New insights into geologic CO₂ sequestration from natural analogues of the Colorado Plateau and Rocky Mountain provinces, USA

Noble gas analysis permits the resolution of the lithospheric mantle contribution from both the crustal and groundwater-derived inputs within natural CO₂ reservoirs (Ballentine et al., 1992). Determination of these crustal and groundwater sources provides new information about the role of the groundwater and the regional focussing and transport of magmatic CO₂ and other inorganic natural gases in the subsurface. Hence, the data presented here from SW US natural CO₂ fields provides an important analogue relevant to understanding the integrity of programs investigating the geological option for sequestration of anthropogenic CO₂. We have extensively studied CO₂ well gases from three systems to the east of the Colorado Plateau uplift province, and two systems from within the uplift area, namely Bravo Dome (Harding Co, NM) (Ballentine et al., 1992), Sheep Mountain (Huerfano Co, CO), McCallum Dome, (Jackson Co, CO), St. Johns Dome (Apache Co, AZ) and McElmo Dome (Montezuma Co, CO). Our study is the first to determine a magmatic origin for the CO₂ in these fields, by comparing ?13C(CO₂) and CO₂/3He ranges observed within the gas fields to those of MORB. Additionally, we have developed an innovative CO₂ redissolution model that can account for the clear reduction of the original magmatic CO₂/3He ratio and the fractionation of ?13C(CO₂) isotopes observed within all of the fields. This enables the processes responsible for lowering the CO₂/3He ratio, namely CO₂ dissolution into the groundwater and CO₂ precipitation as CaCO3, to be distinguished and quantified. Importantly, this model highlights that in several of the reservoirs, precipitation of CaCO3 is not a significant process in reducing the original CO₂/3He ratio compared to dissolution of CO₂ into the groundwater.