The Spontaneous Potential (SP) Log
The spontaneous potential tool measures natural electrical potentials that occur in boreholes. The "battery" mechanism is caused by the drilling of the borehole with drilling mud that has a different salinity from formation waters. The invasion of formations by mud filtrate results in the juxtaposition of two aqueous solutions with different ion concentrations. Ions diffuse from the more concentrated solution (typically formation water) to the more dilute. The ion flow is electrical current, with an associated potential measured in millivolts. The cumulative potential is caused by several effects: the electrochemical components of a liquid-junction potential and a shale membrane potential, together with a minor electrokinetic potential.
Figures & Tables
This manual was created in 1994 to assist the geologist to interpret logs. In the not too distant past, the reading of geology from wireline logs was highly interpretive. The ability of a rock to conduct electrical current or sound waves is several steps removed from traditional outcrop descriptions based on the eye and hammer. However, the range of logging measurements has expanded markedly over the years. In particular, the addition of nuclear tools has introduced log traces that reflect both rock composition and geochemistry in a more direct manner. Taken together, both new and old logs contain a host of keys to patterns of rock formation and diagenesis. The majority of books on log analysis focus on the reservoir engineering properties of formations penetrated in the borehole. The promise of potential porous and hydrocarbon-saturated rocks generally pays for both the hole and the logging run. There are many examples of common log types from a variety of sequences.