Structure Data Structure data relate directly or indirectly to the horizons being modeled. Data relating directly are top picks from
wells, interpreted seismic events (time or depth), and digitized contours. Indirect data include fault polygons or
sticks usually from seismic interpretation and fault cuts from wells.
Top picks and fault cuts are sometimes interpreted in other programs. They can be imported into Petrel as X‐Y‐Z
point files or as points with attributes, in which case there can be many Z‐values linked to one X‐Y location. More
commonly, the
tops
are
imported
and
linked
to
well
bores.
To
do
this
requires
that
the
well
bores
exist
in
Petrel.
Creating well bores involves:
1. Creating a wells folder. 2. Importing the Well Headers which contains: X‐Y, well name, KB and other support information about the well.
3. Importing the deviation survey for the well. Once well bores exist then a tops folder is created and the tops, fault cuts or both are imported. Critical parameters
for this import are the well name (exactly as the well bore is named), the name of the surface or fault, the measured
depth (MD) and (optionally) the data type (horizon or fault). MD is almost always used rather than X‐Y‐Z (another
choice for importing tops) since MD will link the top to the well bore, while X‐Y‐Z data will force the pick to be
located in
that
position
regardless
of
whether
the
well
bore
actually
passes
through
the
location.
Figure: Points as a single X‐Y‐Z file (left), with multiple attributes (center), and linked to a well bore (right).
Tops and fault cuts are often picked in Petrel. To do this requires that logs have already been imported into Petrel.
The logs are displayed in a Well Section window and the desired tops or cuts named and picked using the Make/edit
well tops process. The picks can easily be reset to a different top pick or fault cut using the Well Tops spreadsheet.
Figure: Digitized contours (left) and the grid built from them (right). Note that the points on the contours are widely
spaced. This spacing is important when working with contours in Petrel to allow easy editing but does not impact
the quality of the grid.
Petrophysical Data Petrophysical data representing the average petrophysical value at the location where a well penetrates the zone
can be calculated in Petrel or by other programs and then imported into Petrel. Usually the calculated value is
stored at the location where the well penetrates the top of the zone for which the value is being calculated. If these
values are calculated in Petrel, they are stored with the top picks as zone attribute data. They can be extracted from
the Tops file as a separate point file for each zone. If these data are moved into Petrel from another program, they
will likely be X‐Y points with one or more Z values per point.
Figure: Petrophysical data in the Tops file displayed in a spreadsheet (left) and as a points file (right).
The creation of zone average petrophysical values in Petrel is not intuitive and would be a good subject for a future
TIPS&TRICKS article.
The
general
approach
is:
1. Have a log of the property to be averaged 2. Have a tops file containing tops between which the zone average values are to be calculated (be sure there are no missing tops)
3. Create a new attribute in the Tops file (Continuous or Discrete depending on what you are calculating)
4. Go to Attribute operations tab and calculate the value:
a. Check the radio button To the zones at level :
b. Check the radio button Sample from : Well logs
c. Select the Log to be used:
d. Select the Averaging method:
e. Adjust other parameters as needed f. Click on the Run button
5. Rename the attribute if desired 6. Change the attribute’s template, if needed 7. QC the values in a Well section
Figure: Attribute operations tab used to calculate zone average values (left) and Well section showing the original
log and calculated average value (right).
Build 2D Structure Grid for each Horizon 2D structure grids are built for each horizon to be modeled. The Make/edit surface process is used to build these.
Several data scenarios are used for this work and some of the most common are summarized below. Petrel 2009
and later releases have functionality that allow several files representing the same surface to be blended together
by the Make/edit surface process with each file being given a different weight.
Seismic Interpretation and Tops Seismic interpretation in depth is gridded and that grid tied to top picks in one pass of the Make/edit surface
process. Seismic interpretation data is the Main input. The Algorithm tab parameters control building the grid and
are usually allowed to default. The Geometry tab controls the X‐Y increments, rotation, and X‐Y limits and can be
automatically set using the input data. The Well adjustment tab allows top picks related to the seismic data to be
used to tie the grid. The influence radius for the correction can be controlled and the calculated error data and error grid output along with a report to understand how closely the original seismic was tied to the tops.
Often, the digitized contours are edited using the Make/edit polygons process after the initial grid has been created.
When editing is done the grid can be updated by simply right clicking on the grid and selecting Regenerate. This will
rebuild the grid using all the original parameters and avoids having to open the Make/edit surface process.
Figure: Edited contours in blue (far left), Regenerate option selected as a means to rebuild a grid when data content
changes but the parameters and names of the files do not (left center), grid before regenerate (right center), and
grid after regenerate (far right). Regenerate avoids opening the Make/edit surface process just to rebuild the grid
with the
same
parameters.
Mix of Contours, Seismic Interpretation, and Tops Often several data sets are needed to fully define a structure surface. For example seismic may cover a part of a
surface, hand drawn contours may be needed to define the rest of the surface, and well tops need to be tied to. This
little scenario is best handled by building a small workflow. You would create the main input to the Make/edit
surface process by copying the contours, refining by spline interpolation (add more points to contours), converting
to points, and appending the seismic data with the contour points. From that point on the same process that was
used to grid seismic interpretation and tops above is used. The example in the figure below is for sand thickness and
adds an additional modification step for the point data (eliminate zero valued data) but is basically the same. Note
that the files are always copied before they are changed.
Baselap and Truncation Relationships When structural surfaces intersect due to baselap or truncation, it is best to allow the surfaces to cross one another,
in fact it is desirable. Crossing means to allow the surface that “doesn’t exist” to project past and stay past the other
surface. In the next step of the Mapping Workflow, 2D horizon Grids will be intersected with one another. The
surfaces input to this step should be made to cross. If instead they are made coincident by performing an operation,
then the tool that combines all the surfaces into one framework may, due to re‐interpolation, create a pair of
surfaces that are almost coincident but not quite and that crisscross back and forth slightly.
Figure: Surfaces crossing at a truncation showing how the truncated 2D Grid is allowed to cross the truncating 2D
Grid (left) and what sometimes happens when the two are made coincident too early in the Mapping Workflow
(right). In this case, the truncated grid was edited slightly before being linked with the truncating structures and this
allowed the
two
surfaces
to
separate
in
the
area
of
truncation.
Build Structural Framework (3D Grid) The Mapping Workflow has not traditionally been thought of as a 3D Modeling process. However, Petrel has a
number of tools used for 3D Modeling that can be used for 2D Mapping. These tools make the incorporation of
geologic relationships, generation of isochores and displays, and calculation of volumes easy and quick (hours
instead of days). To use these tools requires that the 2D Mapping elements be moved into the 3D Modeling world.
Petrel has made it easy to do this using the Make simple grid process.
The 2D structure grids built earlier in the Mapping Workflow are linked together into a structural framework using
the Make simple grid process. How this is done and the parameter controls for doing it are described in detail in
SCM’s Tips & Trick entitled “Make Simple Grid”. To quickly summarize that document, you do the following steps:
1. Create 2D structure grids that all cover the same area, have acceptable geologic form, and cross in areas
where baselap
or
truncation
occur.
2. Open the Make simple grid process.
3. Insert the 2D Grids in top down stratigraphic order
4. Define their geological relationships (erosional, conformable, etc.) 5. Use one of the grids to define the X‐Y limits and grid increments to use 6. Build the 3D Grid (structural framework)
Figure: Parameters of the Make simple grid process used to build the structural framework.
Figure: The independent 2D structure Grids (left) and the horizons linked in a 3D grid and cut by a general
Once in a 3D Grid, cross sections can be generated in any direction through the framework, isochores can be
generated as 2D Grids for each zone with a push of a button, and gross rock volumes are calculated in a matter of
minutes.
Build Average 2D Petrophysical Grids for Each Zone Often in the Mapping Workflow a constant average petrophysical value is used for an entire zone. This allows you to
move quickly
into
the
volumetrics
portion
of
the
workflow.
Statistics
on
all
log
values
in
all
wells
for
a zone
are
calculated and the average used (if net‐to‐gross is used, then care is needed to ensure that porosity and saturation
logs used for averaging represent only net, otherwise a double dipping effect will be seen and result in lowered
volumes).
In projects where petrophysical properties vary laterally across the field, zone average grids are commonly used.
Building these grids is similar to building structural grids except you have some additional parameters that are used.
The input data are typically zone average points from a Tops or point file or they are zone average contours, or they
are both. For this discussion, average values in a tops file will be used to build the zone average grid. The general
approach to build a zone average porosity grid using the Make/edit process is:
1. Highlight the zone name from the Stratigraphy folder in the Tops file and arrow it into the Main input:
parameter.
2. Select the Attribute to be gridded.
3. Because Petrel will name all grids built using this approach the same (for the attribute) you must check the box in front of Name: and enter a unique name for the grid (the authors have always considered this a bug
in Petrel and perhaps in some release it will be fixed).
4. Go to the Geometry tab and set the parameters as desired.
5. Go to the Algorithm tab and adjust the parameters.
a. Usually use the Convergent interpolation algorithm
b. Usually check the Maximum value: and allow it to go +10% of input data
c. Usually check the Minimum value:
and
make
it
to
go
‐10%
of
input
datta
6. Go to the Post proc tab and adjust the parameters. a. Usually alter the Min Z ‐value: parameter to be Truncated and set the value to 0.0.
b. Sometimes alter the Max Z ‐value: parameter to be Truncated and set the value to a reasonable
upper limit (e.g., 1.0 for net‐to‐gross).
Using these parameters allows the grid to extrapolate (in Z direction) some but prevents it from violating reasonable
Figure: Parameters used to build a zone’s porosity grid.
Figure: 3D View of the porosity grid with data posted. Note that a little transparency has been added to see the
bottom portions of the text.
Build Petrophysical Properties (3D Grid) If all the structure and petrophysical grids for each zone are used as individuals then a considerable amount of work
is required to combine structures with fluid contacts to create isochores and then to discount those isochores by
N:G, Porosity, 1 – Sw, and so on (volume processing). Each of these operations is prone to errors and each set of
operations must be performed for each zone, a time consuming process. If these structure and petrophysical
elements are
combined
in
one
3D
Grid
then
Petrel
automatically
handles
all
the
volume
processing.
Since the structures were linked together in a 3D Grid in a previous step, it is a simple process to link the zone
average petrophysical grids to the zones of that 3D Grid. This is done using the Geometrical modeling process:
1. Make sure the correct 3D Grid is active
2. Open the Geometrical modeling process (under the Property modeling folder)
4. Set Select method: equal to Constant or surface in segments and zones
5. Select the Template that matches the property you are building
6. Uncheck Same for all zones
7. Check All surfaces 8. Highlight the 2D
petrophysical Grids
and
use
the
blue
arrow
to
insert
them
9. Click OK to create a petrophysical property. Figure: Parameters used to create the petrophysical property (left), location in the data tree of the Models tab
where the property is stored (center), and 3D fence view of the resulting property (right).
Create Fluid Contacts Fluid contacts start out as either constants (if horizontal) or 2D Grids (if tilted). These contacts are linked to the 3D
grid
using
the
Make
contacts
process.
You
will
need
to
determine
the
values
to
use
or
build
the
2D
Grids
prior
to
calling this process. The steps in creating a fluid contact set are:
1. Make sure the correct 3D Grid is active
2. Open the Make contacts process (under Corner point gridding folder)
3. Select or add the desired contact 4. Set the Contact type: 5. Enter the Contact name:
6. Insert the constant or grid representing the contact (note: it can vary for each zone, you did not use faults so it cannot vary by segment)
7. Click OK to build the contact. 8. The contact is stored in the 3D Grid on the Models tab.
