Horizon markers are commonly used to define layer boundaries. Correctly defined horizon markers are critical to the accuracy of the 3D model.
In GridSTAT, you can generate a correlation horizon from well logs, modify horizon markers interactively, or use the autopick function to modify the markers by setting a tolerance.
To generate a correlation horizon from well logs, go to the Options pull down menu and select Analysis. Initially, GridSTAT sets up the default parameters and the depth range includes most of the data. You may modify the depth interval and the correlation parameters such as shift tolerance, dip average (leave as 0 if no clear average dip can be defined) and down dip azimuth, number of neighbor to correlate, and minimum number of data points required for a well to be included. If there are multiple data types in the project, first go to the Options pull down menu and select Data&Unit, then select the primary data to correlate, before coming to the Analysis panel. With all parameters selected, click the Auto 1 Horizon button to perform the correlation. A marker file will be generated and the results can be inspected from the MarkerHeader table, Markers graph, or FanSection graph.
Markers may be edited from the MarkerHeader table in DataQC panel. First, from the Table pull down menu, turn (toggle) Edit On.
To modify the markers interactively, you need to graph FanSection in DataQC panel. Go to Options pull down menu, select FanSection and turn Edit Mode on. Use the left mouse button to drag a trace around to overlay it with other wells. Click the right mouse button at a trace to anchor the new marker depth on the well sticks. To add a new horizon marker, use the Add button on the left of DataQC panel and select New Marker. This will add a null marker to each well and then you can anchor the depth at each well interactively. Note that different traces in the same well can have separate markers. To separate the different traces in the same well on the cross section, go to the Options pull down menu, select FanSection, and turn on Equal Distance.
To help guide the correlation, you may graph Var-Shift, Var-Shifted, Crossplot or C.plotShifted graphs. The Var-Shift graph displays multiple curves for multiple well pairs if a well list is picked and the number of neighbors is larger than 2. If a grid has been made, drawing the contour on the cross section helps determining the tops and the connectivity.
The autopick function searches for a best match between log traces and updates the markers accordingly. Go to the Markers panel from the Options pull down menu and specify shift tolerance for the marker to be modified. If the shift tolerance is 0 for a marker, that marker will not be changed. It is possible to modify multiple markers simultaneously, but the time it takes to search the match can be very long. Therefore it is recommended to modify one marker at a time. To graph the matching variance without modifying the markers, after setting the shift tolerance from the Markers panel, use the Graph button on the left of DataQC panel and select Var-Marker graph.