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Conceptual Model for Basement and Surface Structure Relationships in the Sawtooth Range, MT, and Potential for Trap Assessment


The reactivation potential of pre-existing basement structures affects the geometry of subsequent deformation structures and location of some hydrocarbon traps. Potential hydrocarbon migration routes and their effectiveness, as well as the lateral extents of traps, are heavily influenced by faults and fractures, the intensity of which may be altered by reactivation. The Sawtooth Range, Montana, has been used as a study area. A model for the development of structures close to the Augusta Syncline in the Sawtooth Range is being developed using: 1) a compilation of the basement structures of the belt based on analysis of gravity and aeromagnetic anomalies, seismic data and well logs where available, and 2) a compilation of the surface deformation structures of the belt based on remote sensing images and field data indicating stress directions and age relationships. The final result will be a conceptual model based on the interpretation of the two previous maps including statistical correlations of data and development of balanced cross-sections. Preliminary results indicate that the change in orientation of surface thrust faults observed in the Sawtooth Range, from a NNW-SSE orientation near the Gibson Reservoir to a WNW-ESE trend near Haystack Butte, correlates with the Scapegoat-Bannatyne trend, a pre-existing structure lying within the Great Falls Tectonic Zone. The Scapegoat-Bannatyne trend may be composed of up to 4 NE-SW oriented, en-echelon basement strike-slip faults reactivated from the Proterozoic suture zone between the Medicine Hat and Wyoming Cratons. These multiple reactivated features of deformation influence fracture intensity both at depth and at the surface. This is indicated by the variations between along-dip versus along-strike folding and fracturing observed at the surface. An understanding of these reactivation relationships can lead to more successful unconventional exploration in that an analysis of fault migration or sealing potential can be determined. The fracture intensity will determine the effectiveness of the faults as a hydrocarbon seal versus a migration pathway. In addition, an assessment of the pre-existing structures with respect to the present-day stress direction will contribute to the understanding of which fracture sets will be open or closed and enhance the design of hydrofracturing treatments.