We present outcrop studies of a leaky natural CO2 accumulation in central Utah, U.S.A. The fluid migration history through the faulted stratigraphy is established from field relationships. Fluids charged and diffused through a carrier bed, migrating up-dip to pool in the structural high created by the faulted, Green River Anticline. The fault forms a lateral seal, while overlying clay-rich cap rocks provide a transient top seal. Fractures in the damage zone to the fault compromise the sealing integrity of this top seal and enable vertical migration of fluids, preferentially around structurally complexities where the fracture network is most intense. The fluids subsequently charge a shallower sand-rich carrier bed and the process continues through the faulted sequence. The dependence of folding and fault-associated fracture permeability is paramount in controlling fluid flow in the study area and emphasizes the influence structure plays on the migration of fluids in sedimentary basins. These results emphasise the need for detailed fault analysis of structures within future engineered CO2 storage sites, and consideration of the burial history and timing of fault activity to assess the risk of CO2 leakage through cap rocks, fault cores and fault damage zones.


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