Helium and neon isotope systematics in carbon dioxide-rich and hydrocarbon-rich gas reservoirs

Abstract

The isotopic compositions and elemental abundances of helium and neon were measured in three natural gas reservoirs in the Pannonian sedimentary basin of Hungary. Kismarja (a CO 2-rich reservoir) and Szeghalom-South and Szeghalom-North (both CH 4-dominated reservoirs) are located on topographic basement highs close to the Derecske Sub-Basin in eastern Hungary. Mantle-derived neon has been identified in mixed CH 4-CO 2 reservoirs in the Vienna Basin, Austria. This study establishes that mantle-derived neon and helium are a characteristic feature of gas reservoirs throughout the Neogene extensional basins of Hungary and Austria regardless of the dominant active gas composition. 3He 4He ratios within these samples are attributable to a two-component mixing between mantlederived and crustal-radiogenic helium. The percent contribution of mantle-derived 4He varies from 2.3 to 17%. In contrast, neon isotopic ratios indicate that the gases contain a significant component of atmosphere-derived neon in addition to the mantle- and crustal-derived components. 20Ne, 21Ne and 22Ne abundances can be corrected for this atmospheric contribution. Calculated contributions of mantle- and crustal-derived 21Ne are between 3.6–21% and 1–37%, respectively. 20Ne 22Ne c and 21Ne 22Ne c ratios derived for these atmosphere-corrected components correlate with measured R/Ra values and plot along a single two-component mixing line between crustal and mantle isotopic endmembers. This is consistent with a model in which simple mixing occurs between crustal and mantle endmembers with fixed He/Ne ratios. The mixing line is defined by a hyperbolic constant K (where K = ( 4He 22Ne ) rad/( 4 He 22Ne ) mntl) with a mean value of 67.3 ± 11.8. Based on estimated values of 0.47 for 21Ne 22Ne rad and (1.62 ± 0.03)× 10 7 for ( 4He 21Ne ) rad (Kennedy et al., 1990), values of 7.61 × 10 6 for ( He 22Ne ) rad and 11.3 × 10 4 for ( 4He 22Ne ) mntl can be calculated for the Pannonian Basin gases. This ( 4He 22Ne ) mntl value is indistinguishable within error from the value of 8.04 × 10 4 calculated for rare gases in natural gases from the Vienna Basin. These results clearly establish that the continental expression of mantle-derived rare gases in continental extensional systems in Austria and Hungary is distinct and consistently different from that of gases discharging at the spreading ridges where best estimates of ( 4He 22Ne ) mntl are 8.1–11.3 times higher (9.10 × 10 5; Staudacher et al., 1989). Given the remarkable agreement in the continental expression of mantle-derived gases throughout the Pannonian and Vienna Basins, it is difficult to attribute the observed neon enrichment/helium depletion with respect to MORB gases to fractionation related to lithospheric transport processes. Kinetic fraction ation processes involved in transport through the crust might be expected to produce a much wider variation in the observed He/Ne elemental ratios. The consistent, order-of-magnitude neon enrichment 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.