How to make a contour map

Contour maps represent a three dimensional structure where X, Y, and Z displays how a quality like depth, isopach thickness, or porosity varies by location.

There are a few different methods for creating a contour map in Petra. Contour maps can be drawn by hand, by having the computer calculate and contour grids, or by a combination of hand contours and computer grids. Both methods have their strengths and weaknesses. Directly drawing contour lines allows more control over the map, while creating computer-calculated grids is fast and flexible. Using both hand contours and computer-calculated grids is a good way to harness the strengths of both methods.

Hand-drawn Contour Lines

Hand-drawn maps are best. An interpreter making a contour map knows the geology of an area and is able to incorporate this interpretation in a contour map, while a computer cannot. This method can be time consuming, however, especially with large data sets.

To create a contour line, first click on the pencil icon on the toolbox on the right side of the map module: The mouse cursor will turn to a crosshair. Click the left mouse button where you want the beginning of the line to start, and continue clicking the left mouse button for more control points on the line. Clicking the right mouse button will end the line and bring up the Overlay Line Attributes dialogue box.

The Line Stream method can be significantly faster than creating each line node individually. To enable Line Streaming, select Overlay>Line Stream Mode Activeor select SHIFT + CTRL + S on your keyboard. The normal pencil icon on the toolbar changes to . After selecting this button, mouse cursor turns to a crosshair. Click the left mouse button where you want the beginning of the line to start, and drag the mouse to draw the line. Note that Petra places line nodes automatically. The line stream method creates a large number of nodes, which may be time-consuming to move individually. To end the line, click the right mouse button. Clicking the right mouse button will end the line and bring up the Overlay Line Attributes dialogue box.

In addition to selecting line color and width, you can establish a line as a contour line. Contour lines draw like normal lines, but store and display a contour value. To designate a line as a contour line, click the Line Class dropdown box and select contour line. Designating a line as a contour line creates the Contours tab.

The Contours tab gives further control over the contour line.

Contour Value	This is where you establish the individual lines value. In the example above, the value of the line is 80.
Label Contour Line	Checking this box will tell Petra to label the contour line. Text size is chosen immediately below.
Hatchures	This tells Petra to draw hachures on contour lines. Draw contour lines clockwise for IN and OUT to respectively be inside and outside a closed polygon. From a lines start to finish, Petra will draw an IN hachure on the right and an OUT hachure on the left.

Gridding

In creating a computer-drawn contour map, Petra first extrapolates data between data points into a grid. Petra then draws a set of contour lines from this grid. The process of creating computer-drawn contour lines is generally faster than drawing contours by hand. Computer-created grids can also be used for more advanced calculations, such as volumetric and isopach summations. However, purely computer-created grids do not incorporate any form of geological experience or interpretation.

Creating a grid

The first step of creating a set of computer-drawn contour lines is to create a grid. A grid contains the mathematical extrapolation of your data points over an area. Grids can be calculated on any combination of the following data types:

·zone data items (such as fm tops and reservoir data)

·repeat tops from horizontal and deviated wells

·contour lines and faults displayed on the map module

·control points loaded into Petras memory or from an external file

·interval data

To create a grid, in the map module either go to Contours>Create Grid, or click on the Create a Grid icon: . This brings up the create Contour Grid box.

Selecting Data

First, select the type(s) of data you want to grid under the Zone Data To Grid.

Zone Data	Most grids will be on zone data from each well, such as reservoir thickness or formation tops. In the example above, the net pay thickness for the 2nd Wall Creek reservoir is being gridded. This value is stored under the 2WALLCREEK zone under the NET data item. Petra automatically creates both a grid title and grid file name based on this zone data, though these names can be changed. In deviated and horizontal wells, a data point (such as a formation top) can be at a distance away from the surface location. Selecting the Calc XY for Dev Wells At Top: button will calculate the correct XY location along the wellbore for the data point at the selected formation top. This feature needs survey data to be loaded into deviated wells.
Repeat Tops	Petra can store repeated tops from horizontal and deviated wells in a special format available on the Repeats tab on the Main Module. To use repeat tops while gridding, select the Tops tab on the Create Contour Grid dialogue box.
Contour lines	Contour lines are one of the most powerful ways to directly modify a grid. This option tells Petra to use all displayed contour lines as additional data points in the gridding process. Hand drawing a contour line can be useful for making a more realistic grid. Petra will use all contour lines from all displayed layers, so be sure to turn off the display of irrelevant contour lines. For a more detailed discussion of contour lines, see Hybrid Method below.
Fault Polygons	This tells Petra to use lines and polygons designated as faults in the gridding process. Petra will use all faults from all displayed layers, so be sure to turn off layers containing irrelevant faults.
Control Points	Control points are XYZ data that are not tied to wellbores. This is often seismic data.
Interval Data	Interval data can be particularly useful for storing data outside of Petras normal zone/data item database particularly for gridding petrophysical data along deviated and horizontal wells. To grid with interval data, select the Interval tab on the Create Contour Grid dialogue box.

After youve selected the data type, click on the Well Dist button. This tells you statistical information on distances between wells. This information on well spacing is useful in determining the grid size.

Generally, its best to make your grid size ½ the average distance between nearest neighbors. Its worth noting that this tool will calculate distance statistics between all wells displayed on a map without discerning if they have data or not. In other words, if you have a large number of closely spaced wells but only a few of those wells have data, this well distribution box will give you an average distance that is too small. When making a grid, it is generally best to select only the wells that have the right data and exclude wells with null values. In the example above, average distance is 806, so a grid size of 400 xy units is appropriate.

Selecting a Grid Size and Surface Style

The next step is to select grid size and surface style in the Method tab. Grid size simply governs the spacing of grid points, which can influence the smoothness of a grid. Small grid spacing generally better honor data points but are prone to bullseyes. Large spaced grids with fewer points are smoother but usually dont honor the data as well.

Though there are 4 ways to select a grid size, the best way for a new grid is to use ½ the average well spacing (see above). In the above example, average well spacing is 860, so grid size is 400 by 400.

Next, select a Surface Style. The surface style determines how Petra interpolates the data to create a surface for contouring. For most data, Highly Connected Features works best and handles faults well. For isopach maps, select Adjust Zero Contour for Isopach Surface. This option moves the zero line midway between a zero point and the closest non-zero data points. In effect, this option creates a grid with the more reasonable assumption that the interval pinches out before the wellbore penetration.

Other Advanced Features

One of the best options is Adjust Grid To Better Fit Control Points, which adds an additional processing step to force rectangular grids to honor the data better. Its worth noting that this feature still depends on a reasonable grid size. More specifically, a large grid size will break this feature.

There are many more options for manipulating the grid file available on the other tabs, but ultimately the best way to do that is to use contour lines. See Hybrid Method below.

Contouring a Grid

Now that the data has been extrapolated into a grid, Petra can draw a set of contours on that grid surface.

To contour a grid, in the map module either go to Contours>Display Contours, or click on the Contour a Grid icon: . This brings up the create Contour Grid box.

First, select the grid you wish to display. The grids title and a color range will be selected automatically. At the bottom of the Contour Range section, you have the option of selecting Normal colors and Enhanced Colors. Both of these options select colors for display, but Normal Colors is a little simpler for new users. Select Normal Colors, and select the Colors button.

Here, set the minimum and maximum values for the contours in the MIN and MAX boxes. The Interval sets the contour interval between colors. Note that normal colors can have up to 47 different intervals, while Enhanced colors can have many more intervals. The default color scheme is a rainbow from purple to red, but Petra comes with several other color schemes available through the Default button on the right side of the screen. You can also set your own color scheme by clicking on each individual interval and then clicking on a color from the palette for that interval. Click OK when the color bar is set, and OK again to contour the grid. The contour map should now show up on your map.

Hybrid Method

A hybrid method of contouring takes advantage of a computers speed and a human interpreters knowledge. It is fairly easy to modify a computer-calculated grid with overlay contour lines, as well as convert a grids contours to overlay contour lines. This allows for iterative improvement of a contour map.

Modifying a Grid with Contour Lines

Using hand-drawn overlay contour lines to modify a computer-calculated grid is a great way to add a more geologic interpretation to a contour map.

In the map below, a net pay map is discontinuous at the thickest part of the reservoir, and is only limited to the area immediately around the wells. A better interpretation of this data would smooth the contours and extend the northwest-southeast trend of the map.

The first step is to draw contour lines, as discussed above. Petra will incorporate these lines during regridding, and will modify the grid to fit them. This set of contour lines can be comprehensive (as shown below) or can be only a couple of short lines over a problem area. Make sure that these lines are designated as contour lines and have the correct contour value.

Once the contour lines are drawn, the next step is to recreate the grid using the contour lines. In the map module either go to Contours>Create Grid, or click on the Create a Grid icon: . This brings up the create Contour Grid box. Just like calculating a regular grid, put in the zone data to be contoured. This time, also add Use Overlay Contour Lines. Petra will use these drawn overlay contour lines.

The newly contoured grid now reflects the drawn contour lines to have a more realistic geological interpretation, as shown below. Hand-drawn overlay contour lines are easy to change, so this grid can be iteratively modified to create better and better maps.

Creating Contour Lines from a Grid

You can capture gridded contours to contour lines in an overlay layer. Once these lines are overlay contour lines, you can edit them just like any other line. This can be useful for letting Petra build a quick first pass at contour lines, and then using your skills as an interpreter to modify them into a more reasonable geological interpretation. Depending on the complexity of your contours, this can be very labor and time-intensive. For most contour maps, its probably faster and better to hand-draw overlay contour lines from scratch and use them to modify your grid as discussed above.

After creating a grid file, open the Contour a Grid dialogue box. Go to the Options tab, and turn on Draw Contour Lines and Draw Lines in Default Color Only. This will tell Petra to draw black contour lines.

Next, go to the Advanced tab. Select Capture Contours Directly Into Layer, and then select the Overlay layer you want to contain these contours. The R button allows you to rename the layer. In this example, the overlay layer has been renamed to CONTOURS.

This draws the gridded contours (in colors) and stores the computer-generated contour lines as overlay contour lines in the designated layer.

Turn off the grid, by either going to Contours>Hide Contours, or by clicking on the Hide a Grid icon: .

The grid is off, and now you are left with a set of fully editable contour lines stored in the designated layer. In the example above, one contour line has been selected and all the line points are highlighted in blue.

Notice that the computer-generated contour lines have a lot of points, and editing them one-by-one will take a long time. Its generally easier to reduce the number of points on the line by using the Simplify tool on the tool box. Its icon looks like this: . When youve completed modifying these contour lines, you can use them again in recalculating your grid (see Modifying a Grid with Contour Lines above).