*3D Horizon/Fault Interpretation Workshop

Geology 493M

Day 3 - Horizon Interpretation and Correlation

42. Creation of a depth map is a two-stage effort that begins with construction of an average velocity map. To create a velocity map, selectToolsfrom the main Menu Bar and then Depth from the drop down list. Under Depth there areseveral selections. Click on Compute Average Velocity Map. For Type, select Horizon. The program computes the average velocity at each well using one of three options (Apparent, Time Grid or Formation Top) (Figure 27).

Figure 27: Method used to compute the Average Velocity Map of a selected horizon is selected in this menu.

1)The Apparent method used to construct the average velocity map uses the horizon time and formation top depth. You must provide a velocity map name (Figure 27). Gridding parameters can be tailored to individual needs (Figure 28). Time and depth pairs are then combined to form an average velocity grid.

Figure 28: Gridding parameters selections menu.

Average velocity in this (Apparent) approach is computed by dividing formation top depth by half the horizon time. Whether you Extrapolate (Figures 27 and 29) or not (Figure 30) will yield two different results. Extrapolation will project the resulting velocity grid outside the area of well control.

Figure 29: Velocity map formed by extrapolation.

Figure 30: Velocity grid of C38 horizon without extrapolation. Velocities were derived using the Apparent method.

2)The Time Grid method of deriving the velocity map uses the horizon time picks, converts them to depth using the well time/depth function and then generates a velocity grid. This velocity map (Figure 31) is considerably different from that obtained using the apparent method.

Figure 31: Velocity grid from Time Gridding (no extrapolation).

Figure 32: Velocity grid from time griddingwith extrapolation.

Comparison of Figures 29 and 32 reveal notable but minor differences in this example.

3)The Formation Top method of deriving a velocity map starts with the formation top depth, converts it to time using the well time/depth function and then generates a velocity grid. If the horizon and formation top do not tie, three different velocities can be generated. Use the default grid parameters as a first pass for each velocity map. Depths obtained from this approach (Figure 33 and 34) reveal subtle differences.

Figure 33: Velocity grid obtained from Formation Top method with extrapolation to the borders of the survey.

Figure 34: Formation Top conversion without extrapolation yields this velocity grid, which has been extended to incorporate well #10 along one of the 2D lines external to the 3D survey.

43. Once the velocity map has been generated, you are ready to convert times to depth. From the top menu bar click on Tools then Depth, then Depth Map by Average Velocity Map. This will open up the menu shown below (Figure 35).

Figure 35: Depth Map by Average Velocity Map menu.

Supply the appropriate information in the above menu (Figure 35) and then click OK. The Grid Parameters window (not shown) will then appear. Parameters can be tailored to your specific project. Click OK to generate the Depth Map (Figure 36).

Figure 36: Depth map derived from the average velocity map.

Edge effects may be noticeable on your depth map. These areas can easily be removed using the Grids, Polygon Erase utility. Polygon erase will provide you with a "rubber band" that you can use to define an enclosed region to delete. The operation is identical to that used to digitize fault polygons. Digitize the points around the region you wish to delete and double click to complete the operation.

44. Contour the depth map and display the amplitudes under the contours. Remember that you can contour your maps using the Map Select Contour Overlay options. Contours of velocity for the C38 horizon are shown below (Figure 37).

Figure 37: Depths obtained from time-gridding (Figure 32) have been contoured for the C38 horizon.

Amplitudes of the C38 reflection event can then be followed along the structure by dragging the amplitude horizon from the project menu onto the map (see montage Figure 38). Note the association of amplitude anomalies with the faults in this example.

Figure 38: Reflection event amplitudes combined with a depth contour overlay for the C38 Horizon.