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Petrel TIPS&TRICKS from SCM

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© 2011 SCM E&P Solutions, Inc.

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Fault Editing Editing Fault models often involves a lot of time. How much time depends upon how efficiently you apply the editing tools. This TIPS&TRICKS article discusses many of the editing tools as well as methods for using the tools and your data to quickly create a fault model that both honors the data and creates a good quality pillar grid in only a few iterations.

Fault Editing Tools

Petrel has many fault editing tools. Some are in the Fault Modeling process and some are general tools used for fault editing and many other modeling tasks. The tools most commonly used by the authors are listed below.

Viewing mode: Used to rotate or shift the 3D view to best position for editing.

Select/pick mode: Used to select and edit objects in the view. Clicking on a fault (any object) tells you its name.

– Key used to toggle between the mode and the mode. Allows quick shifting from editing to repositioning and then back to editing.

Zoom: Used to focus the point of rotation on the fault being edited to allow rapid re‐positioning of the display while editing.

Undo: Removes the last edit or edits. Will not undo fault joins and cuts.

Delete: Deletes the highlighted object. You can also use the Delete Key to do this.

Toggle fill between pillars: Used to color the fault plane. This allows you to easily see where the fault is and when data is on one side of the fault or the other.

Make point size bigger/smaller: Used to make the shape points large enough to easily pick and the edit widget easy to click on and move.

Add pillar to end: Used to extend a fault by adding new Key pillar. Want last two existing Key pillars to be of similar height and correctly aligned because their spacing and trends are projected to make the next Key pillar.

Add pillar between: Used to insert a Key pillar between two existing and selected Key Pillars. The new pillar is half way between the existing two.

Connect two faults: Highlight Key pillars on each fault that are to be joined and click on this icon. You will be asked to join or merge and which faults to do it to.

Disconnect fault: Highlight two Key pillars and click this icon to cut the fault(s) between the two pillars.




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New branched fault: Click on a Key pillar and click this icon to create a new fault running at right angles the current fault. The new Key pillar can then be moved to any side of the existing fault and used as the beginning Key pillar of the new fault.

New crossing fault: Click on the Key pillar and click this icon to create a new fault that cuts across the current fault and is connected to the selected Key pillar.

Select shape points: Have this icon toggled on when editing individual shape points (normally is on).

Select pillars: Toggle this icon on to move an entire Key pillar. You can also move an entire Key pillar by clicking on the line between the pillar’s shape points and moving the widget that is displayed.

Set number of shape points: Highlight the Key pillar (or one of its shape points) and click on the icon having the desired number of shape points. Remember if 5 shape points are used in the fault model, then the number of shape points used in Pillar Gridding must be changed from the default of three to five.

Fault Editing Tips

There are probably a hundred tricks and conventions that help speed the fault editing process. Unfortunately, we can only discuss a few in this article. We have selected tricks we use over‐and‐over during our fault editing.

Use Two Pass Editing

The first pass of fault modeling uses the primary data. Data such as fault cuts, fault polygons, fault sticks, or even faults as interpretations are considered primary data. This data actually represents the fault surface. Often you will have more than one primary source of fault data and sometimes those data will be in conflict with one another. Your job is to identify the best data for each fault and build the initial fault model from that data.

Figure: First pass example, fault model built from fault sticks.

The second pass of fault modeling uses secondary data that indirectly defines the fault position. Data such as seismic interpretations for horizons and top picks from wells represent secondary data. Display this data with the initial fault models, study the position of the fault models relative to this data, and edit the faults to make the two compatible.




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Figure: Second pass example, using the seismic surface interpretation to tune the fault position.

Smooth Top and Base of Faults

Smoothing the top and base of the fault models makes them easier to view and edit and produces a skeleton that is easier to QC. The smoothing is done automatically using the Fault modeling process operation Cut/extend. You will need to create two 2D Grids that are smooth and extend a little farther in the X‐Y direction than the model or faults do. One of the grids represents the smooth top of model and the other the smooth base of model. The easiest way to build these is to create a grid from the top most horizon data in the model and another from the base most data, both extending a sufficient distance past where the model will be. Then smooth these considerably and shift them up and down respectively until they are just above and below the top and base of the structures to be modeled.

Figure: The Fault modeling operation Cut/extend set to trim the displayed faults (top left) the faults before trimming (top right) and after trimming (bottom).

Use Few Shape Points

Shape points are the means by which faults are edited. You can have two, three, and five shape points per Key Pillar. Often modelers will use five shape points when the fault is planar and only two or at most three are needed. When you start to edit, if the Key Pillar is defined by 5 points then you will need to edit several of those five points to reposition the fault. If the fault is defined by only two then you usually will only need to edit one point and at most two. Although this sounds like a minor point, when you are editing hundreds




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of Key Pillars, keeping the number of Shape Points to as few as possible can save hours and avoid a sore wrist.

Figure: Fault with too many shape points (left) and a more acceptable number of shape points (right).

Use Few Key Pillars

Too many Key Pillars is worse than too many Shape points. You need only enough Key Pillars to define the curvature of the fault in the X‐Y dimensions. When fault models are automatically created they often have too many shape points. When you see this, undo the fault construction, double click on Fault modeling and change the Settings Increment parameter (increase it), and rebuild the fault model.

Figure: Grid for which a fault is to be automatically created (top left), initial fault model with too many Key Pillars (top right), fault model with about the right number of Key Pillars (bottom left), and Fault modeling Auto insert pillars style tab showing the parameters used to set the spacing (increment) for automatically created Key Pillars (bottom right).




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Set Point of Rotation

The point of rotation is set using the Zoom icon. About half of fault editing involves moving the display to see the fault from an angle that lets you perform a specific edit. To speed and simplify this display rotation process, you should set the point of rotation to the center of each new area you are to edit.

Display only Faults You are Editing

Most projects have many dozens or even 100s of faults. Display only a small sub‐set of the faults you are editing. If possible, group the faults in sub‐folders by area so that a sub‐set of the total is easily displayed at one time.

Figure: All faults displayed while trying to edit the intersection of only three faults (left) and only those three faults displayed (right). Note how much easier it is to see the faults and to get to the shape points you want to edit.

Use Escape Key to Switch Between Editing and Screen Movement

Most fault editing requires bouncing between moving the view to a better editing position with the

icon and actually doing the edits with the icon. The key toggles between these two modes. Experienced modelers work with one finger on this key while they are editing to allow for fast movement and editing.

Color‐fill Between Key Pillars

Although a fault model is defined by Key Pillars, it actually represents a plane or mathematical surface. To know whether one fault crosses another and to more easily visualize the faults, use the

Toggle fill between pillars icon to color‐fill the fault surface between the Key Pillars.




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Figure: Fault models without (left) and with (right) color‐fill between Key Pillars.

Make Shape Points Large for Easy Edit

Mouse clicking during fault editing can be excessive. Anything you can do to reduce the number of missed clicks and to make clicking on the right spot easy will speed your editing. Be sure to increase the size of the shape points so they are easy to click on and so the displayed editing widget is easy to move. Use the Make

the point size bigger/smaller icons to increase and decrease the size of the shape points.

Figure: Small, hard to edit shape points (left) and large, quick, and easy to edit shape points (right).

View Down Key Pillar When Editing X‐Y Position

Probably the most common editing problem new users have is getting the Key Pillar shape points to quickly move to where they want them in the X‐Y direction. The usual problem is that they are not looking down (along) the Key Pillar when they are making the move.

Figure: Wrong: Key Pillar shape point edited using the widget’s plane when viewing from the side (left) and the result of the edit (right).




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Figure: Right: Key Pillar shape point looking down the axis of the shape point (left) and an edit made using the widget plane and viewing from that position.

View From Side when Editing Z Position

Another common error is to move the edit widget cylinder when viewing from above or below the Key Pillar.

Figure: Wrong: Shape point edited using the widget’s cylinder when looking from above or below the Key Pillar (left) and a view of the resulting edit (right).

Figure: Right: Shape point edited using the widget’s cylinder when viewing from the side (normal to the Key Pillar) before (left) and after (right) editing.

Move Multiple Points When Possible

All shape points that are highlighted at the time the edit is made will be moved. Any time several shape points need basically the same edit then they should be moved together. If, after the move, small adjustments are still needed, then finish with individual edits. When making faults from a single polygon line, the initial fault will be vertical. You can then click on the line connecting the top or base shape points and drag all shape points along that line to the desired position, changing the dip or height of the fault.




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Figure: Clicking on the top‐of‐fault line selects all points along that line (left) and moving the widget will move all the selected points (right).

Delete Problem Pillars

Problem Key Pillars are sometimes difficult to edit. Since Key Pillars on either side of the problem are often acceptable or more easily edited, deleting the problem Pillar might be the easiest way to fix it. Highlight the problem Pillar and use the Delete key to eliminate it.

Use Insert Pillar Between or at End

When faults are built from fault polygons the end Key Pillars are often incorrect. By deleting them, adjusting the next Pillar’s shape points and then adding new pillars to the end (using the Add pillar to end

icon) will create acceptable or more easily edited Key Pillars. A similar approach works when a pillar in the middle of the fault is bad. Delete the bad Pillar, and, if the form is not quite right, then insert a Pillar

between the remaining two Key Pillars using the Add pillar between icon.

Figure: Problem Pillars at end of fault (left), deleted pillars (center), and two new pillars added to the end of the fault (right).

Figure: Problem Pillars in the center of a fault (left), are deleted (center) and new ones inserted and edited (right).




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Join Faults, then Edit Their Form

When joining faults at bifurcations it is often easier to join the faults first and then edit the form of the Key Pillars to make them look geological.

Figure: Two faults that need to be joined (left), blue fault’s end Pillar deleted and the faults joined (center), the connecting Pillar converted to three shape points and moved to match form of blue fault while preserving shape of red fault (right).

Use High Angle Bifurcations

When fault data for two bifurcating faults are nearly coincident with one another near the bifurcation, problems can result. The faults are likely to cross one another, if not at the pillars then between them. The horizons and cells of the model created in this very narrow fault block often result in wild extrapolations and strange shapes. Delete the end Pillar(s) from the subordinate fault, join the fault to the main fault at an angle of 20 to 30 degrees or more. Check the secondary data (seismic horizon interpretation) to make sure the data are honored as well as possible.

Figure: Example of bifurcation with faults too close and crossing near the bifurcation (left) and corrected version of the same bifurcation (right).

Avoid Narrow Gaps Between Parallel Faults

Extremely narrow fault blocks make it difficult to insert horizons between the faults. The Make Horizons process parameters are usually set to blank some distance back from the faults and this often wipes out all the structure data in the narrow fault block. Opening the space between the faults even slightly often will allow a much better result. Joining the faults into one (especially if one or both have low displacement) will eliminate the problem. Doing no blanking in the Make Horizons process will help (you will likely have to blank the input data using a polygon to correct for data on the wrong side of the faults).




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Figure: Narrow fault block viewed from above with only one or two cells between the faults.

Extend Non‐tip‐out Faults Past Pillar Grid Boundary (Model edge)

Often the model’s edge will coincide with or be slightly outside the seismic data edge. In these situations the interpreted fault data for many faults will stop before the model edge. If the fault has displacement at the point where it stops, then it is not tipping out but is only out of data. These faults must be projected beyond the end of data out past the edge of the model. If the fault stops before the model edge, the data from opposite sides of the fault will “leak” around the end of the fault and create an incorrect structural form.

Figure: Three faults that should all extend with displacement to the edge of the model (left). The fault models for the top two faults did not extend to the edge of the model while the third fault’s did (right). Therefore, the structures on the high and low side of the upper faults join, creating a dramatic displacement reduction to zero.

Edit Mid‐Skeleton Line on Fault if Crossing in Pillar Grid

The mid‐skeleton line on the fault models is used by the Pillar Grid algorithm to decide if faults cross. In most cases the faults do not cross, only the mid‐skeleton lines (center‐lines) when seen in a 2D window appear to cross. To get Pillar gridding to work, you will have to edit these lines on the fault models. To edit the lines, you will need to have Key Pillars with 3 or 5 shape points, click on the middle line shape point, and move it along the Key Pillar to the desired position.

Figure: 2D showing center‐lines of faults (for Pillar gridding) crossing (top left), 3D showing the same faults and that they do not actually cross (top center), 3D straight down showing the position of the center‐lines of the faults (top right), 3D straight down showing edited center‐lines (bottom left), 2D showing center‐




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lines of faults no longer appear crossing (bottom center), and 3D showing that the actual fault geometries were not changed during the edits (bottom right).

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