The use of natural He, Ne and Ar isotopes as constraints on hydrocarbon transport

Abstract

Hydrocarbon accumulations contain rare gases derived from the atmosphere, the crust and, in some cases, the mantle. The distinctive isotopic structure of these different rare gas components allows them to be resolved. The relative abundances of the He, Ne and Ar in the crustal-, mantle- and atmosphere-derived components provides information on the physical processes which have operated in the sub-surface. When combined with mass balance considerations, the rare gases provide powerful constraints on fluid provenance and transport. The results of case studies from the Pannonian Basin in Hungary, and the Vienna Basin, Austria are reviewed. Natural gas reservoirs in the Pannonian and Vienna basins provide samples from depths between 0.5 and 5.5 km. The atmosphere- and crustal-derived rare gas components show systematic patterns of He/Ar and Ne/Ar fractionation which are in the sense predicted by rare gas partitioning between liquid, such as oil and water, and a gas phase. This could occur either during transport or upon emplacement into the reservoir. The role of diffusive processes in fractionation of rare gases cannot be resolved and is assumed to play only a minor role in hydrocarbon transport in the accumulations investigated. The rare gases also convey information on the extent of interaction between groundwater and hydrocarbons. The relationship between major species, such as CH 4 and rare gas tracers, provides estimates for the minimum volume of groundwater that has interacted with the hydrocarbon phase. In the case of the Pannonian Basin, the results are consistent with a model in which the natural gas has been transported in a CH 4 -saturated water phase, from which the gas has separated upon cooling and/or decompression. The mass balance demonstrates that the interaction between the hydrocarbon gas and groundwater has been on a scale much greater than often envisaged in models of hydrocarbon formation and migration.