Rare gas constraints on hydrocarbon accumulation, crustal degassing and groundwater flow in the Pannonian Basin

C. J. Ballentine, R. K. O'Nions, E. R. Oxburgh, F. Horváth, J. Deak

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Abstract

The isotopic composition and abundances of He, Ne and Ar have been measured in a sequence of vertically stacked gas reservoirs at Hajduszoboszlo and Ebes, in the Pannonian Basin of Hungary. The gas reservoirs occur at depths ranging from 727 to 1331 m, are CH4 dominated and occupy a total rock volume of approximately 1.5 km3. There are systematic variations in both major species abundances and rare gas isotopic composition with depth: CO2 and N2 both increase from 0.47 and 1.76% to 14.1 and 30.5%, respectively, and 40Ar 36Ar and 21Ne 22Ne increase systematically from 340 and 0.02990 at 727 m to 1680 and 0.04290 at 1331 m. A mantle-derived He component between 2 and 5% is present in all samples, the remainder is crustal-radiogenic He. The Ar and Ne isotope variations arise from mixing between atmosphere-derived components in groundwater, and crustally produced radiogenic Ar and Ne. The atmosphere-derived 40Ar and 21Ne decreases from 85 and 97% of the total 40Ar and 21Ne at 727 m to 18 and 68% at 1331 m. The deepest samples are shown to have both atmosphere-derived and radiogenic components close to the air-saturated water and radiogenic production ratios. The shallowest samples show significant fractionation of He Ar and Ne Ar ratios in atmosphere-derived and radiogenic rare gas components, but little or no fractionation of He Ne ratios. This suggests that diffusive fractionation of rare gases is relatively unimportant and that rare gas solubility partitioning between CH4 and H2O phases controls the observed rare gas elemental abundances. The total abundance of atmosphere-derived and radiogenic rare gas components in the Hajduszoboszlo gas field place limits on the minimum volume of groundwater that has interacted with the natural gas, and the amount of crust that has degassed and supplied radiogenic rare gases. The radiogenic mass balance cannot be accounted for by steady state production either within the basin sediments or the basement complex since basin formation. The results require that radiogenic rare gases are stored at their production ratios on a regional scale and transported to the near surface with minimal fractionation. The minimum volume of groundwater required to supply the atmosphere-derived rare gases would occupy a rock volume of some 1000 km3 (assuming an average basin porosity of 5%), a factor of 670 greater than the reservoir volume. Interactions between groundwater and the Hajduszoboszlo hydrocarbons has been on a greater scale than often envisaged in models of hydrocarbon formation and migration.

Original languageEnglish
Pages (from-to)229-246
Number of pages18
JournalEarth and Planetary Science Letters
Volume105
Issue number1-3
DOIs
Publication statusPublished - 1991

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Noble Gases
Groundwater flow
degassing
Degassing
ground water
Hydrocarbons
Radiogenic gases
groundwater flow
rare gases
hydrocarbons
hydrocarbon
gas
basin
Fractionation
atmospheres
fractionation
atmosphere
Groundwater
groundwater
Gases

ASJC Scopus subject areas

  • Earth and Planetary Sciences (miscellaneous)
  • Geochemistry and Petrology
  • Geophysics
  • Earth and Planetary Sciences(all)
  • Environmental Science(all)

Cite this

Rare gas constraints on hydrocarbon accumulation, crustal degassing and groundwater flow in the Pannonian Basin. / Ballentine, C. J.; O'Nions, R. K.; Oxburgh, E. R.; Horváth, F.; Deak, J.

In: Earth and Planetary Science Letters, Vol. 105, No. 1-3, 1991, p. 229-246.

Research output: Contribution to journalArticle

Ballentine, C. J. ; O'Nions, R. K. ; Oxburgh, E. R. ; Horváth, F. ; Deak, J. / Rare gas constraints on hydrocarbon accumulation, crustal degassing and groundwater flow in the Pannonian Basin. In: Earth and Planetary Science Letters. 1991 ; Vol. 105, No. 1-3. pp. 229-246.
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AU - Deak, J.

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N2 - The isotopic composition and abundances of He, Ne and Ar have been measured in a sequence of vertically stacked gas reservoirs at Hajduszoboszlo and Ebes, in the Pannonian Basin of Hungary. The gas reservoirs occur at depths ranging from 727 to 1331 m, are CH4 dominated and occupy a total rock volume of approximately 1.5 km3. There are systematic variations in both major species abundances and rare gas isotopic composition with depth: CO2 and N2 both increase from 0.47 and 1.76% to 14.1 and 30.5%, respectively, and 40Ar 36Ar and 21Ne 22Ne increase systematically from 340 and 0.02990 at 727 m to 1680 and 0.04290 at 1331 m. A mantle-derived He component between 2 and 5% is present in all samples, the remainder is crustal-radiogenic He. The Ar and Ne isotope variations arise from mixing between atmosphere-derived components in groundwater, and crustally produced radiogenic Ar and Ne. The atmosphere-derived 40Ar and 21Ne decreases from 85 and 97% of the total 40Ar and 21Ne at 727 m to 18 and 68% at 1331 m. The deepest samples are shown to have both atmosphere-derived and radiogenic components close to the air-saturated water and radiogenic production ratios. The shallowest samples show significant fractionation of He Ar and Ne Ar ratios in atmosphere-derived and radiogenic rare gas components, but little or no fractionation of He Ne ratios. This suggests that diffusive fractionation of rare gases is relatively unimportant and that rare gas solubility partitioning between CH4 and H2O phases controls the observed rare gas elemental abundances. The total abundance of atmosphere-derived and radiogenic rare gas components in the Hajduszoboszlo gas field place limits on the minimum volume of groundwater that has interacted with the natural gas, and the amount of crust that has degassed and supplied radiogenic rare gases. The radiogenic mass balance cannot be accounted for by steady state production either within the basin sediments or the basement complex since basin formation. The results require that radiogenic rare gases are stored at their production ratios on a regional scale and transported to the near surface with minimal fractionation. The minimum volume of groundwater required to supply the atmosphere-derived rare gases would occupy a rock volume of some 1000 km3 (assuming an average basin porosity of 5%), a factor of 670 greater than the reservoir volume. Interactions between groundwater and the Hajduszoboszlo hydrocarbons has been on a greater scale than often envisaged in models of hydrocarbon formation and migration.

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