Material Balance for Shale Content
Computing shale volume from several methods is a good idea and
doesn't cost much when the computer software is set up for it.
Trimming results between 0.0 and 1.0, and then selecting the "best"
method is done based on sensible rules, or by taking the minimum of
all reasonable answers.
The material balance for shale content prevents impossible
values and should be applied to each shale volume method
used.
1:
IF Vsh < 0.0
2: THEN Vsh = 0.0
3: AND VshNEG = VshNEG + 1
4: IF Vsh > 1.0
5: THEN Vsh = 1.0
6: AND VshPOS = VshPOS + 1
To
simplify the above (if the software supports the Min and Max
commands):
7: Vsh = Min(1, Max(0, Vsh))
WHERE:
Vsh = shale content from any method (fractional)
VshNEG = counter for Vsh less than zero
VshPOS = counter for Vsh greater than one
COMMENTS:
If too many values fall below zero or above 1.0, the analyst
should review the choice of clean and shale base lines.
Less than 10% of all individual data points should fall
outside the material balance constraints. Vsh from the
density neutron crossplot will always be negative if gas
crossover occurs. In this case, an alternate method should
be used. Improper matrix offsets may also cause erroneous
gas crossover.
Selecting the Minimum Shale Volume
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 to high
a value for Vsh. Heavy minerals force the density neutron
method to likewise give too high a value for Vsh.
1:
IF PHIN < PHID
2: THEN Vshx = 10^6
3: Vsh = min (Vshg, Vshs, Vshxnd, ......)
WHERE:
PHID = density log reading (fractional)
PHIN = neutron log reading (fractional)
Vsh = shale volume from minimum method (fractional)
Vshg = shale volume from gamma ray method (fractional)
Vshs = shale volume from SP method (fractional)
Vshxnd = shale volume from density neutron crossplot method
(fractional)
COMMENTS:
This algorithm needs to be modified to include the methods
actually used to calculate shale volume..
Selection of Shale Volume Method
These methods provide some independent approaches
for calculating
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 the 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 GR (if sandstone or carbonate is not radioactive).
2. Vsh from density neutron (only if hole condition is
good, there is no gas crossover, and no heavy minerals).
3. Vsh from SP (only if SP has sufficient character or resolution
to be believed).
4. Vsh from sonic density crossplot (not the Q method).
5. Vsh from minimum of above if there is no reason to prefer
one method over another.
6. Use linear methods unless local correlations have shown
a need for a nonlinear relationship.
Answers
should be rounded to the nearest 5% (0.05 fractional) for
hand calculations, and to the nearest percent (0.01 fractional)
for computer work. Set negative values to zero and those
greater than 1.0 equal to 1.0. Too much precision in an
imprecise number is unnecessary and confusing.
Computer
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
