Computer Science
Scientific paper
Dec 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.5279s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 23, pp.5279-5290
Computer Science
3
Scientific paper
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. 3 He / 4 He ratios within these samples are attributable to a two-component mixing between mantlederived and crustal-radiogenic helium. The percent contribution of mantle-derived 4 He 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. 20 Ne, 21 Ne and 22 Ne abundances can be corrected for this atmospheric contribution. Calculated contributions of mantle- and crustal-derived 21 Ne are between 3.6-21% and 1-37%, respectively. 20 Ne / 22 Ne c and 21 Ne / 22 Ne 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 = ( 4 He / 22 Ne ) rad /( 4 He / 22 Ne ) mntl ) with a mean value of 67.3 ± 11.8. Based on estimated values of 0.47 for 21 Ne / 22 Ne rad and (1.62 ± 0.03)× 10 7 for ( 4 He / 21 Ne ) rad (Kennedy et al., 1990), values of 7.61 × 10 6 for ( He / 22 Ne ) rad and 11.3 × 10 4 for ( 4 He / 22 Ne ) mntl can be calculated for the Pannonian Basin gases. This ( 4 He / 22 Ne ) 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 ( 4 He / 22 Ne ) 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.
Ballentine Chris J.
Keith O'Nions R.
Sherwood Lollar Barbara
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