FAULT SEAL ANALYSIS FOR CO2 STORAGE: FAULT ZONE ARCHITECTURE, FAULT PERMEABILITY, AND FLUID MIGRATION PATHWAYS IN EXPOSED ANALOGS IN SOUTHEASTERN UTAH

Upwards migration and leakage of injected fluids along natural fault and fracture networks is a key risk factor for potential injection of CO₂ or sealing for hydrocarbons. We examine exposed natural analogs of faulted clastic rocks on the eastern edge of the San Rafael Swell to evaluate the impacts of faulting and fracturing on reservoir and top-seal pairs and to evaluate evidence for paleo-migration of fluids along the fault zone. We examine the Iron Wash fault, a 25-km long, WNW striking down to the north normal fault which cuts Jurassic sedimentary rocks and has throws that range from 20-120 m, to examine how a fault may affect seal integrity. Field mapping, kinematic analysis, petrographic analysis, characterization of the fault zone facies and fault architecture, analysis of altered and mineralized rocks in and around the fault zone, and modeling of fault seal capacity was conducted to provide an understanding of the Iron Wash fault zone. Field data and observations were combined with well log and borehole data to produce three types of models for the Iron Wash fault: 1) geometric model of the fault in the subsurface, 2) predictive models of fault zone behavior and fault seal analysis, and 3) predictive geomechanical models of the response of the fault zone to an imposed stress field and increasing the effective stress on the fault.

We propose that the western sections of Iron Wash fault zone formed in the presence of a three-dimensional strain field resulting in a more complex and dispersed fault zone. We describe the internal deformation patterns of two fault relay zones at different stages of fault linkage. We conclude that the Iron Wash fault zone has low sealing capacity and will likely not behave as a seal for fluids against the fault zone due primarily to modest throw on the fault and high frequency of fractures associated with the fault zone. Analysis of fluid alteration and mineralization around the fault zone indicates that the fault zone was conduit for paleo-fluids. We conclude that the fault is not likely to develop a sealing membrane and therefore will most likely fail as a seal to fluids moving through the reservoirs modeled here. Modeling results indicate that a reduction in the effective normal stress on fault surfaces may result in induced failure of existing faults or increased hydraulic conductivity of fractures.