This article is based on
Chapter 6 of "The Log Analysis Handbook" by E. R. Crain, P.Eng., published by Pennwell Books 1986 Updated 2004,
webpage version is the copyrighted intellectual
property of the author.
Do not copy or distribute in any form without explicit
Selection of Shale Volume Method
The methods shown on other pages of this Handbook provide
numerous independent approaches
shale volume, as well as a number of correction factors
which could be applied. If the results differ significantly
from each other, then the log scales should be checked,
and then the clean lines, shale lines, and log picks reviewed in an attempt to reconcile
the differences. If no reconciliation can be made, discard
the least trustworthy result. In order of preference, it
is suggested you use:
1. Vsh from minimum of feasible results (at any depth
point) if there is no reason to prefer
one method over another.
2. Vsh from gamma ray (GR) if sandstone or carbonate is not radioactive
due to uranium.
3. Vsh from thorium (TH) if sandstone or carbonate is
radioactive due to uranium.
4. Vsh from density neutron if hole condition is
good. Best suited to feldspar or quartz sands with no
heavy minerals and no gas.
Use linear methods unless local correlations have shown
a need for a non-linear relationship.
METHODS WITH LIMITES USE
5. Vsh from SP. Use only if SP has sufficient character or resolution
to be believed.. May be useful if SP is the only shale
indicating curve available.
6. Vsh from sonic density crossplot. Do not use the Q method..
7. Vsh frrom deep resistivity log. Useful only in hydrocarbon bearing
8. Numerous alternate methods may work in specific areas.
should be rounded to the nearest 5% (0.05 fractional) for
hand calculations, and to the nearest 0.1 percent (0.001 fractional)
for computer work. Too much precision in an
imprecise number is unnecessary and misleading.
generated plots of the results for the mixed lithology example
are displayed below. The crossplot shale values
are not valid because heavy minerals affect the results.
Comparison of Shale Calculation Methods for
Mixed Lithology Example
Minimum Shale VolumeMETHOD
The usual approach for deciding which of the several available
shale volume results to use is to find the minimum value
of the feasible results. Feasible results do not include
answers from the crossplot methods if gas crossover occurs.
The minimum is chosen because most errors for any one method
tend to increase the apparent shale volume. For example,
radioactive sandstone would appear very shaly from the gamma
ray but reasonable from the SP and density neutron crossplot.
High resistivity zones subdue the SP, making it give t9o high
a value for Vsh. Heavy minerals force the density neutron
method to likewise give too high a value for Vsh.
A simple model for handling this is:
IF PHIN < PHID
2: THEN VSHxnd = 10^6
3: VSHmin = min (VSHgr, VSHsp, VSHxnd, ......)
PHID = density log reading (fractional)
PHIN = neutron log reading (fractional)
VSHnin = shale volume from minimum method (fractional)
VSHgr = shale volume from gamma ray method (fractional)
VSHsp = shale volume from SP method (fractional)
VSHxnd = shale volume from density neutron crossplot method
Add other VSH models if they seem to work for a particular
project or zone.