CORE ANALYSIS FROM LITTLE GRAND WASH FAULT, UTAH, AND APPLICATION FOR FAULT RISK ASSESSMENT FOR CO2 STORAGE IN THE NORWEGIAN NORTH SEA

Fault zones contain a mixture of lithologies, various fault structures, and evidence for fluid-rock interaction events, representing a record of multiple deformation events during the faulting history. The resulting complex and highly variable mixture of properties makes faults challenging structures to model, and specifically assessing the fault zone permeability and probability of reactivation is difficult. The ability of fault zones to transport fluids has received renewed attention during the last 10 years of developing subsurface, geological storage reservoirs for CO₂. The North Sea sedimentary basin offshore Norway has an enormous potential for storing CO₂. However, to qualify fault traps in saline aquifers for storage, improved workflows for fault risk assessment focusing on fault zone flow properties and mechanical stability is needed.

An improved approach for fault modelling is under development: we quantify the uncertainties in fault rock properties to address the probability of failure and subsequent changes in permeability. To support our understanding of subsurface faults in the North Sea, we utilize knowledge from the Little Grand Wash (LGW) fault, located in Emery County, Utah. The LGW fault zone provides a natural laboratory to study fault zone development in geological analogues, failure mechanisms, and fluid migration. As a supplement to already published well data and core descriptions from this area, three new cores were retrieved from the fault in 2019. In total 16.5 meters of core, covering both the hanging wall (Jurassic Brushy Basin Member of the Morrison Formation) and footwall (Jurassic Summerville Formation) damage zone, have been logged and sampled for petrophysical analysis and mechanical testing. The current work presents new data on variation of permeability, porosity, strength, and sonic velocity inside the fault damage zone, including measurements on different lithologies, various types of cementation and selected deformation bands, fractures, and veins. Petrophysical and chemical analysis combined with visual identification of oil stains and bleaching provide the possibility for a holistic understanding of the system and we derive valuable knowledge for improving fault risk workflows applicable for CO₂ storage projects in the Norwegian North Sea.