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.