PRELIMINARY RESULTS FROM A STRUCTURAL ANALYSIS OF A 3D SEISMIC REFLECTION VOLUME OVER THE EASTERN EDGE OF THE CORDILLERAN THRUST BELT, WYOMING, USA

A proprietary high-quality 3-D seismic reflection data volume has been procured by Brigham Young University over the eastern edge of the Cordilleran Thrust Belt (Sevier overthrust) in western Wyoming, USA. The volume spans the boundary between the frontal thrust and the Green River Basin situated along the Moxa Arch. Seismic recording used a spread geometry of 6 lines of 120 channels resulting in a nominal fold of 3600% for a total processing travel time of 5 s. We have prepared iso-traveltime “structural” contour maps of prominent seismic horizons as well as time-slice maps based on the 3D migrated version of the data. The recognition of major structural elements is enhanced by application of seismic attribute analysis (e.g., coherence) to selected travel time slices. The shallow part of the section along the frontal thrust is dominated by large-scale east-vergent complex folding that appears to be locally facilitated at depth by a steep, west-dipping fault. This feature represents the expression of the frontal edge of Cordilleran deformation in the western Green River Basin. Smaller-scale (e.g., < 1 km breadth), strongly asymmetric anticlines with counter-regional vergence (i.e., west-vergent) appear further east as elongate structures striking parallel to the overthrust front, but with a limited along-strike extent. These shallow, relatively tight folds are facilitated by listric, east-dipping faults cutting through the core of the folds. At depth, these faults become shallow-dipping and offset dipping strata that are only weakly folded. A complex pattern of deeper high-angle faulting is expressed as at least two sets of trends (ENE and NW). These faults, which are accompanied by monoclinal and anticlinal flexure, may penetrate the entire basin sequence and the top of basement. The high-quality of the 3-D seismic data volume allows subtle features of fault-facilitated folding to be observed and mapped in detail. We thus provide a strategy for analyzing other areas of the USA Midcontinent (e.g., Illinois Basin) that are only weakly deformed based on available 2D seismic profiles and 3D volumes that show limited fault-facilitated folding. For “mature” basin areas such as the Illinois Basin, such subtle structural details are becoming increasing important for issues of deep gas storage and CO2 sequestration.