HELIUM AND NEON ISOTOPE SYSTEMATICS IN CARBON-DIOXIDE RICH AND HYDROCARBON-RICH GAS-RESERVOIRS

The isotopic compositions and elemental abundances of helium and neon were measured in three natural gas reservoirs in the Pannonian sedimen tary basin of Hungary. Kismarja (a CO2-rich reservoir), and Szeghalom- South and Szeghalom-North (both CH4-dominated reservoirs) are located on topographic basement highs close to the Derecske Sub-Basin in easte rn Hungary. Mantle-derived neon has been identified in mixed CH4-CO2 r eservoirs in the Vienna Basin, Austria. This study establishes that ma ntle-derived neon and helium are a characteristic feature of gas reser voirs throughout the Neogene extensional basins of Hungary and Austria regardless of the dominant active gas composition. He-3/He-4 ratios w ithin these samples are attributable to a two-component mixing between mantle-derived and crustal-radiogenic helium. The percent contributio n of mantle-derived He-4 varies from 2.3 to 17%. In contrast, neon iso topic ratios indicate that the gases contain a significant component o f atmosphere-derived neon in addition to the mantle- and crustal-deriv ed components. Ne-20, Ne-21, and Ne-22 abundances can be corrected for this atmospheric contribution. Calculated contributions of mantle- an d crustal-derived Ne-21 are between 3.6-21% and 1-37%, respectively. N e-20/Ne-22c and Ne-21/Ne-22c ratios derived for these atmosphere-corre cted components correlate with measured R/Ra values and plot along a s ingle two-component mixing line between crustal and mantle isotopic en dmembers. This is consistent with a model in which simple mixing occur s between crustal and mantle endmembers with fixed He/Ne ratios. The m ixing line is defined by a hyperbolic constant K (where K = (He-4/Ne-2 2)rad/(He-4/Ne-22)mntl) with a mean value of 67.3 +/- 11.8. Based on e stimated values of 0.47 for Ne-21/Ne-22rad and (1.62 +/- 0.03) x 10(7) for (He-4/Ne-21)rad (Kennedy et al., 1990), values of 7.61 x 10(6) fo r (He-4/Ne-22)rad and 11.3 x 10(4) for (He-4/Ne-22)mntl can be calcula ted for the Pannonian Basin gases. This (He-4/Ne-22)mntl value is indi stinguishable within error from the value of 8.04 x 10(4) calculated f or rare gases in natural gases from the Vienna Basin. These results cl early establish that the continental expression of mantle-derived rare gases in continental extensional systems in Austria and Hungary is di stinct and consistently different from that of gases discharging at th e spreading ridges where best estimates of (He-4/Ne-22)mntl are 8.1-11 .3 times higher (9.10 x 10(5); Staudacher et al., 1989). Given the rem arkable agreement in the continental expression of mantle-derived gase s throughout the Pannonian and Vienna Basins, it is difficult to attri bute the observed neon enrichment/helium depletion with respect to MOR B gases to fractionation related to lithospheric transport processes. Kinetic fractionation processes involved in transport through the crus t might be expected to produce a much wider variation in the observed He/Ne elemental ratios. The consistent, order-of-magnitude neon enrich ment observed throughout these gas fields instead implies that mantle- derived fluids in these continental extensional systems may be sourced in a region of the mantle distinct from that supplying the mid-ocean spreading ridges.