IMPROVING SUBSURFACE STRUCTURAL AND STRATIGRAPHIC ANALYSIS USING TREND SURFACE RESIDUAL ANOMALY MAPS

Well log data has historically been analyzed using structure contour and isopach maps. Trend surface residual anomaly (TSRA) maps are an especially useful method for identifying high frequency/low amplitude variations (such as small-displacement faults or localized folds), especially in sub-horizontal strata. A TSRA map is derived by subtracting a low-order trend surface (generated from structure contour data) from the structure contour map. Positive values indicate that the vertical location of a pick is higher in elevation than the calculated trend surface value; likewise, negative values indicate the opposite situation. The quantitative values represent a deviation from the expected. The only two factors that influence the residual anomaly values are the accuracy of the structure contour data and the order of the trend surface. The most reliable residual anomaly values are achieved when known fault displacements are slightly smaller than the residual values. Typically, first- or second-order trend surfaces are used because higher order surfaces tend to remove the desired correlation possibilities of the residual. Overall, the TSRA method can be used to improve detection and correlation of individual fault systems, subsurface deformation, structural trends, and sole horizons; whereas it can be more difficult to achieve these results using standard subsurface maps. After TSRA maps are used to identify a structure, standard in-depth well log mapping and seismic data can be used to define the detailed characteristics of the structure.