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Using Structural Restoration Techniques and Strain Tracking to Predict Fracture Distributions


Structural restorations and forward modeling of the Teapot Dome anticline (Wyoming, USA) show that the evolution of fracture proxies over time can be tracked through the formation of a structure. This allows to determine where and when fracture proxies such as curvature, dip, or strain were at their highest values and how these zones of potentially high density fractures developed over time. Knowledge of the time and boundary conditions for individual fracture sets can then be used to develop predictive 3D models in present-day fractured reservoirs. Strain can be used as a fracture proxy whereby maximum and minimum shear strains as well as the cumulative shear strains may be indicative for different types of fractures and permeabilities. The cumulative strain is calculated as the summation of the individual finite strains at every modeling step and therefore best represents the total damage to the rocks over the course of the deformation history Using the fractured reservoir field of the Teapot Dome anticline (Wyoming, USA) as a case study, the anticlinal structure is restored by trishear on the main fault and then forward modeled in a sequence of time steps while tracking cumulative strain and evaluating fracture formation conditions based on strain intensities and orientations. Tracking maximum and minimum elongations and shear strains reveals the patterns that potentially govern fracture densities. The fracture system at Teapot Dome consists of several sets that typically develop in an anticline with one set parallel and one set perpendicular to the fold axis. A third set that runs oblique to the fold hinge has been described as predating folding. Permeability is most increased in areas close to the fold hinge and along cross faults that transect the fold hinge in a high angle. Most wells have been drilled in a wide zone along the hinge zone following maximum curvature, and wells away from the hinge into the steep western limb are sparse. However, well data in the area of the key section and highest structural culmination shows highest production rates slightly west of the fold hinge and coinciding to a large extent with the highest cumulative shear strain. When comparing the strain modeling results with other studies, there is also a good match with observed seismic average energy maps.