute solution (concentration cell). Negatively charged chloride ions have greater mobility than positive sodium ions and an excess negative charge tends to cross the boundary, resulting in a potential difference. The shale potential (also called the shale membrane, Nernst, or Mounce potential) results because shale acts as a cationic membrane permitting sodium cations to flow through it but not chloride anions. The liquid-junction and shale potentials are additive. See also SSP and electrokinetic potential.