He and Ne as tracers of natural CO₂ migration up a fault from a deep reservoir

Capture and geological storage of CO<sub>2</sub> is emerging as an attractive means of economically abating anthropogenic CO<sub>2</sub> emissions from point sources. However, for the technology to be widely deployed it is essential that a reliable means to assess a site for both storage performance and regulation compliance exists. Hence, the ability to identify the origin of any CO<sub>2</sub> seepage measured at the near-surface and ground surface and determine if it originates from a deep storage site or a different source is critical. As an analogue for post-emplacement seepage, here we examine natural CO<sub>2</sub> rich springs and groundwater wells in the vicinity of the St. Johns Dome CO<sub>2</sub> reservoir located on the border of Mid-Arizona/New Mexico, USA. Extensive travertine deposits in the region document a long history of migration of CO<sub>2</sub> rich fluids to the surface. The presence of CO<sub>2</sub> rich fluids today are indicated by high levels of HCO<sub>3</sub>− in surface spring and groundwater well waters. We document measurements of dissolved noble gases and carbon isotopes from these springs and wells. We show that a component of the He fingerprint measured in gaseous CO<sub>2</sub> sampled in the deep reservoir, can be traced along a fault plane to occur in waters from both groundwater wells and the majority of springs emerging at the surface above the reservoir. Our results show for the first time that CO<sub>2</sub> can be fingerprinted from source to surface using noble gases and illustrates that this technique could be used to identify dissolved CO<sub>2</sub> migration from engineered storage sites