Crain's Usage Rules For Shale Volume MethoDS
The methods shown on other pages of this Handbook provide numerous 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 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 Minimum Method
Choose from minimum of feasible fsresults (at each depth point)  if there is no reason to prefer one method over another.

2. Vsh from gamma ray (GR)
Use if sandstone or carbonate is not radioactive due to uranium.

Use linear methods unless local correlations have shown a need for a non-linear relationship.

3. Vsh from thorium (TH)
Use if sandstone or carbonate is radioactive due to uranium

4. Vsh from density neutron
Use if hole condition is good. Best suited to feldspar or quartz sands with no heavy minerals and no gas.

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 from deep resistivity log.
Useful only in hydrocarbon bearing zones DO  NOT use in water  zones. Useful in heavy oil and bitumen oil zones..

8. Numerous alternate methods
May work in specific areas.

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

TRIMming Shale Volume RESULTS
Trims may be needed to remove Vsh results below 0.00 and above 1.00 before display or use in other calculations, for example:
      1: Vshg = Max(0, Min(1, Vshg))

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 too 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:

1: IF PHIN < PHID                     ! If gas crossover
      2: THEN VSHxnd = 10^6            ! Vshxnd cannot be the minimum
      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 (fractional)

Add other VSH models if they seem to work for a particular project or zone.



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