FRACTURE ANALYSIS OF THE STEWART PEAK CULMINATION IN THE FOLD AND THRUST BELT OF WESTERN WYOMING: IMPLICATIONS FOR STRUCTURALLY CONTROLLED FLUID MIGRATION

The Stewart Peak culmination (SPC) is a duplex fault zone involving the Absaroka thrust sheet, a major thrust system in the Sevier fold-and-thrust belt of western Wyoming. The SPC has been breached by erosion, exposing the deep architecture of the duplex and the highly deformed Paleozoic succession that accommodated brittle deformation during thrust emplacement. The exhumed nature of the SPC facilitates outcrop-scale investigation of fracture systems and an analysis of the relative timing of faulting, fracturing, fluid migration and structurally controlled diagenesis. The geometry, connectivity and extent of fracture systems are important features that control fluid-migration pathways in complex structural traps. Fractures in the SPC have systematic geometric patterns associated with Sevier faulting and folding. Therefore, the SPC serves as an analogue for similar subsurface structural traps that are present in the Sevier fold-and-thrust belt across the Intermountain West. Understanding the mechanisms of fluid migration and trapping at multiple scales of observation is important for evaluating hydrocarbon plays found in structural traps of the fold-and-thrust belt. Petrographic analysis demonstrates that fractures have served as fluid conduits at all scales of observation. Chemical cements, secondary mineralization and dissolution within reservoir rocks indicate that fractures have controlled the location and extent of late-stage diagenetic alteration for numerous episodes of fluid migration including hydrocarbons, hydrothermal fluids, and CO₂ brines. Hydrothermal fluids likely enhanced structurally controlled conduits via the processes of dolomitization, dissolution, fracturing and brecciation. Fracturing associated with development of the SPC created pathways for migrating fluids, increased the vertical connectivity and enhanced the secondary porosity and permeability of reservoir rocks. Areas with a high intensity of fracturing and brecciation, such as fault zones, breccia pipes and fracture swarms along the crest of the culmination, facilitated focused fluid flow through the SPC. Faults in the SPC continue to serve as fluid conduits as evidenced by active travertine springs.