The Spontaneous Potential Log

Part 3, Chapter 3: Gamma Ray and Spontaneous Potential

Learning objectives

Explain the electrochemical origin of the spontaneous potential
State how the static SP depends on Rmf and Rw
Predict the SP deflection direction from the Rmf-Rw contrast
Use the SP as a permeability and bed-boundary flag

A Voltage the Rock Makes

The spontaneous potential is the one curve the formation generates by itself, with no source in the tool. Where the fresh mud filtrate in the borehole meets the salty formation water across a permeable bed, the difference in salt concentration sets up an electrochemical cell, and a small current flows. The voltage that current produces is the SP, and it shows up as a deflection off an otherwise flat shale baseline.

The Static SP

The maximum deflection a thick clean bed can produce is the static SP,

SSP = -K\,\log_{10}\frac{R_{mf}}{R_w}, \qquad K = 60 + 0.133\,T

with $T$ in degrees Fahrenheit. The whole signal comes from the ratio $R_{mf}/R_w$ . With a fresh mud against a saline brine (Rmf greater than Rw) the SP kicks strongly negative; make the mud as salty as the formation and it flattens; make it saltier still and the deflection reverses.

The SP as a Flag

Because the cell needs a permeable bed to drive current, the SP deflects only opposite permeable rock and stays flat against shale. That makes it a clean permeability flag and a sharp picker of bed boundaries, a second opinion alongside the gamma ray. Its limits are worth remembering: it needs a water-based mud (there is no SP in oil-based mud), it needs a real Rmf-Rw contrast, and shale or thin beds blunt it. Its real prize, though, is in the next section, where that same SSP is turned back into a value of Rw.

References

Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.
Doll, H. G. (1948). The SP log: theoretical analysis and principles of interpretation. Trans. AIME.