Noble gases in natural CO₂ gas deposits: Identifying and quantifying natural CO₂ sequestration processes

Natural CO₂ gas reservoirs provide an important analogue for studying sequestration systems. Noble gases are used here to identify CO₂ source and quantify interaction with groundwater in a naturally occurring CO₂ gas field. 10 samples were taken from a CO₂-rich natural gas reservoir in Jackson Dome, Mississippi, USA. We present compositional, stable isotope and noble gas results of Jackson Dome samples. 3He/4He ratios are between 4.27 and 5.01Ra, indicating a strong mantle signature. Crustual radiogenic 4He increases locally with water-derived 20Ne, suggesting that 4He is pre-mixed with the groundwater before contact with the CO₂ gas phase. N₂ concentrations also correlate with 20Ne and appear to be sourced from the groundwater as well. 40Ar/36Ar ratios are all above air ratio, ranging between 4071 and 6420. Air corrected 40Ar* vary between 92.7 and 95.4%, to give 4He/40Ar* ratios of between 1.26 and 2.52. This range is comparable with values estimated for the upper mantle. CO₂/3He values are between 1.09_109 and 4.62_109, and also fall in the mantle range, indicating that the CO₂ gas in Jackson Dome is also predominantly mantle in origin. A strong anti-correlation between 20Ne and CO₂/3He, is indicative that groundwater plays the principle control in changing the CO₂/3He ratio. CO₂/3He ratios also correlate with _13C(CO₂). Water seems responsible for 25% CO₂ loss and ~1‰ change in _13C(CO₂). Our results show that the first charge of CO₂ migrates through the groundwater filled sedimentary rock and fills the crest of the trapping structure (CO₂~98%, N₂+CH4~2%, high 20Ne and 4He). Later charges of CO₂ migrate through the groundwater filled sedimentary rock to the trapping structure with pure CO₂ occupies the margins of the gas field (CO₂~99.3%, N₂+CH4~0.7%, low 20Ne and 4He). There is little transfer of low-solubility gases from the water to the new CO₂ gas as these have already been “stripped out” of the water phase. From 20Ne we calculate the minimum volume of water responsible for removing CO₂ from the gas phase to be 1.05×109 m3. This quantifies for the first time the importance of the groundwater system in CO₂ gas sequestration.