Structural controls on subsurface fluid migration through thrust sheets of the Stewart Peak culmination, northern Salt River Range, Wyoming

Abstract

The Stewart Peak culmination is a duplex fault zone of the Absaroka thrust sheet, which is part of the Sevier fold-and-thrust belt in western Wyoming. Duplex structures can serve as subsurface oil, gas and carbon dioxide (CO 2) traps. The culmination lies east and up-dip from naturally occurring CO 2 traps in Idaho and west of the Moxa arch in Wyoming, another naturally occurring CO 2 trap and potential target for CO 2 sequestration. The culmination has been uplifted and breached by erosion, exposing traps and reservoir rocks analogous to proximal subsurface structures, thus allowing for outcrop-scale investigation of the elements that comprise a complex trap and an analysis of the relative timing of faulting, fracturing, fluid migration and structurally-controlled diagenesis. The purpose of this study was to characterize structural elements of the Stewart Peak culmination that controlled fluid flow. Field-based analyses of fractures, fault damage zones and breccia pipes were conducted in order to assess how these structures affected fluid flow. Rocks were sampled in order to elucidate the diagenetic characteristics of alteration associated with episodes of fluid migration and document the qualities of reservoir rocks. Faulting has led to extensive fracturing and brecciation of rocks in the culmination. The geometry of faults and fracture sets initially controlled fluid migration pathways. Brecciated fault zones of large-displacement thrusts served as focused fluid conduits. Fracturing also facilitated fluid flow, locally enhancing porosity and permeability. The protracted history of deformation in the culmination helped maintain fluid flow pathways through fractures and fault zones. Fault zones and fractures display complex diagenetic alteration as a result of multiple eposodes of deformation and fluid migration. Sub-vertical fracture swarms and breccia bodies dissect some fault zones and represent discrete vertical fluid pathways through which CO 2-charged hydrothermal fluids were focused. Hydrothermal brines may have enhanced structurally-controlled fluid migration pathways through interrelated processes of effervescence-induced brecciation and dolomitization. Faulting, fracturing, brecciation and diagenetic alteration generally enhanced the quality of reservoir rocks and increased the hydraulic connectivity within the culmination. The enhanced porosity and permeability of the Madison Limestone and Bighorn Dolomite indicate that these reservoirs have good potential for CO 2 sequestration.