Regional groundwater focusing of nitrogen and noble gases into the Hugoton-Panhandle giant gas field, USA |
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| Authors: | Chris J Ballentine Barbara Sherwood Lollar |
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| Institution: | 1 Institute of Isotope Geology and Mineral Resources, ETH Zurich, NO C 61.7, Sonneggstrasse 5, CH-8092 Zurich, Switzerland
2 Department of Geology, University of Toronto, 22 Russell Street, Toronto, Ontario M5S 3B1, Canada |
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| Abstract: | The Hugoton-Panhandle giant gas field, located across SW Kansas and the Texas and Oklahoma panhandles in the USA, is the case type example of high nitrogen concentrations in a natural gas being linked with high helium concentrations. We collected 31 samples from producing wells in a north-south traverse of the 350-km-long field. The samples reflect the previously observed north-south change in 4He/N2, with values changing from 0.020 to 0.049 respectively. 3He/4He, 21Ne/22Ne, and 40Ar/36Ar vary between 0.14-0.25 Ra, 0.0373-0.0508, and 818-1156 respectively, and are caused by quantifiable contributions from mantle, crustal, and atmosphere-derived sources. The atmosphere-derived 20Ne/36Ar ratios are indistinguishable from groundwater values. The crustal 4He/21Ne* and 4He/40Ar* ratios show a 60% excess of 4He compared to predicted production ratios in the crust and are typical of noble gases released from the shallow crust. The mantle 3He/N2 and groundwater-recharge 36Ar/N2 ratios enable us to rule out significant magmatic or atmosphere contributions to the gas field N2, which is dominantly crustal in origin.Correlated 20Ne/N2 and 4He/N2 shows mixing between two distinct crustal N2 components. One N2 component (N2*) is associated with the crustal 4He and groundwater-derived 20Ne, and the other with no resolvable noble gas contribution. Measured δ15NN2 values vary from +2.7‰ to +9.4‰. The N2* and non-He-associated N2 endmembers are inferred to have δ15NN2 = −3‰ and +13‰ and contribute from between 25-60% and 75-40% of the nitrogen respectively. The non-He-associated nitrogen is probably derived from relatively mature organic matter in the sedimentary column. The δ15NN2* value is not compatible with a crystalline or high-grade metamorphic source and, similar to the 4He, is inferred to be from a shallow or low metamorphic-grade source rock. 4He mass balance requires a regional crustal source, its association with significant magmatic 3He pointing to a tectonically active source to the west of the Hugoton system. The volume of groundwater required to source the 20Ne in the gas field demonstrates the viability of the groundwater system in providing the collection, transport, and focusing mechanism for the 4He and N2*. The N2*/20Ne ratio shows that the N2* transport must be in the aqueous phase, and that the degassing mechanism is probably contact between the regional groundwater system and the preexisting reservoir hydrocarbon gas phase. |
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