Seismic Interpretation in Petrel

▼ Petrel

In this article we review seismic interpretation in Petrel 2009
The input data can be downloaded from http://www.mediafire.com/?ci6exih18ng9ysn
You can also find seismic data for Gullfaks oilfield in the public domain: http://www.ipt.ntnu.no/rose/doc/syntlib/Gullfaks/index.html

Seismic format

The standard format for 2D and 3D seismic data is SEGY, a binary format that can easily be imported into Petrel.
The SEGY data are not copied into the Petrel project, rather a link to the seismic data is created. Any type of SEGY cube can be imported (e.g. full-fold data, near- and far offset cubes, etc). Certain operations are more efficient when seismic data are resident in memory (e.g. autotracking) but large SEGY volumes may be too large to load the entire survey into memory. Cropping and realizing seismic data allow you to work effectively with large volumes of seismic data.
In addition, data in ZGY bricked format can be imported. ZGY format cannot be realized.

Import/Export Seismic Data

Import Seismic Data

Seismic data is stored under a specific Seismic Survey folder. You can have multiple and rename Seismic Survey folders.

How to import seismic data:

Insert a New Seismic Survey folder.
Right click on the Survey 1 folder and choose Import (on Selection).
Select the file, choose Files of type: SEG-Y seismic data (*.*) then click "Open".
In the Input Data Dialog window, select the proper color template and correct domain.

Export Seismic Data

How to export seismic data:

Right click on a seismic survey and choose Export.
Choose SEG-Y as the format and specify the output file name, then click Save.

Seismic Data Visualization

3D Window

Right click on a seismic volume to access the pull-down menu. Here you can choose to create seismic lines, crop a volume, realize a volume, generate attribute volumes etc.
Double-click on survey to open Settings dialog where the basemap annotation and other display parameters can be modified.

To display seismic lines:

Open the seismic cube folder. Immediately available are Inlines and Crosslines. To add other seismic line types right click on the seismic cube folder and select one of the ’Insert ...' Intersection options. Objects are added to the Input tab under the seismic cube folder.
Toggle on the check box in front of a seismic line.

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2D Window (‘Base Map’)

A 2D window will serve as a basemap when the seismic interpretation process is active. Within a “basemap” window you can:

Display the frame of the seismic cube(s) with annotations.
Display 2D lines with annotation.
Display inline, crossline, randomline and other seismic line traces.
Select seismic lines.
Create various arbitrary seismic lines.
Display interpretations of horizons and faults.
Display other data such as wells, tops etc.

To create a seismic basemap:

Open a 2D window.
Toggle on the checkboxes in the Input tab to display seismic lines.
Double click on the Survey folder to open the Settings dialog (Or right-button click → Settings).
Under the Style tab specify the Base Map annotation.

Interpretation Window

Note: You can interpret in a 3D window or an Interpretation window.

The seismic interpretation window gives the user a classical 2D view of seismic data. Inlines, crosslines, time slices and other line types can be displayed in an interpretation window. All interpretation tools are available in this window and the view can be zoomed, stretched, and squeezed in either direction. Inline, crossline and randomline annotations are posted at the top of the window. These lines can be selected by clicking on the small triangle at the top with the left mouse button. Dragging the triangle allows you to change the position of the annotated line.
The time-depth scale will show the time or depth in accordance with the project unit settings. Time seismic will be in either seconds or millisecond and depth seismic will be in meters or feet.
If multiple windows are open the displays can be linked.

Note: TimeSlices cannot be displayed in an Interpretation Window.

There are 2 ways to create an Interpretation window:

Method 1:

Right click on a seismic line and choose Create Interpretation window.
The line will automatically be displayed in the window.

Method 2:

Go to the Window menu on the top tool bar and select New Interpretation Window. An empty Interpretation window will appear.
To display a seismic line in the Interpretation window toggle on the check box in front of a seismic line.

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Work in Interpretation Window

To Pan position cursor on the lower/right white bar (horisontal/vertical movement) or anywhere else (free movement) - move using the left mouse button. Pan sideways or vertical until a grey area appears (lower or right).
To Zoom position cursor between the white and the grey lower/right area in the bars and move using left mouse button (or use Left mouse+Ctrl+Shift).
To Squeeze hit the line between the grey/white area. Use Shift key + left mouse button to stretch/squeeze.

Notes: It is also possible to zoom using the Magnifying glass in Function bar. Drag a square using the left mouse button. To un-zoom use Shift+Z.

Seismic Lines

Selecting Seismic Lines

Method 1:

Activate line (inline, crossline, randomline etc) in Input tab.
Use the player at the bottom of the window.

Method 2:

Activate line (inline, crossline, randomline etc) in Input tab.
Activate the Seismic Interpretation process
Click Manipulate Plane icon.
Drag in window using Left mouse button.

Method 3:

Activate the Seismic Interpretation process and activate the proper cube (bold).
Use the icons in the Function bar.

Note: these methods work for both 2D and 3D windows.

Random/Arbitrary Seismic Lines

In addition to displaying Inlines, Crosslines and Time slices, you can create random or arbitrary lines through a seismic volume. There are five methods:

Automatically. Similar to an inline or crossline in that it is a vertical intersection extending through the seismic volume without any bends or turns.
Using an arbitrary polyline intersection. In a 3D or 2D window, you define the desired path of the line.
Using a seismic aligned polyline intersection. Similar to an arbitrary polyline intersection, you define the desired turning points but the path between points is snapped to either the inline or crossline direction.
Using a combination of arbitrary and aligned polylines. Same process as for 2, use the Ctrl key to snap to either the inline or crossline direction.
Between selected wells. This is called a Well Intersection Fence.

Arbitrary Polyline Intersection
To create data in a 3D window, there needs to be an object to supply X, Y and Z reference points.

The arbitrary polyline intersection is composed of 2 parts, the polygon trace and the vertical seismic intersection. The polygon trace is stored under the General Intersection object in the Input tab. Remember these “General Intersections” can be renamed to something more meaningful.

How to digitize an arbitrary polyline intersection:

Activate the Seismic Interpretation process.
If using a 3D window, post an object (e.g., timeslice). Not required in a 2D window.
Click on the ”Create arbitrary polyline intersection” icon.
Draw the polygon using the left mouse button and stop by double clicking. The vertical seismic intersection will automatically be displayed.
The general seismic intersection together with the polygon will be put under the Input tab in the Petrel Explorer window.
To edit the intersection activate the Make/Edit Polygons process, activate the Arbitrary polygon and toggle on the ”Select and Edit/Add points” icon in the Function tool bar.
Edit a point or the whole polygon line and the seismic intersection will be changed according to the polygon.

Seismic Aligned Polyline Intersection
How to digitize a Seismic aligned polyline intersection:

Activate the Seismic Interpretation process.
If using a 3D window, post an object (e.g. timeslice). Not required in a 2D window.
Click on the ”Create seismic aligned polyline intersection” icon.
Draw the polygon using the left mouse button and stop by double clicking. The vertical seismic intersection will automatically be displayed.
The seismic intersection together with the polygon will be put under the Input tab in the Petrel Explorer window.
To edit the intersection activate the Make/Edit Polygons process then activate the polygon and toggle on the ”Select and Edit Line” icon in the Function tool bar.
Edit the polygon line and the seismic intersection will be changed according to the polygon.

Well Intersection Fence
This is an arbitrary seismic line connecting selected wells. You can interpret horizons and faults on a well intersection fence.
How to create a Well Intersection Fence:

Go to Window and create New section window.
Go to the Wells folder and toggle the wells you want to include.
Open 3D Window. The Well Section Fence is placed in the Well Section folder in the windows pane, only active when 3D window is open.
Activate Well Section Fence.
Click on the ”magic” blue button: Toggle visualization on plane.
Toggle on the blue checkbox in front of the Seismic 3D cube.
The seismic will be displayed on the Well Intersection Fence. Logs and Well Tops can be displayed in time if a Time-Depth relation is available.

Wells

Wells – Displaying Data in Time

In oder to display well data in time, a time-depth relationship for each well is required. The general procedure is:

Load velocity data for wells (e.g. check shots, sonic logs etc).
Create time log for each well. This will determine the time-depth relationship to use for displaying well data in time.
Display well bores. (Toggle on the checkbox next to the Wells folder to display all wellbores or expand the Wells folder and toggle on individual wells. You can adjust display parameters by double-clicking on the Wells folder and going to the Style tab.)
Set domain and display picks and fault cuts.

Wells – Import Check Shots

Check shots are time-depth (MD) pairs for a borehole obtained by analyzing the first seismic arrival for known depths in the borehole. They are used to obtain a time-depth relationship for the borehole and are the best source of time-depth data for generating synthetic seismograms. Check shot data can be imported with one file for each well or with a single file for all wells. Like Wells and Well Tops, check shot data must be loaded into a Petrel specific folder. The process for importing check shots is identical to that for loading well tops. If the file does not have a column containing a well name, Petrel will assume that a single file is used for each well. It will then attempt to attach the check shot file to a well based on the file name; the user can override this and manually select the well if necessary.

Import Check Shots:

Create check shot folder by going to Insert -> New Check Shots.
Right click on the Check Shot folder under Wells and select Import (on Selection).
Select file(s) and format and click Open.
Adjust parameters if necessary and click OK.

As with deviation surveys and well tops, check shot data can be edited using the spreadsheet. Right click on the Check Shot folder and select Spreadsheet.

Wells – Create time logs

Create time logs:

Right click on the Wells folder and select Settings... (or double-click on Wells folder).
Go to Time tab. If there are multiple entries, make sure check shots are first on the list and click Run.
A time log will be created for each well.

Wells – Display Well Tops in Time

This will determine the time depth relationship used for displaying the wells in time. All the possible sources of such a relationship (check shots, well tops, sonic logs etc.) are listed in the window and can be set in order of preference. For each well, the highest available item on the list will be used as the time depth relationship.
Manual adjustment can be used to stretch or squeeze the time section by adjusting the time depth relationship. This is used when events can be correlated between the synthetic seismic and the 3D seismic cube.
Override settings on all wells in this folder can be used to change the settings for a particular folder of wells.

In the top toolbar, change the domain to TWT.
Toggle on the check box next to the Well Tops folder. Expand the Attributes folder under the Well Tops folder.
Toggle on TWT Auto attribute.

Horizon Interpretation

There are different methods for horizon interpretation in Petrel:

Manual interpretation (drawing)
Guided autotracking
Seeded 2D autotracking
Seeded 3D autotracking
Paintbrush autotracking
Active box autotracking

The autotracking settings for each of the interpreted horizons are held in the settings for that horizon. To begin a new interpretation, you must first add an interpretation folder if one is not already present in the project.

Insert Seismic Horizon

To start interpretation seismic horizon in Interpretation folder must be activated. If there is no Interpretation folder or horizon it must be inserted.

Insert a New Interpretation Folder. The folder will appear in the Input tab in the Explorer window.
Right click on the Interpretation folder and select Insert New Horizon.
The new horizon will appear under the Interpretation folder

Both the Interpretation folder and the New Horizon(s) can be renamed as necessary. Double click on the folder to open the Settings dialog and change the name on the Info tab. You can have multiple interpretation folders and add sub-folders if necessary.

Horizons can be interpreted in 3D and Interpretation windows and on most seismic line/displays (e.g. inline, crossline, randomline, arbitrary line, well intersection fence etc). The exception is timeslices, horizons cannot be interpreted on timeslices.

Horizon interpretation points will be added to the active horizon (bolded in the Explorer). To change to a different horizon, use the Deactivate Current Interpretation icon on the Function bar or select the horizon to use by clicking on it in the Explorer. If you start interpreting without an active horizon, a new horizon will be created and added into the Interpretation folder.

Autotracking Settings

Autotracking settings are very important for seismic interpretation.

Open the settings for the horizon (right click on the Seismic horizon folder and select Settings... or double-click on Seismic horizon folder) and choose the Autotracking tab.
Set the tracking constraints, such as Seed confidence, parameters under Geometry and Constraints tab and the type of seeds to use as input, or use the default settings.

3D Autotracking Settings (for Petrel'07. To correct see Petrel Help )

Signal tracking feature: Controls the feature to be tracked; Peaks, Troughs, S-crossing, Z-crossing, Peaks or Troughs, Any Zero Crossing and None (flat).
Seed Value Confidence: Specify the minimum value for the seismic amplitude, as a percentage of the seed point(s) values.
Value range: Specify a minimum and/or maximum amplitude value seismic attribute to be tracked. Hence, picks outside of these amplitude limits will be rejected.
Max Value Delta: Controls how much the amplitude is allowed to change from one trace to the next. Picks with higher amplitude change will be ignored during autotracking.
Vertical range: Set absolute vertical limits for the autotracked horizon in project units.
Max Vertical Delta: Controls how much the vertical position are allowed to change in from one trace to the next.
Trend Data: Select a surface or a previous interpretation as a trend to guide when autotracking.
Expansion Quality: Controls how stringently seed points are checked before being expanded. None: readjust existing points. Basic 3×3: check the 8 closest points against the seed point. Validated 3×3: check the 8 closest points against the seed point and two additional neighbors. Validated 5×5: check the 24 closest points against the seed point and their neighbors.
Boundary polygon: Specify a closed polygon which the autotracking is not permitted to expand beyond.
Stop at visible faults: Stop autotracking at interpreted faults or fault in the fault model.
Seismic Volume: Select the seismic volume to use in the autotracking. If not selected, the volume associated with the active lines will be used.
Visible points: Use all the visible points in the current interpretation as seed points.
Selected points: Use the selected points in the current interpretation as seed points.
Adjust seed points: Determine whether or not seed points can be reinterpreted (off as default).
Lock filter: Use an interpretation filter to specify points that can’t be reinterpreted.
Override filter: Use an interpretation filter to specify points that should be reinterpreted.

Tip! Use a boundary polygon when investigating the parameters before the tracking. That way you can test the parameters on a smaller area. When you are content with the parameter settings you may use the interpretation within the boundary polygon as seeds for the complete volume.

Autotracking

Interpretation is performed by horizon interpretation tools under the Interpret grid horizons icon in the Function bar.
3D Autotracking in a 3D window

Insert a new interpretation horizon in the Interpretations folder if one doesn't already exist.
Make sure Interpret grid horizons is on (shortcut key H).
Click on the Seeded 3D autotracking icon (shortcut key Shift + A) in the Function bar.
Start the autotracking by clicking on an event on the seismic.
If some interpretation already exists for the seismic horizon, pressing 3D Track will expand the interpretation based on the multiple seeds.

The seeded autotracking will follow the picked event until it reaches a disturbance in the data quality of some sort (e.g. fault, noise, etc.).

Guided Autotracking

To specify the tracking mode (e.g. peak or trough), double-click on the Horizon to open the Settings dialog. Go to the Tracking tab and make the appropriate selection under Signal tracking feature.
Toggle on the Guided autotracking icon.
Using the left mouse button, select the starting and ending points along a reflector. The interpretation will follow the event between the 2 points.To end an interpretation segment, for example at a fault, press N or H on the keyboard.

Note: If you have not selected the type of event to pick on (e.g. peak or trough) on the tracking tab, the polarity of the event you start on will be used (e.g. where you click first with the mouse).

Seeded 2D Autotracking

Method 1:

Start Seeded 2D autotracking by clicking on the Seeded 2D Autotracking icon.
Then left click on an event. The interpretation will follow the event in both directions of the point selected and will stop when it encounters an abrupt change in the amplitude value.

Method 2:

Highlight line direction to interpret on, for example Inline (make bold).
Select Seeded 2D autotracking.
Interpret on the inactive line direction (crossline).

Edit the Interpretation

The Undo/Redo steps are limitless. These are saved for each horizon and can be retrieved when Petrel is reopened (if the project is saved). For each interpreted horizon, it is possible to remove undo history. This may improve process speed if too many undo steps in history. Hint; always check to see which horizon or fault is active (bold in the Input tab) before using Undo/Redo. Pausing the mouse over the icons will display exactly what the icon will do, for example after autotracking, the Undo icon will read: “Undo Add XXX points (autotracked)”.

To edit the interpretation use the following:
Selection Paintbrush – use a box to ‘paint’ the points you wish to select. Works in either a 2D or 3D window. Increase the size of the box using the ‘+’ or ‘-’ keys on your keyboard.
Boundary Box Select – select a rectangular area by dragging a box using the left mouse button; multiple areas can be selected by holding down the Shift key while dragging the box (Ctrl while dragging the box de-selects points). All points or lines inside the box or boxes will be selected.
Select and Edit/Add Points – individually select points using left mouse button; use Ctrl left mouse button to select multiple points. To move selected points horizontally, click again with left mouse button and drag the points to a new location. To move vertically, hold down the Shift key while dragging the points.
Eraser – like the Selection Paintbrush, use a box to paint points you wish to delete. Works in either a 2D or 3D window. Increase the size of the box using the ‘+’ or ‘-’ keys on your keyboard.
Delete icon or Delete key – delete selected points.

Manual Interpretation

In some areas it is necessary to do manual interpretation, for example areas without a continuous reflector or areas with poor seismic data quality.
To do manual interpretation:

Activate the Manual Interpretation icon and start interpreting using the left mouse button. To insert a flag use the shortcut keys N or H.
When the reflector is stronger and more continuous, use Guided or Seeded 2D autotracking.
To step a seismic plane backwards, use the Player and specify the Increment.

Fault Interpretation

Faults can be interpreted in 3D and Interpretation windows and on any kind of seismic line/display (e.g. inline, crossline, timeslice, randomline, arbitrary line, well intersection fence etc). Faults can also be interpreted directly into a Petrel fault model. This will be discussed in the section on fault modeling.
Fault interpretation will be added to the active fault (bolded in the Explorer). To change to a different fault, use the Deactivate Current Interpretation icon on the Function bar or select the fault to use by clicking on it in the Explorer. If you start interpreting without an active fault, a new fault will be created and added into the Fault Interpretation folder. Faults can be renamed by opening the Settings dialog and changing the name on the Info tab.
There are two formats commonly used for faults in Petrel, fault interpretations and fault sticks. Both formats consist of points connected by lines, however fault interpretations can be displayed with a triangulated surface and fault sticks cannot. Fault sticks are characterized as polygons while fault interpretations have their own format (illustrated by icons in Petrel Explorer). You can only interpret faults as fault interpretations, however fault interpretations can be converted to fault sticks and visa versa. Both fault interpretations and fault sticks can be used when creating a fault model. More on this in the section on fault modeling.

Fault Interpretation Workflow

Insert a New Interpretation Folder. It will be placed in the Input tab in the Petrel Explorer window.
Double click on the Interpretation folder and change the name to ”Fault Interpretation” in the Info tab. Click OK.
Right click on the Fault Interpretation folder and choose Insert New Fault.
Activate the Fault Interpretation icon in the Function bar and start interpreting.
Faults are digitized as segments, which are triangulated to display as a surface.
To step a seismic plane forward or backwards, use the Player and specify the step increment.

Using Faults as Constraint when Autotracking

Go to the Tracking tab in the settings for a Horizon and toggle on Stop at visible faults in the Lateral constraints section.
Interpreted faults will be used to stop the Autotracking.

Note: Fault interpretations and fault pillars can be use to constrain the 3D autotracker, fault sticks cannot.

2D Survey Manager

The survey manager in Petrel can be found by right clicking on the survey folder and selecting Survey Manager. The Survey manager is a tool used to sort and manage the seismic data in Petrel. Within the survey manager the user can control all parameters regarding 2D lines, like CDP, SP, Trace number, navigation file assignment, and can sort and move data into folders. The Survey Manager can also be used to visualize data.

Mis-tie Analysis

The Mis-tie Manager in Petrel can be found by right clicking on the survey folder and selecting Insert Mis-tie Set. The Mis-tie Manager is used to set vertical, phase and/or gain mis-ties and make corrections.