Fracture characterization and analysis along the Blaine Escarpment of northwestern Oklahoma

Abstract

Detailed fracture mapping and analysis along the Blaine Escarpment of northern Oklahoma demonstrates that zones of deformation previously assigned to solution collapse and desiccation failure appears to play a role but may not be the only or even the primary driving stressor of the region. This implication is strengthened by a thematic strike orientation in fracture sets, mineral veins and regional linear features including surface geomorphology and subsurface fault systems. The maximum compressive stress orientations of the area trend almost exactly E/W, at about 85±5°. This regional stress drives the compressional strike-slip regime of the basement and sedimentary faults systems in this portion of the state. These strike-slip faults of northwestern Oklahoma display a dominant NE/SW trend which is heavily reflected in the linear features of the surface. Large scale, relatively shallow, westward retreating salt bodies are present in the area as well, with certain zones exhibiting active subsurface dissolution and surface precipitation. This active dissolution suggests subsidence at an unknown scale. The surface influence is dependent upon the scale of vertical and horizontal salt front retreat and likely manifests itself in upward propagating sets of extensional fractures. Of the 1,406 mapped fractures and fracture traces, there is a primary trend with a NE/SW orientation. Dominant fracture trends throughout the research stations that are oriented in nearly the same direction as sub-surface features suggests a relationship between the two. However, fracture characteristics, including bed-boundedness, bedding-parallel veining, and others that occur in regions of heavy sub-surface salt dissolution suggests subsidence-related deformation. This study demonstrates, in lieu of the aforementioned characteristics, that: (1) Dissolution subsidence played an active role in shale fracture, as inferred through magnitude of deformation that varies with proximity to dissolving salt fronts; and (2) Fault activity or contemporary stress trends influenced shale fracture propagation and orientation, as inferred through strong trends of fracture strike in association with regional sub-surface fault systems and previous stress field measurements.