The SP is a passive measurement of very small electrical voltages resulting from electrical currents in the borehole caused by the differences in the salinities (resistivities) of the formation connate water (Rw) and the drilling mud filtrate (Rmf), and by the presence of ion selective shale beds. The voltage changes are measured by a downhole electrode relative to a surface ground. The unit of measurement is millivolts (mV)
Unlike other logging tools which are displayed on a specific scale with a specified reference value, the SP has no specified origin and values used for computation are referenced to deflection from the nearby shale baseline established by the interpreter.
Interpretation Goals:
- Correlation of formations from well to well
- Gross lithology
- Formation water resistivity (Rw) estimation
- Shale/clay content estimation
- Qualitative indication of permeability
- Identification of depositional environments
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