NON-HYDROCARBON COMPONENTS OF CARBONIFEROUS-SOURCED GAS IN THE SOUTHERN PERMIAN BASIN, NORTHWEST EUROPE

Natural gas sourced from Carboniferous coal-bearing strata is produced from Carboniferous, Permian and Triassic reservoirs in the Southern P ermian Basin in northwest Europe. The composition of this gas has been reviewed. In all reservoirs a distinct area rich in non-hydrocarbons, principally nitrogen, coincides with the basin depocentre in northern Germany. Furthermore, the average nitrogen concentration increases up wards in the stratigraphy. Carbon dioxide is a minor component which i s geographically restricted and occurs mainly in Carboniferous and Zec hstein reservoirs. It is generated indigenously in these reservoirs an d does not migrate into other reservoirs. Hydrogen sulphide occurs onl y significantly in the Zechstein and is areally restricted to the basi n margin facies of this reservoir. It is formed by thermochemical sulp hate reduction. The maturity of the source rock, timing of the trap fo rmation relative to the gas source maturation and reservoir lithology appear to be the dominant controls on the non-hydrocarbon gas composit ion in the Southern Permian Basin. For very high nitrogen gases (50-10 0 vol. %), an admixture of inorganic nitrogen to the thermogenic gas f rom organic sources cannot be excluded.