1. n. [Formation Evaluation]

The ideal spontaneous potential (SP) that would be observed opposite a permeable bed if the SP currents were prevented from flowing and any shaliness in the bed were ignored. The static spontaneous potential (SSP) is equal to the electrochemical potential. When current is flowing, the SP measures only that fraction of the potential drop that occurs in the borehole. In normal conditions, this potential drop is much higher than the drop in the formation because the cross-sectional area of the borehole is much smaller, and hence its resistance much higher. It is for this reason that in the middle of a thick, clean bed whose resistivity is not too high, the SP reads close to the SSP. However, in other conditions the SP is significantly less than the SSP. As well as ignoring shaliness in the sand, the SSP ignores other sources of potential and assumes a surrounding shale that is a perfect cationic membrane.

See related terms: electrokinetic potential,  liquid-junction potential,  membrane potential,  pseudostatic spontaneous potential,  shale baseline