SP
SchlumbergermVSpontaneous Potential
The natural electrical potential between a moving borehole electrode and a surface reference, driven by salinity contrasts between mud filtrate and formation water. SP deflects opposite permeable, water-bearing beds and stays flat across shales — one of the oldest permeability and correlation indicators.
- Property
- Spontaneous Potential
- Electric Potential
- Quantity
- electric potential difference
- mV · fV · µV · kV · nV · MV · cV · pV · V · TV · dV · GV
- Typical range
- ~-225.3–169.2 mV
- millivolts, relative to the shale baseline
Logging tool
No logging tool is associated with this curve.
Service company
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How it's measured
A single electrode records the natural potential (in millivolts) versus a fixed surface reference as the tool moves up the hole. There is no active source — the signal comes from electrochemical and electrokinetic effects at bed boundaries.
How it's used
Classic permeable-bed and shale indicator, a way to estimate formation-water resistivity (Rw), and a bed-boundary picker. The static SP deflection scales with the Rmf/Rw contrast.
Common gotchas
SP is nearly useless when mud filtrate and formation water have similar salinity (fresh muds, or Rmf ≈ Rw), and it is suppressed in shaly sands and by hydrocarbons. It is a relative curve — only deflections from the shale baseline carry meaning.
In LAS files
Usually recorded as SP near depth, in millivolts. The baseline is arbitrary, so absolute values vary well to well; interpret deflections, not levels.
Also seen with SP in real logs
Curves that appear alongside SP in the same wells across the public corpus.