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Paleotectonic Controls on Structure in Southern Sand Wash Basin and the Potential Influence on Fracture Orientation and Distribution within the Niobrara Formation: An Integrated Seismic and Geologic Approach

Lin, Vivian; Sarg, Rick; Sonnenberg, Stephen

The structures in the southern Sand Wash Basin in Northwestern Colorado have been identified as having anomalous orientations that are influenced by the transition from the E-W trending Uinta Mountain Arch to the West to the primarily NNW trending structures that dominate the rest of Colorado. The tectonic history of this area is complex with large compressional features that were influenced by previous rifting structures that run oblique to the younger principal stress orientations. The paleotectonic controls on the region were determined from an integrated study of a 25 mi2 area using a 3-D seismic survey and well log data. This was accomplished by mapping the structures observed within the survey area and analyzing the geometries and relationships of the stratigraphy.

Based on this structural analysis and 3-D Move forward modeling, a principal maximum stress direction was determined for Laramide and post-Laramide tectonics, which have the most influence on Niobrara Formation fracture orientations. The five major thrust faults identified in this region were shown to be a linked fault system with two tear faults connecting two, large parallel thrust faults. These thrust faults were determined to have some strike-slip component as well. Although high density fracture zones were not resolvable directly using the processed seismic data available, possible regional fracture orientations were predicted from the stress orientations determined and the structure present. Since the study area is also a transition zone between two major NW trending thrust faults, high density fracture zones are expected to be more prevalent particularly at the tips of faults and around the tear faults.

With the high quality seismic data, much more can be extracted about the structural history of this anomalous region than previously understood. The stress orientations and predicted fracture orientations may be extrapolated to most of the southern Sand Wash Basin where structures roughly trend parallel to the structures observed in the study area. These predictions will facilitate high-grading of similar regional structures for further data acquisition and analyses.


AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013