Structural controls on leakage from a natural CO2 geologic storage site : Central Utah, U.S.A.

Dockrill, Ben and Shipton, Zoe (2010) Structural controls on leakage from a natural CO2 geologic storage site : Central Utah, U.S.A. Journal of Structural Geology, 32 (11). pp. 1768-1782. ISSN 0191-8141 (https://doi.org/10.1016/j.jsg.2010.01.007)

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Abstract

Faults and associated fracture networks can significantly influence regional flow of groundwater, hydrocarbons and other fluids. The distribution of CO2 springs and seeps along the Little Grand Wash fault and Salt Wash faults in central Utah is controlled by along-fault flow of CO2-charged groundwater from shallow aquifers (<1 km deep). The same faults are the likely conduits that charge the shallow aquifers with CO2 from depth. We document fault zone trace geometry and architecture, and evidence for palaeo-fluid flow within the footwalls of both faults. Evidence for palaeo-fluid flow consists of extensive bleaching of sandstones and some siltstones, mineralisation of carbonates and celestine veins and minor hydrocarbon staining. The field evidence shows that the pathways for multiple phases of fluid flow were structurally controlled utilising the fracture network developed in the damage zone of the faults. To investigate the likely effect of these faults on the regional fluid-migration pathways at depth, a 3D model of the faulted systemwas generated and a fault seal analysis applied to predict the cross-fault sealing capabilities of the studied faults. Due to the scarcity of subsurface data, the results are not conclusive but suggest probable multiple cross-fault leak points for fluids to migrate across the fault, in contrast to the field observations that indicate fault-parallel flow. This comparison of field observations to the modelling approach demonstrates the inability of conventional seal analysis techniques to predict fault-parallel fluid leakage and highlight the effects fracture networks in the damage zone, especially at structural complexities along the fault, have in producing pathways for vertical flow. Multiple fluids have utilised similar fault-parallel pathways over geological time demonstrating that such pathways have the potential to cause long-term leakage from hydrocarbon reservoirs and CO2 storage sites.

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