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→Spontaneous potential
===Spontaneous potential===
===Spontaneous potential===
[[file:basic-open-hole-tools_fig1.png|thumb|left|{{figure number|1}}A typical log showing SP, gamma ray, dual Induction, and sonic measurements. © Schlumberger, 1983.]]
[[file:basic-open-hole-tools_fig1.png|thumb|{{figure number|1}}A typical log showing SP, gamma ray, dual Induction, and sonic measurements. © Schlumberger, 1983.]]
''Spontaneous potential'' (SP) is a natural voltage or electrical potential that arises due to differences in the ionic activities (relative saltiness) of the drilling mud and the formation waters. This potential can be used to correlate formations between wells, to indicate permeability, and to estimate formation water resistivity. No SP occurs when oil-based mud is used in the borehole. Hydrocarbons and shaliness in the formation suppress the SP. The magnitude of the SP decreases as the resistivity of the mud filtrate and formation waters approach a common resistivity. The direction of SP deflection reverses as the ratio of the resistivity of the mud filtrate (''R''<sub>mf</sub>) to that of the formation water (''R''<sub>w</sub>) reaches 1.0 or more. If there is no contrast in the mud filtrate and formation water salinities, there is no measurable SP. A typical presentation of SP is shown on the left of the log in [[:file:basic-open-hole-tools_fig1.png|Figure 1]].
''Spontaneous potential'' (SP) is a natural voltage or electrical potential that arises due to differences in the ionic activities (relative saltiness) of the drilling mud and the formation waters. This potential can be used to correlate formations between wells, to indicate permeability, and to estimate formation water resistivity. No SP occurs when oil-based mud is used in the borehole. Hydrocarbons and shaliness in the formation suppress the SP. The magnitude of the SP decreases as the resistivity of the mud filtrate and formation waters approach a common resistivity. The direction of SP deflection reverses as the ratio of the resistivity of the mud filtrate (''R''<sub>mf</sub>) to that of the formation water (''R''<sub>w</sub>) reaches 1.0 or more. If there is no contrast in the mud filtrate and formation water salinities, there is no measurable SP. A typical presentation of SP is shown on the left of the log in [[:file:basic-open-hole-tools_fig1.png|Figure 1]].