Interpretation of wrench faulting and faultrelated pressure compartmentalization, Wattenberg Field, Denver Basin Colorado
A structural interpretation of a 128 km2 3D seismic data set from Wattenberg Field, Colorado indicates a northeast-trending step-over fault system in the Niobrara Formation. These step-over faults are the product of a fault block rotation that is caused by two large-scale dextral wrench faults: the Longmont and the Lafayette wrench fault zones. An investigation into different fault orientations between the Niobrara and the overlying Pierre Shale indicates that a rotation of the maximum horizontal stress has occurred during the Laramide Orogeny. The step-over faults and stress rotation result in reservoir compartmentalization in the Niobrara. Seismic inversion shows the different stress field compartmentalization. Low impedance occurs in a lower effective stress environment caused by high pore pressure. These compartments may constitute ‘sweet spots’ in exploration and production in the Niobrara Formation. The Niobrara Formation is a major hydrocarbon reservoir deposited during the Late Cretaceous (Figure 1a). This formation consists of the Smoky Hill Member and Fort Hays Limestone. The Smoky Hill member consists of A, B, and C chalks, and A, B, and C marls. Bratton (unpublished work, 2015) indicates that the overpressure regime extends into the overlying Sharon Springs Member of the Lower Pierre Formation. Figure 1B shows the geologic events of the Late Cretaceous Petroleum System (Higley and Cox, 2007). The Laramide Orogeny occurred during the Late Cretaceous and Early Tertiary (between 70-50 ma.). This event is thought to be responsible for the structural features in the study area (Higley and Cox, 2007). The products of this movement include several wrench fault zones located in the area mapped by Weimer (1996).