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# Fault Modeling in Petrel

Petrel

In this article we review fault modeling in Petrel. (Note: Petrel 2009 has been used)

## Fault modeling overview

Fault modeling is the process of generating a faulted 3D grid and inserting the horizons, zones and layers into it. The process takes 3 steps:

1. Fault Modeling - in Petrel this is done by first defining the faults in the process called Fault Modeling. The purpose of this step is to define the shape of each of the faults that should be modeled. This is done by generating “Key Pillars”.
2. Pillar Gridding - the next step is to make a grid based on the defined faults. This is done in the Pillar Gridding process. In this process, a set of pillars will be inserted in the entire project area. The pillars are inserted in between the faults, and there will be one pillar in every corner of each grid cell. The result of the pillar gridding process is a “Skeleton grid”, defined by all the faults and all the pillars. This is not associated with any other input than the faults.
3. Layering - the final step is to insert the horizons into the faulted 3D grid. At this point, the 3D grid will be attached to depth by associating it with inputs such as time or depth maps and/or well tops. The input horizons (that can be surfaces based on seismic or well points, or it can be seismic interpretations (lines or point data) directly) will honor the defined fault planes. After the horizons are inserted, the zones (based on geological input such as isochors) are inserted, and the final step will be to make the fine-scale layering, suitable for property modeling.

## Fault Shapes

The faults are grouped into 4 main groups:

• Vertical faults - Consist of 2 shape points. Will always be vertical.
• Linear faults - Consist of 2 shape points. This is in most cases the best type to use.
• Listric faults - Consist of 3 shape points. All the three shape points can be edited separately in order to get a listric shape of the fault.
• Curved faults - Consist of 5 shape points, and can be used in order to define a curved, or S-shaped, fault.

A fault in Petrel is defined by a set of Key Pillars. Each Key Pillar consists of a set of shape points. A Key Pillar can have 2, 3, or 5 shape points, and a fault can consist of Key Pillars with varying number of shape points. Each Shape Point can be edited in 3D.

## Fault Input

The faults can be defined based on different types of input. The input can be:

• Fault sticks - this is a set of vertical lines representing the slope of the faults.
• Fault polygons - which is a set of lines, representing the hangingwall and footwall of the fault for each horizon.
• Faulted surfaces - the faults can be defined by digitizing directly on the surface.
• Seismic interpretations (3D lines) - the faults can be digitized directly on the line data.
• Seismic - the faults can be digitized on a vertical section along the crossline, inline or along a random line through the seismic.
• Cross Section - A cross section can be made showing the available surfaces. This cross section can be cut anywhere through the surfaces and it can be a tilted plane or a vertical plane. Then it is possible to digitize the faults on this plane and jump stepwise with a certain increment, digitize a new fault pillar, jump a new increment, digitize a new fault pillar, etc.
• Based on Well Cuts - Well cuts can be used both for conditioning and if there are many enough, i.e. if you have so many well cuts that they represent a plane, they can be used as input by itself.

## Fault Modeling General Workflow

1. Define a Fault Model
2. Model the initial set of faults (generate Key Pillars)
3. Edit any problem areas
4. Trim/Extend to the top and base of the model
5. Connect faults (required for Pillar Gridding)
6. Add more faults by repeating steps 2-5

… then do final edits

## Define a new model

Before starting Fault Modeling, you need to define and name a model that will be used to store the fault model and all 3D grids related to that fault model.

1. Open Define Model in the Process Diagram and type in the name of the new model.
2. The new model is stored in Models tab (empty; no faults are created yet).
3. Activate the Fault Modeling process.
4. A set of fault modeling tools is available to start creating faults (Key Pillars).

Fault Polygons. Fault polygons are line data representing the hanging-wall and foot-wall for each horizon. The polygons could be separated into lines for each fault or lines for each horizon. In the latter case, the lines should be separated with flags to prevent that two faults are not defined by the same line. If there are no flags between the line data, you should do some editing in the Make/Edit Polygons process in order to split the lines.
Defining Key Pillars based on fault polygons
Display polygons for all horizons in 3D window. Click on Fault Modeling in the Process Diagram.
Select the lines that represent the fault that you want to make. You have to be in the “Select/Pick mode” in order to be able to select the lines. To select more than one line, hold the Shift Key while selecting the new line. The selected lines display a lighter color. Once all the lines that define your fault are selected, click on the icon called Create fault from polygons. Key Pillars will now be created based on a given increment between them and a given height.
The increment and height of Pillars can be changed by double-clicking on the Fault Modeling process, go to Settings Tab and change the Increment parameter.
If too many, two few, or strange Pillars are built, then adjust their spacing by changing the Increment, click on the Undo icon, then click on the Create fault from polygons icon.