STRUCTURAL ANALYSIS OF CO2 LEAKAGE THOUGH THE SALT WASH AND THE LITTLE GRAND WASH FAULTS FROM NATURAL RESERVOIRS IN THE COLORADO PLATEAU, SOUTHEASTERN UTAH

The Salt Wash and the Little Grand Wash normal faults in the Colorado Plateau of southeastern Utah emit large amounts of CO₂ gas from abandoned drill holes, springs, and a hydrocarbon seep. The leakage of CO₂-charged water has also occurred in the past as shown by large carbonate deposits localized along the fault traces. These deposits consist of thick veins and travertine terraces extending up to 50 meters from the fault damage zones. The faults cut natural CO₂ reservoirs and provide an excellent analogue for geologic CO₂ sequestration. The faults cut a north plunging anticline of siltstones, shales, and sandstones, and the fault rocks are fine-grained with clay-rich gouge, suggesting they should be effective barriers to cross-fault flow. The Little Grand Wash fault displaces these rocks approximately 290 m and the Salt Wash graben offsets rocks approximately 130 m. Well log, geologic surface and geochemical data indicate the CO₂ reservoirs have been cut by the faults at depth providing a conduit for the vertical migration of CO₂ to the surface, but not horizontal flow across the fault plane. Even though lateral cross-fault migration may be impeded, this study clearly indicates that there are possible migration pathways for the escape of CO₂ from faulted subsurface aquifers, including aquifers faulted by “low-permeability” faults with clay gouge. Three-dimensional flow models show how the fault’s damage zone permeability is adjacent to the faults and the leakage though the damage zones is localized near the regional anticline's fold axis. The faults appear to be active in the Quaternary, responding to the north-northeast extension of the area. In the event of injecting CO₂ into deep aquifers careful design and monitoring of the geological structure and stress regimes must be considered to avoid leakage.