Ellenburger Formation hydrocarbon reservoirs in West Texas produce world-class reserves from fractured carbonates. These fractures are related to both intense karstification or cave development, and a tectonic fracture system that pervasively overprints the karst-related fractures. Many tectonic fractures are related to high-angle reverse and wrench faults found along the margins of the Central Basin Platform. A major problem in modeling Ellenburger Formation fracture genesis, is delineating the magnitude of crustal shortening that caused tectonic fracturing. We have developed a rapid, innovative approach to quantify regional shortening that permits us to more easily model crustal shortening, and allows us to identify and predict zones of subsurface faulting.

To calculate regional shortening, we performed a computer-based calculation of the horizontal and vertical components of regional displacement. We calculated the total strain shortening partitioned between flexures or folds, and fault displacements. Using an Ellenburger datum composed of several thousand well penetrations, we used computer-based algorithms to calculate the incremental X,Y and Z direction shortening on an enveloping surface overlying the data grid. From this, we plotted maps showing the magnitude and orientation of the local crustal shortening. Although the calculation is strongly influenced by grid spacing and data availability, somewhat limiting its universal applicability, we tested the method using tight well control available along the Central Basin Platform margin. Given the absence of major overthrusting along the eastern margin of this structure, we have been able to calculate the importance of flexure or fold-related shortening relative to fault displacement. We have found a strong correlation between subsurface fault locations and high horizontal shortening gradients. This shortening, when mapped over the general Permian Basin, locates almost all major faults and related structural features as reported in the structural geology literature of the area. Preliminary results show that faulting is the dominant mode of crustal shortening and that shortening due to flexure or folding is subordinate.

Work in progress is refining the model and applying it to data sets from other basins to verify the transferability of the method. Preliminary results suggest that subsurface fault zones can be readily identified with the method, provided well control is adequate relative to the spatial geometries of the fractures and fault systems. This approach promises to be a rapid method to calculate regional shortening and identify the location of most major fault systems in areas with sufficient subsurface well control.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas