1. n. [Formation Evaluation]

The ideal spontaneous potential (SP) that would be observed opposite a shaly, permeable bed if the SP currents were prevented from flowing. In the middle of a thick, permeable bed whose resistivity is not too high, the SP reads close to the pseudostatic spontaneous potential (PSP). In other conditions, however, the SP may be significantly less than the PSP. The PSP ignores other potential sources and assumes that a surrounding shale is a perfect cationic membrane. The ratio of the PSP to the static spontaneous potential is known as the SP reduction factor, alpha. Alpha is less than 1 and is a function of the shaliness, or cation-exchange capacity, within the sand. The higher this cation-exchange capacity, the larger the internal membrane potential. The latter has the opposite polarity to the liquid-junction potential and reduces the SP. The PSP, and alpha, are reduced when hydrocarbons are introduced into shaly sands, because the cation-exchange capacity in the sands is forced into a smaller conductive pore volume and therefore has a larger relative effect.

Synonyms: SP

Alternate Form: PSP

See: cation exchange capacity,  electrokinetic potential,  hydrocarbon,  liquid-junction potential,  membrane potential,  shale baseline,  spontaneous potential