Regional groundwater focusing of nitrogen and noble gases into the Hugoton-Panhandle giant gas field, USA

摘要

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 He-4/N-2, with values changing from 0.020 to 0.049 respectively. He-3/He-4, Ne-21/Ne-22, and Ar-40/Ar-36 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 Ne-20/Ar-36 ratios are indistinguishable from groundwater values. The crustal He-4/Ne-21* and He-4/Ar-40* ratios show a 60% excess of He-4 compared to predicted production ratios in the crust and are typical of noble gases released from the shallow crust. The mantle He-3/N-2 and groundwater-recharge Ar-36/N-2 ratios enable us to rule out significant magmatic or atmosphere contributions to the gas field N-2, which is dominantly crustal in origin. Correlated Ne-20/N-2 and He-4/N-2 shows mixing between two distinct crustal N-2 components. One N-2 component (N-2*) is associated with the crustal He-4 and groundwater-derived Ne-20, and the other with no resolvable noble gas contribution. Measured delta(15)N(N2) values vary from +2.7parts per thousand to +9.4parts per thousand. The N-2* and non-He-associated N-2 endmembers are inferred to have delta(15)N(N2) = -3parts per thousand and +13parts per thousand 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 delta(15)N(N2)* value is not compatible with a crystalline or high-grade metamorphic source and, similar to the He-4, is inferred to be from a shallow or low metamorphic-grade source rock. He mass balance requires a regional crustal source, its association with significant magmatic He-3 pointing to a tectonically active source to the west of the Hugoton system. The volume of groundwater required to source the Ne-20 in the gas field demonstrates the viability of the groundwater system in providing the collection, transport, and focusing mechanism for the He-4 and N-2*. The N-2*/Ne-20 ratio shows that the N-2* 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. Copyright (C) 2002 Elsevier Science Ltd. [References: 68]