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Spontaneous potential (SP), also called self potential, is a naturally occurring electric potential difference in the Earth, measured by an electrode relative to a fixed reference electrode. Spontaneous potentials are often measured down boreholes for formation evaluation in the oil and gas industry, and they can also be measured along the Earth's surface for mineral exploration or groundwater investigation.
The Spontaneous Potential (SP) log measures the natural or spontaneous potential difference between the borehole and the surface, without any applied current. It was one of the first wireline logs to be developed, found when a single potential electrode was lowered into a well and a potential was measured relative to a fixed reference electrode at the surface.[11]
The most useful component of this potential difference is the electrochemical potential because it can cause a significant deflection in the SP response opposite permeable beds. The magnitude of this deflection depends mainly on the salinity contrast between the drilling mud and the formation water, and the clay content of the permeable bed. Therefore the SP log is commonly used to detect permeable beds and to estimate clay content and formation water salinity
ADDED ON THE TOP aNWSER
the SP method has been employed in the search for minerals, whereas anomalous surface potentials are commonly measured in the vicinity of sulphide mineralization (pyrite marcasite, chalcopyrite, pyrrhotite, and sphalerite), and graphite. These potentials are measured in millivolts (mV) relative to a "survey base", where the potential is arbitrarily assigned to be zero volts. Amplitudes of SP anomalies in mineralized areas range from a few millivolts to one volt or more. The potentials of interest are always negative above a mineralized body relative to a point outside the mineralization.
Generally, The SP potentials are set up due to electrochemical action in the subsurface rocks or/and in any bodies which could be occurred within these rocks and can be separated into two basic categories. Firstly, small background potentials which can be either positive or negative and lies with the range of a fraction of a millivolt to a few millivolts. Secondly, large mineralization potentials which may hundreds of millivolts and are always negative
