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Shale Volume from the Spontaneous Potential
The
two most common shale indicating logs are the gamma ray
(GR) and spontaneous potential (SP) logs. The units of measurement
for GR are API units or counts per second, and for SP are
millivolts.
The algebraic formula to solve for shale volume from the
SP log is a linear interpolation between the minimum and
maximum log readings, similar to the GR solution.
1:
VSHsp = (SP - SP0) / (SP100 - SP0)
2: VSHsp = Min(1, Max(0, VSHsp))
Where:
SP = spontaneous potential log reading in zone of interest
(mv)
SP0 = spontaneous potential log reading in 100% clean zone
(mv)
SP100 = spontaneous potential log reading in shale (mv)
VSHsp = shale volume from spontaneous potential (fractional)
 COMMENTS:
The SP method is the second most popular approach to shale
volume. The SP is reduced by high resistivity, often associated
with hydrocarbons or tight zones, so these may appear too
shaly by this approach. The method has poor resolution in
zones with fresh formation water, or in wells drilled with
salty mud. The method works well in radioactive sands, but
not in carbonates.
References:
1. The SP Log: Theoretical Analysis and Principles of Interpretation,
H.G. Doll, AIME, 1948.
2. SP Log in Shaly Sands,
H.G. Doll, JPT, 1950.
Shale Volume from ALPHA Method
(OBSOLETE)
In
older charts and technical papers, the SP is called the
pseudo static SP (PSP) and SP0 is called the static SP (SSP).
The same formula was used for shale volume as given above,
with SP100 taken as zero. The result is always called ALPHA,
the SP reduction factor, and was often equated to shale
volume:
3: VSHpsp = ALPHA = PSP / SSP
4: VSHpsp = Min(1, Max(0, VSHpsp))
This
formula is no longer commonly used.
RECOMMENDED
PARAMETERS:
Range Default
SP0 -100 to -45 -80 mv
SP100 -10 to +10 0 mv
Choose from crossplot or depth plot
NUMERICAL
EXAMPLE:
Assume SP = -50, SP0 = -90, SP100
= 0 mv.
1. Vsh from spontaneous potential log:
VSHsp = (-50 - (-90)) / (0 - (-90)) = 0.44
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