HR: 0830h
AN: V51H-0379    [PDF]
TI: Noble Gas Tracing of Subsurface CO2 Origin and the Role of Groundwater as a CO2 Sink
AU: * Zhou, Z
EM: zhou@erdw.ethz.ch
AF: ETH Zurich, IGMR, Department of Earth Sciences, Sonneggstrasse 5, Zurich, 8092 Switzerland
AU: Ballentine, C J
EM: cballentine@fs1.ge.man.ac.uk
AF: The University of Manchester, Department of Earth Sciences, The University of Manchester, Oxford Road, Manchester, M13 9PL United Kingdom
AU: Schoell, M
EM: schoell@pacbell.net
AF: GasConsult International L.L.C., 693 St. George Rd., Danville, 94526 United States
AU: Stevens, S H
EM: sstevens@adv-res.com
AF: Advanced Resources International, 4501 Fairfax Drive, Suite 910, Arlington, 22203-1661 United States
AB: The source, generation, migration and accumulation of CO2 gas associated either alone or with hydrocarbons are unclear and therefore hard to predict. So far, noble gases provide one of the best tools to resolve this question, because they are conservative within the subsurface system. The atmosphere-derived noble gases dissolved in groundwater do not react with the rock system, while noble gases produced in the rock phase by radioactive decay or input from magmatic source are isotopically distinct and can be resolved from the dissolved air-derived noble gases. 10 samples were taken from a CO2-rich natural gas reservoir in Jackson Dome, Mississippi, USA to investigate its origin and extent of interaction with the groundwater system. The area lies within the Mississippi Interior Salt Basin. It is bounded on the north by the Pickens-Gilbertown fault system, the updip limit of the Jurassic Louann Salt unit, and on the south by basement highs of the Wiggins, South Mississippi, and Lasalle uplifts. We present compositional, stable isotope and noble gas results of Jackson Dome samples. Gas composition is 98.75-99.38% CO2, with small amounts of methane and nitrogen. CO2 content increases linearly with the decrease of CH4. d13C(CO2) in all samples ranges between -3.55 and -2.57 per mil, increasing with the increase of the CO2 content. Atmosphere-derived He contributions are negligible in all cases. 3He/4He ratios are between 4.27 and 5.01Ra, indicating a strong mantle signature. Crustal 4He in these samples therefore accounts for between 7.0% and 20.8%, the remainder being magmatic in origin. 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 1.09E+9 and 4.62E+9, and also fall in the mantle range, indicating that the CO2 gas in Jackson Dome is also predominantly mantle in origin. 20Ne is dominantly sourced from the groundwater. A strong anti-correlation between 20Ne and CO2/3He, is indicative that groundwater plays the principle control in changing the CO2/3He ratio. Because 3He is conservative, this is probably by CO2 loss into the water phase and seems to account for the 75% reduction from initial CO2/3He
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 1099 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting