The three methods available for computing the Shale and Clay volumes are
- from GR curve
- from Neutron Porosity/Bulk Density
- from SP
Each of these can be enabled or disabled in the "Shale/Clay Volume" processing section, as shown below. The left side shows on the GR method enabled. The right side shows both GR and NPHI/RHOB methods enabled. If 2 or more methods are enabled, there is also a final option to allow computation of the VSH-minimum and VCL-minimum, which will the minimum value of all methods used.
9.4.1 V-Shale / V-Clay from GR Method
The GR values for clean sand and Shale need to be picked. Where the GR curve is less than or equal to the GR-min, the VSH will be zero. Where the GR curve is greater than or equal to GR-max, the VSH will be 1.0.
The five common transforms from GR to VSH are provided. These are Linear, Larionov (x2), Clavier and Steiber.
The GR Clay Factor is a multiplier applied to the GR-max for computing VCL. In the example shown, the VCL will be computed using a GR-max of $150 \times 1.25 = 187.5$, and the resulting VCL will be less than the VSH. The transform is applied after the GR-max is adjusted. The GR Clay Factor will always be greater than 1.0.
9.4.2 V-Shale / V-Clay from SP method
The method for computing VSH and VCL from the SP log is similar to the GR method, where SP-Clean and SP-Shale values are selected.
The Clay/Shale ratio is a factor that is applied to VSH to derive the VCL, and will always be less than one. Converting VSH to VCL is a linear transform.
Note that the Clay/Shale ratio for SP is NOT the reciprocal of the GR Clay Factor, since the GR Clay Factor only modifies the GR-max, where-as the Clay/Shale ratio is applied to the difference between the SP-max and SP-min. However, it is expected that there will be an inverse correlation between the two values.
9.4.3 V-Shale / V-Clay from Neutron/Density logs
The V-Shale is determined by picking a "Shale Point" on the Neutron Density crossplot.
The V-Clay is determined by picking a "Clay Point" on the Neutron Density crossplot.
The points picked are independent of each other, but realisitically, the Clay Point needs to be picked further to the right than the Shale point, else the VCL will be greater than the VSH, which is not possible.
To assist with picking these points, a link to the crossplot is provided, and the shale and clay points can be picked interactively, as shown in the right in the figure below.
To pick a Shale or Clay point:
- first click on the crossplot with the left mouse button at the location where the new shale or clay point is to be, and a cross will appear on the crossplot at that location.
- second, right click on the crossplot to show the drop-down menu. Select "Pick Shale/Clay points", then next select either "Select Shale Point" or "Select Clay Point", depending on which point you were selecting.
Once this is done, the updated values will appear in the processing window.
After the parameters for all the active V-Shale / V-Clay methods have been set, then click compute. The newly computed curves will appear in the datalist (in this example, there are 6 new curves), and a plot of the results is presented.
A Clay/Shale ratio will be computed from the Neutron Density method, and written to the message panel. This should be less than one and also be comparable to Clay/Shale ratio used for the SP method. As mentioned above in the SP Method, it is expected that there will be an inverse correlation between GR Clay factor and Clay/Shale ratio.
By default, doing repeat computations will remove the earlier computations. If an earlier computation is to be retained, then uncheck the option to remove earlier computations.