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Using Structural Relief/Missing Section Growth Plots to Predict Missing Section on Unconformities on the Steeply Dipping Flanks of Salt Bodies

Richard E. Bischke, Daniel J. Tearpock, and Joseph L. Brewton
Subsurface Consultants & Associates, LLC, Houston, TX

Structural Relief/Missing Section growth plots were initially introduced to distinguish faults from unconformities in well logs on steeply dipping flanks of large salt bodies. They can also be used to more accurately predict the interval of missing section on an unconformity from well logs prior to drilling a step out or infill well. This method should allow the geoscientist to better predict where a large unconformity may eliminate the objective reservoirs or where the updip limit of a reservoir should be. The Structural Relief/Missing Section growth plot is generated by comparing the sub-sea depths of all the correlation markers in all the wells to each other to determine the differences in depth for the markers for each well. This results in two square matrices, one for missing section and the other for structural relief. These matrices are used to construct the plot with structural relief as the x-axis and missing section as the y-axis. In the example unconformity from a Gulf Coast salt dome, the data show a near linear, increasing trend. When missing section for faults are plotted against structural relief, the missing section tends to increase logarithmically until it stabilizes about a constant value. Estimated missing sections using this method for wells not used to generate the plot are shown to fall on the same near linear curve at their respective structural relief points. Standard log correlation methods can then be used to identify which reservoirs are likely to be missing at a given structural relief point.