Helium isotopes in North Sea gas fields and the Rhine rift

Abstract

The popular assumption that all natural hydrocarbon accumulations are biogenic in origin has recently been questioned^1,2 with the suggestion that deep source (mantle-derived) hydrocarbons may contribute to accumulated biogenically produced hydrocarbons, and also give rise to hydrocarbon deposits in unexpected parts of the crust. The possibility that deep-source volatiles are present in significant quantities in the hydrocarbon-bearing parts of the crust of western Europe is assessed here using analyses of ³He abundances in groundwaters and natural gas accumulations within the UK mainland and continental shelf. For comparative purposes, analyses are also presented for groundwaters from the southern part of the Rhine Graben, a region that has been relatively active tectonically, and the site of Tertiary and Quaternary volcanic activity. In contrast, the North Sea sedimentary basins and Central Graben have no recorded Tertiary or younger volcanic activity. Helium associated with North Sea gas fields and groundwaters on the UK mainland is dominantly radiogenic, produced by decay of U and Th in the continental crust, even where heat flow exceeds 100 mW m⁻². A minor component (< 1%) of mantle-derived helium with promordial ³He, may be present in some gas fields close to the North Sea Central Graben. Further south along the more active part of the same fault system in the Rhine Graben this component reaches >15% in groundwater samples. Fault structures seem to be important for the transport of mantle helium through the brittle upper crust. The results are entirely consistent with a biogenic origin for North Sea hydrocarbons.