Noble Gas Partitioning in Bravo Dome Natural Magmatic CO2 field: Implications for Crustal CO2 accumulations and Gas Migration

Abstract:

We report a combination of field data, experiments, and analytical theory investigating the dissolution of the Bravo Dome natural CO₂ field into the underlying brine aquifer. The Bravo Dome reservoir contains 1.3Gt±0.6Gt of magmatic CO₂ trapped above a shallow brine aquifer. We compute that 22%±7% of the original CO₂ has dissolved since its migration, and that the CO₂ entered between 1.2Ma and 1.5Ma. The CO₂ also contains trace amounts of noble gases, used by previous authors and this study to identify the migration direction and quantify mass of CO₂ dissolved. We present a simple analytical model and experimental verification showing that noble gases in even trace amounts will not move uniformly with a migrating gas plume in the crust. Due to varying degrees of solubility in groundwater, the more volatile gases will tend to enrich at the front of the migrating CO₂ plume. This effect can be used to improve interpretations of subsurface noble gas concentration gradients.