Computing Aggregate Reserves for a Site with Two Isolated Carbonate Units

This paper describes how to use RockWorks to compute total economic reserves for a site that includes two carbonate units: an upper limestone and a lower dolomite, separated by a shale unit. It involves creating separate I-Data models using the Stratabound filter, combining the models, and checking them against the observed log data.

Link to original paper: http://www.rockware.com/assets/products/165/casestudies/6/9/computing_aggregate_reserves.pdf

 

 Introduction

 The purpose of this study is to compute the total economic reserves for a site that includes two carbonate units; an upper limestone and a lower dolomite separated by a shale unit. Quality analyses have been obtained at one-foot intervals within the carbonates. The following diagram depicts a typical log showing the lithology, stratigraphy, and aggregate quality.

Figure 1: Typical log depicting aggregate quality (bargraph on left), stratigraphy (patterns in center), and lithology (subdivisions within stratigraphy)

Step 1. The Problem

Modeling the rock quality en-masse is problematic because the node values would include the quality values for both the limestone and the dolomite. The following diagrams depict a solid model based on the rock quality and a stratigraphic block model. Notice how the rock quality (I-Data) model interpolates quality values where there is no corresponding carbonate.

Figure 2: Problematic “Bulk” Rock Quality Model
Compare the rock quality model with stratigraphy model below and note how quality values are interpolated where there is no carbonate.
Figure 3: Stratigraphic Model

 Compare this stratigraphic model with bulk rock quality model above and note how quality values were interpreted within overburden (light yellow) and interburden.

Step 2. The Solution

The solution to this problem is to use the “Stratabound” option within the I-Data / Model menu. Two rock-quality models were created; one for the upper limestone and another for the lower dolomite.

In the example below, the I-data model is confined to points and nodes within the Hanford Limestone unit.

Figure4: Hanford Limestone Rock-quality Model

 In this example, the I-Data model is confined to points and nodes within Shuller Dolomite.

Figure5: Shuller Dolomite Rock Quality Model

Step 3. Combining the Models

The next step involved adding the two models together and removing all voxels with a quality value less than 50 (the minimum acceptable quality).

Figure6: Fence diagram depicting combined rock-quality models for upper limestone and lower dolomite.

Figure 7: Block Model depicting voxels with a quality value greater than 50.

Figure 8: Block model depicting zones from previous model in which the thickness for any single contiguous ore zone is more than 6 feet thick for any given column.

Figure 10: Block model depicting zones from previous model in which the stripping ratio is less than 1.2. This area represents a good place to start mining in order to gain the highest return on investment.

 Step 4. Checking the Model

The final, and most important step, is to create a 3D log diagram, combine it with the final ore model, and examine the data to see if it make sense.

Figure 11: 3-Dimensional Lithology/Quality Logs Combined With Final Ore Model.

Figure 12: Enlargement of area around highest-ROI ore depicting lithology and quality logs.

Step 5. Conclusion

By combining the preceding approach with increasingly more tolerant filter cutoffs, it is possible to create a mining strategy that will yield the highest return on investment from the onset.

 

Calculating the Volume of a Stratigraphic Unit within a Polygonal Boundary

A customer recently sent this inquiry to our support staff:
Q: Can I calculate the volume of a stratigraphic unit within an outline that doesn’t match the model extents and is not square?
A: There are a couple of ways to do this.
First, create a Polygon Table in your project database (see the Misc. Project Tables in the Project Manager) that represents the outline of the area you’re interested in.  You can hand-enter the XY coordinates into the Polygon Table, paste them in, or even draw a polygon in RockPlot2D onto a map and save those coordinates to the Table.  Then, here are some options:
1. Isopach Grid Model, Automatic:  Use the Stratigraphy / Stratigraphic Thickness / 2D menu option to interpolate a thickness grid for a selected formation (it does this by gridding the formation top and base, then subtracting the base surface from the top surface). BE SURE to activate the Save Grid Model option and assign the thickness grid model a name (e.g. formation_a_thickness.grd).  Then clip this grid model using your Polygon Table (Utilities Grid / Filters / Polygon Clip), setting the filter type to Exterior with replacement=null.  (e.g. formation_a_thickness_clipped.grd). You can turn on the Create Grid Statistics Report option, and the volume of the grid will be displayed at the bottom of this report:
Cell Area .............................. 100.0
Map Area (X*Y) ......................... 738,000.0
Grid Area (Sum(Cell Area)).............. 755,300.0
Model Volume (Sum(Cell Area*Z)) ........ 5,493,616.411554  *
Non-Zero node area ..................... 119,600.0
2. Isopach Grid Model, Manual:  If you don’t want the program to reinterpolate the formation top/base to generate the isopach grid (e.g. you already have top and base surfaces that you’re pleased with), you can use the Utilities Grid / Math / Grid & Grid Math tool to subtract your existing base surface from the top surface, generating a thickness grid.  Then clip this model with the polygon and generate the report, as described above.
3. Solid Model:  You can create a solid block model (MOD) of your surface-based stratigraphy model by running the Stratigraphy / Model option and activating the Save Numeric Model option and entering a name (e.g. strat_solid_model.mod).
NOTE: if you don’t want to re-interpolate the stratigraphic surfaces – you want the program to build the solid using your already-interpolated grids – be sure to turn OFF the Interpolate Surfaces option.  Then RockWorks will just build a block model of your stratigraphy.
ANOTHER NOTE: How well this block model will represent your stratigraphy surfaces (and how good the volume computation will be) will depend on the vertical node spacing defined in your project dimensions.  If you have skinny units, be sure the vertical spacing of the nodes is tight enough to represent them to your satisfaction.  Generally surface-based computations will be more accurate.
Here’s a surface-based strat model (left) and a voxel representation of a solid stratigraphy model (right), for your reference:
Once you have the solid strat model generated, you can clip that solid model using a polygon which represents the outline (Utilities Solid / Filter / Polygon Clip), replacing the nodes outside the polygon with null (e.g. strat_solid_model_clipped.mod).  Then, you can use the Stratigraphy / Volumetrics Based on Solid Model option to generate a report from that clipped model.
Or you can simply view the clipped model in RockPlot3D as a voxel model, adjust the viewing filter for a specific unit, and see the volume there.