Abstract
CO2 sources, sinks and migration mechanisms in natural CO2 gas fields provide critical analogues for developing the safe application of anthropogenic CO2 sequestration technologies. Here we use noble gas and carbon isotopes, together with other gases, to identify and quantify the origin, transport and trapping mechanisms of CO2 in the Late Cretaceous Jackson Dome CO2 gas deposit (98.75% to 99.38% CO2). Located in central Mississippi, USA, and producing from >5000m, it is one of the deepest commercial CO2 gas fields in the world. 10 gas samples from producing wells were determined for their noble gas, chemical and stable carbon isotope composition. 3He/4He ratios range between 4.27Ra and 5.01Ra (where Ra is the atmospheric value of 1.4×10−6), indicating a strong mantle signature. Similar to CO2 deposits worldwide, CO2/3He decreases with increasing groundwater-derived 20Ne (and 4He). We model several different processes that could account for the Jackson Dome data, and conclude that, similar to other CO2 dominated deposits, a Groundwater Gas Stripping and Re-dissolution (GGS-R) process best accounts for observed 20Ne/36Ar, 84Kr/36Ar, CO2/3He, δ13C(CO2), 4He, 20Ne and 36Ar. In this context, crustal and magmatic CO2 components contribute 57% and 43%, respectively. Changes in CO2/3He across the field show that groundwater contact is responsible for up to 75% loss of original emplaced CO2. δ13C(CO2) variance limits the degree of precipitation to be less than 27%, with the remaining CO2 loss being accounted for by dissolution only. A higher degree of dissolution gas loss and evidence for water contact at the reservoir crest compared to the reservoir flanks is used to argue that CO2 in this system has not undergone subsequent loss to either dissolution or precipitation since shortly after reservoir filling at over 60Ma.
Published Version
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