To evaluate sites for long-term geological storage of CO 2 and optimize techniques for monitoring the fate of injected CO 2 , it is crucial to investigate potential CO 2 migration pathways out of a reservoir and surface leakage magnitudes. For the fi rst time, we calculate CO 2 leakage rates and volumes from ancient fault-related travertines and from an abandoned borehole. U-Th-dated travertine along two faults near Green River, Utah (western United States), shows that leakage has occurred in this area for over 400 k.y. and has switched location repeatedly over kilometer-scale distances. One individual travertine was active for at least 11 k.y. Modern leakage is predominantly through the active Crystal Geyser, which erupts from an abandoned exploration well. Using age data and travertine volume, we calculate magnitudes and rates of CO 2 emission. Fault-focused leakage volume is twice as great as diffuse leakage through unconfi ned aquifers. The leakage rate from a poorly completed borehole is 13 times greater than the long-term time-averaged fault-focused leakage. Although magnitudes and rates of any leakage from future storage sites will be highly dependent on local geology and pressure regime, our results highlight that leakage from abandoned wells is likely to be more signifi cant than through faults.

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SUMMARYWe 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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