Using the Make Simple Grid Process to Build Un‐faulted Frameworks
Un‐faulted reservoirs are relatively rare but do occur and provide a wonderful break from the tedium of modeling faulted reservoirs. Constructing un‐faulted structural frameworks is easy using the Make simple grid process. Because it is so easy, initial models of faulted reservoirs are sometimes built using un‐faulted techniques to allow quick checks of data, interpretation, volumes, modeling methods, and even issues related to up‐scaling for simulation.
The Make simple grid process is used in place of the Pillar gridding process or in place of both the Pillar gridding and the Make horizons processes to build un‐faulted pillar grids and un‐faulted structural frameworks. Frameworks that are faulted but for which the faults have not been modeled can also be built using the Make simple grid process. In fact, this approach is used repeatedly by seismic interpreters while making their interpretation to get a quick visual of their progress and to test gross rock volumes.
Figure: Make simple grid process is found in the Utilities folder of the Processes Explorer.
The common uses for the Make simple grid process are described in this document.
Make Simple Grid, a Replacement for Pillar Gridding
The Make simple grid process should be used to define the cells of a 3D Grid (pillar grid) when faults are not being modeled and when trends are not needed to control the orientation or size of the 3D Grid’s cells. If faults and trends are needed, then the Pillar gridding process should be used. Petrel users like using the Make simple grid process for building pillar grids because it is quick and simple to use.
Figure: Pillar grid skeletons built using the Make simple grid process with constant Zmin and Zmax (left) and with grids for Zmin and Zmax (right).
The steps and/or parameters used to create a pillar grid using Make simple grid are:
Make simple grid Dialog
Start: double‐click on the Make simple grid process (Utilities process folder).
General Parameters
Create new or Edit existing: allows naming a new 3D Grid or writing over an existing one.
Boundary: not always used but allows a 2D Grid or polygon to define the boundary of the pillar grid. If the Automatic radio button is checked on the Geometry tab, this boundary will also be used to define X‐Y limits and rotation parameters.
Input Data
Skeleton only: Check this radio button to only build a pillar grid skeleton (don’t add horizons).
Top limit: a grid or constant defining the top of the 3D Grid (top skeleton).
Base limit: a grid or constant defining the base of the 3D grid (base skeleton).
Figure: General and Input tab parameters for Make simple grid process when building only a pillar grid skeleton.
Geometry
Automatic: Check this radio button to collect the X‐Y limits from either the supplied boundary or the Top limit and Base limit grids.
User defined: Check this radio button to use the X‐Y limits entered in the limits parameter boxes.
Xmin: X coordinate of lower left corner of the pillar grid. Ymin: Y coordinate of lower left corner of the pillar grid.
Xmax: X coordinate of upper right corner of the pillar grid. Ymax: Y coordinate of upper right corner of the pillar grid. Width: Xmax‐Xmin Height: Ymax‐Ymin
Rotation: used when checked. You supply the angle or get the angle from a Petrel object having a stored angle by inserting (blue arrow) the object or by highlighting the object and clicking the Get all settings from selected button.
Grid increment: always used. You supply the Xinc and Yinc or get the increments from a 2D Grid or seismic cube by inserting (blue arrow) the object or by highlighting the object and clicking the Get all settings from selected button.
Figure: General and Geometry tab parameters for Make simple grid process used to build a pillar grid.
Make Simple Grid, a Replacement for Pillar Gridding and Make Horizons
The Make simple grid process should be used as a replacement for the Make horizons process when:
The pillar grid can be defined as described in the previous section (no faults no trends)
The input data used to make the horizons of the 3D Grid are 2D Grids (Note, these grids may or may not contain fault discontinuities and that is discussed in the section below.)
The most common scenario is to have a set of 2D Grids defining the horizons of a structural framework. The grid limits and increment of one of these grids is used to define the geometry of the pillar grid. The 2D Grids are input in top down order and their geologic relationships specified. At this point, all the information needed to both build the pillar grid and make the horizons of the 3D grid are known to the Make simple grid process.
Figure: One of several 2D Grids used as input to Make simple grid (left) and the horizons of a 3D Grid built by Make simple grid using un‐faulted 2D Grids as input.
The steps and/or parameters used to insert horizons into a 3D Grid using Make simple grid are described below. The parameters needed to build the pillar grid into which the horizons are inserted are not discussed as they were covered in the previous section.
Make simple grid Dialog
See previous section
General Parameters
See previous section
Input Data
Insert surfaces: Check this radio button to open the 2D Grid input table
2D Grid input table:
Highlight one or all of the 2D Grids that are to be inserted and click the Append item in the table
icon.
Use the blue up or down arrow icons to arrange the 2D grids in top‐down stratigraphic order.
Use the , , and icons to add or delete 2D Grids to the table.
Horizon type: Highlight the horizon in the table and then select the geologic relationship that horizon is to have. Initially all horizons default to conformable.
Figure: Petrel help describing the geologic relationships that can be set for each input horizon.
Figure: Input tab parameters for Make simple grid process used to define input data and relationships for horizons.
Geometry
See previous section for description of Automatic and User Defined. Optionally, highlight one of the input surfaces used in Input Data above and click either Get all settings from selected or Get limits from selected. If you choose the latter, be sure to check the grid increments; both Xinc and Yinc.
Building Faulted Frameworks Using Make Simple Grid
Modeling faulted structures in Petrel or in any modeling package requires considerable time and attention to detail. When input data are interpreted seismic events, fault discontinuities are contained in the data. 2D Grids can be built from these data that clearly show the fault discontinuities and are adequate for the goal of many projects. Project goals where 2D Grids might be adequate include:
QC looks at the seismic interpretation to identify areas needing more work
Simple surfaces used as input for velocity processing
Simple surfaces to get a rough estimate of gross rock volumes.
Input to building a quick structural framework and property models to evaluate an exploration project’s feasibility for continued development.
When more than one seismic event is involved, it is best to incorporate all the 2D event Grids into a structural framework. This allows sections to be cut through the framework, zone volumes to be calculated, isochores or isochrons to be created, and simple velocity models to be built.
The fastest approach to create structural frameworks of interpreted seismic events is to:
1. Use the Make/edit surface process to build 2D Grids of each horizon.
a. Place all the seismic data in one folder in top‐down stratigraphic order (best if each file is a set of points with no attributes)
Figure: Folder with seismic events arranged in top‐down stratigraphic order.
b. Use Make/edit surface to define the parameters needed to build a 2D Grid for the first file in the folder
c. Check the box at the top of Make/edit surface that will Run for all main input in the same folder. This will create a new folder containing all the seismic events as 2D Grids.
Figure: Make/edit surface process with parameters set to build 2D Grids for all files in the input data folder (left) and the resulting folder filled with 2D Grids (right).
Figure: One of the 2D seismic event Grids (left) and all the 2D Grids (right).
2. Use the Make simple grid process to combine the 2D Grids into a structural framework in a 3D Grid.
a. Set parameters to create a new 3D Grid and give it a name.
b. Use one of the 2D grids to define all the geometry parameters.
c. Insert all the 2D Grids into the horizons table in top‐down stratigraphic order
d. Define the geologic relationships.
e. Build the 3D Grid.
Figure: Make simple grid parameters for defining the horizon inputs (left) and defining the pillar grid (right).
Figure: 3D Grid folder in the Models tab built by Make simple grid (left) and structural framework of horizons contained in that 3D Grid (right).
Building a non‐regular (non‐orthogonal) grid Using Make Simple Grid
Using any of the above techniques builds a Pillar grid that is orthogonal (consistent in Xinc and Yinc over the entire grid skeleton). Using the Tartan grid option lets you vary the size of the grid cells. Start off naming the new 3D grid and gathering the input data the same as before.
Next, go to the Geometry tab and set the X‐Y limits using any of the methods described earlier. Don’t bother setting the Xinc and Yinc parameters, these will be overridden in the Tartan grid. Finally, go to the Tartan grid tab. After clicking to Make tartan grid, the grid is constructed using the following rules:
Figure: Making a tartan grid using Uniform density. Make simple grid dialog (left) and model skeleton (right); makes a very regular grid. The difference is that this grid has 20 rows and 20 columns and not a number set by a prescribed Xinc and Yinc.
Figure: Making a tartan grid using Logarithmic, Central density. Make simple grid dialog (left) and model skeleton (right); cell sizes get bigger logarithmically in the I direction, symmetric from the center.
Figure: Making a tartan grid using Logarithmic, Custom density. Make simple grid dialog (left) and model skeleton (right); cell sizes get larger logarithmically around the chosen well.
Figure: Making a tartan grid using Logarithmic, I+ density. Make simple grid dialog (left) and model skeleton (right); cells get larger logarithmically, starting from the minimum “I” location.
