Spontaneous Potential logging
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The Spontaneous Potential log, commonly called the "SP log", is a measurement taken by oil industry well loggers to characterise rock formation properties. The log works by comparing the voltage on the wall of the well bore to a grounded voltage at the surface. The change in voltage through the well bore is caused by a build up of charge on the well bore walls. Clays and shales will generate one charge and porous sands will generate an opposite one. This build up of charge is, in turn, caused by differences in the salt content of the well bore fluid (drilling mud) and the connate water. Whether the mud contains more or less salt than the connate water will determine which way the SP curve will go.
