Plumbing the depths – testing natural tracers of subsurface CO2 origin and migration, Utah, USA

The effective geological storage of liquid CO2 in porous subsurface rock will require the ability to track, and identify the origin of, any CO2 seepage measured at the near-surface. In a study relevant to postemplacement seepage, we examine natural CO2 springs around Green River, on the Colorado Plateau USA, in an attempt to determine the CO2 origins. We use a combination of natural geochemical and isotopic tracers to fingerprint the origin of CO2 and water erupting at the surface. Stable oxygen and hydrogen isotope data enable estimation that 80 – 90 % of the springwater is derived from shallow groundwater, with the Wilkinson et al 2007 AmericanAssociation Petroleum Geologists Special Publication Ed: M Grosse Page 1 remainder originating from a deep saline aquifer. CO2 and water volumes imply ascent of CO2 as a discrete phase, not just in solution in the porewater. Stable carbon isotopes of CO2 are unable to discriminate between possible sources. However the CO2 contains trace amounts of noble gas isotope, which can be used to demonstrate that approximately 0 – 20 % of CO2 erupted at the surface originates from the mantle, with the remainder originating from the crust. Within the study area there are diverse noble gas signatures at different springs, implying that there is no homogeneous free-phase accumulation at shallow depths as has been previously proposed. Natural tracers are, in this case, capable of resolving CO2 origins and migration pathways. From this study we conclude, that the errors and uncertainty in tracing anthropogenically stored CO2 will be reduced dramatically if a baseline survey is conducted to unambiguously characterise aquifer water chemistry, CO2 and noble gas content before addition of anthropogenic CO2 to the subsurface