FRACTURE ANALYSIS OF LITTLE SHEEP MOUNTAIN ANTICLINE: STRUCTURAL CONTROLS ON FLUID MIGRATION THROUGH A FAULT-CONTROLLED LARAMIDE STRUCTURE

Faults and fractures are critical factors in enhancing the porosity and permeability of otherwise tight carbonate reservoirs which have important implications for hydrocarbon migration. Fracturing associated with Laramide deformation created pathways for migrating fluids and enhanced the secondary porosity and permeability of reservoir rocks in the Paleozoic carbonate rocks of Little Sheep Mountain Anticline in the Bighorn basin of Wyoming. Fracture analysis of the anticline is being used to characterize the porosity, permeability and structural history of the anticline. The anticline was breached by the down-cutting of the Bighorn River and hosts the active Lower Kane Cave system within the Madison Limestone in its core. The cave is still forming through sulfuric acid speleogenesis, a mechanism in which limestone dissolves due to sulfuric acid instead of carbonic acid, aided by microbial processes. This phenomenon may be much more common than previously recognized and could play a role in evaluating potential subsurface sites for CO₂ storage in terms of reservoir quality. The fractures in the limestone allow warm fluids and gases, including CO, CH₄, and H₂S, to migrate from depth in the Bighorn Basin. Breccia zones and fluid-flow pathways in carbonate rocks of Little Sheep Mountain Anticline are hypothesized to be spatially controlled by fracture and joint sets that are geometrically and kinematically related to the Laramide folding of the anticline. Fractures are consistent with regional stresses during Laramide deformation and breccia zones within dominant fractures are visible in outcrop. Little Sheep Mountain Anticline is a well-exposed structure which may provide an analogue for similar underground structures being considered for carbon sequestration. In selecting sites for carbon sequestration, it is important to evaluate and avoid structures that pose a risk of leakage. Structures that contain pervasive faults and breccia zones can create permeability networks that cause anisotropy in fluid flow, and if these networks breached the seal on a potential carbon trap, there would be a high risk of leakage. Thus, an evaluation of the well exposed Little Sheep Mountain Anticline will allow a better understanding of similar types of underground structures under consideration for carbon sequestration.