Spontaneous potential, self potential. The difference of potential (DC voltage) between a movable electrode in the borehole and a distant reference electrode usually at the surface. The SP results from the IR drop measurable in the borehole produced by the flow of SP currents in the hole. These currents are generated by the electrochemical and electrokinetic potentials.
In impermeable shales, the SP tends to follow a fairly constant shale base line. In permeable formations, the deflection depends on the contrast between the ion content of the formation water and that of the following: drilling mud filtrate, the clay content, the bed thickness and resistivity, hole size, invasion, and bed boundary effects, etc. In thick, permeable, clean, nonshale formations, the SP value approaches the fairly constant static SP value which will change if the formation water salinity changes. In dirty reservoir rocks, the SP will not reach the same value, and a pseudo-static SP value will be recorded.
The SP is most useful when the mud is fresher than the formation water, a good contrast exists between mud filtrate and formation water resistivities, and formation resistivity is low to moderate. In these cases, it indicates permeable beds by large negative deflections, permits easy sand-shale discrimination, is useful for correlations, and under favorable conditions, can be used for the estimation of formation water resistivity.
The curve still remains useful in some saline muds. If the formation water is less saline than the mud filtrate, the SP deflection will be positive. However, when the mud column becomes so conductive it will not support a demonstrable IR drop, the SP curve becomes featureless. See electrochemical potential, electrokinetic potential, SSP, activity, and differential SP.
