Fracture analysis of Little Sheep Mountain anticline, eastern Bighorn Basin, Wyoming : structural controls on fluid migration through a fault-controlled Laramide structure
Little Sheep Mountain is a doubly-plunging asymmetric anticline in the northeastern Bighorn Basin, Wyoming. Anticlines in the basin provide subsurface structural traps for oil and natural gas, several of which produce from the Mississippian Madison Formation, a world-class fractured carbonate reservoir. Little Sheep Mountain anticline has been uplifted and the hydrocarbon trap has been breached by erosion, exposing reservoir rocks analogous to these proximal subsurface structures. An outcrop-scale investigation of fracture geometry and geochemical analysis of breccia bodies in the anticline provides insight into the history of fluid migration and structurally-controlled diagenesis within the anticline. A combination of field techniques and analytical methods were used to characterize the relationship between fracture patterns and subsurface paleofluid migration. Field-based analyses of fractures and breccias were conducted to characterize structural elements of the anticline with respect to fluid flow. Geochemical analysis was used to constrain the origin and migration history of paleofluids. Diagenetic characteristics and alteration associated with fluid migration within the fracture network were examined using strontium and stable isotopes in conjunction with petrography. Faulting related to Laramide deformation has led to extensive fracturing and brecciation in Little Sheep Mountain anticline. Fracture network geometry was identified as a major control on fluid migration within the structure that has provided conduits for episodic hydrothermal fluid flow. Progressive deformation during Laramide fold growth maintained and enhanced fluid flow networks and locally enhanced porosity. Mississippian-age collapse breccia bodies were identified as preexisting weak horizons for focused hydrothermal fluid flow during the Laramide. Low-temperature hydrothermal mineralization resulting from episodic fluid flow led to complex diagenetic alteration. Fracture sets parallel to and perpendicular to the Laramide shortening direction acted as primary fluid flow conduits within Little Sheep Mountain anticline and provided enhanced porosity and permeability in the subsurface reservoir rocks. Hydrothermal breccia bodies represent discrete vertical fluid conduits controlled by fracture geometry through which hydrothermal fluids migrated. Brecciation associated with Laramide deformation and late-stage diagenetic alteration was found to generally reduce the porosity of Madison reservoir rocks adjacent to breccia bodies by the infilling of fractures and occlusion of pores with late-stage calcite cement.