Vclay from SP

Summary

The SP method scales the measured Spontaneous potential between a clean reservoir value and the shale baseline to give Clay volume. It does not depend on radioactivity, so it is an independent check on gamma-ray-based clay volume. Use it in thick beds with a clear salinity contrast between the mud filtrate and the formation water.

Inputs and outputs

Equations

The clay volume is the linear position of the SP between the clean and clay values, clamped to the interval 0 to 1:

Single-value calculator

Behavior

Clay volume rises in a straight line from 0 at the clean SP to 1 at the clay SP. Because the clean SP is usually a large negative value, a small change in the shale baseline moves the answer for the whole interval. The plot shows three clay baselines with the same clean value.

Parameter guidance

The two picks are read from the log itself. Clay SP is the shale baseline, and many analysts shift the curve so the baseline sits at zero before picking. Clean SP is the largest deflection in a thick, clean, water-bearing sand in the same zone. Pick them per zone, and avoid hydrocarbon-bearing sands, where the SP is suppressed. The general principles are in the Clay Volume page.

Worked example

An SP reading of -45 mV, with a clean value of -80 mV and a shale baseline of 2 mV:

sp, sp_clean, sp_clay = -45.0, -80.0, 2.0
vcl = min(1.0, max(0.0, (sp - sp_clean) / (sp_clay - sp_clean)))
print(f"Vcl = ({sp:g} - ({sp_clean:g})) / ({sp_clay:g} - ({sp_clean:g})) = {vcl:.3f}")

Output

Vcl = (-45 - (-80)) / (2 - (-80)) = 0.427

Assumptions and limitations

• The SP deflection is proportional to clay content, which holds only approximately.
• The beds are thick enough for the SP to reach its full deflection. In thin beds the SP is attenuated and clay volume is overestimated.
• There is a contrast between mud filtrate and formation water salinity. With a near-zero contrast the SP is flat and the method fails.
• The mud is water-based. In oil-based mud there is no SP.
• The sand is water-bearing. Hydrocarbons suppress the SP deflection, which reads as extra clay.

QC checks

• The clean and clay picks are on the same side of any baseline shift, and the clean value is clearly separate from the clay value.
• Clay volume is high in thin beds that the other indicators call clean. This is the signature of bed-thickness attenuation, not of clay.
• Compare with the gamma ray result. Large, systematic differences in hydrocarbon zones point to SP suppression.
• Check the baseline for drift with depth. A drifting baseline gives a slowly changing clay volume that does not follow the geology.

Going Deeper

The SP is generated mainly by electrochemical effects at the shale-sand boundary and by the streaming of fluid. Its use as a clay indicator is historical, from the era when it was often the only curve available besides resistivity. In quantitative work it is more often used to estimate formation water resistivity than clay volume, because the corrections for bed thickness, invasion and shaliness are substantial. The quantity used in many shaly-sand schemes is the pseudo-static SP relative to the static SP, which this simple scaling approximates.

References

References will be added once verified.

Python reference implementation

Related methods

• Vclay from GR (Linear Index)
• Vclay from GR (Larionov, Tertiary)
• Vclay from GR (Larionov, Older Rocks)
• Vclay from GR (Clavier)