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In contrast to predominantly hydrocarbon-rich natural gases in the western part of the Central European Basin (CEB), accumulations of natural gases from the eastern part of the North German Basin (NGB) are nitrogen-rich with up to 90% N 2 . This study is focused on the behaviour of fixed ammonium in clay minerals of organic-rich Palaeozoic sediments in the eastern part of the NGB as a major source of nitrogen-rich natural gases. Carboniferous shales have been investigated for a better understanding of nitrogen fixing during diagenesis, storage during burial and release during devolatilization processes or fluid-rock interactions. The total nitrogen contents in the studied Carboniferous shales of the NGB reach up to 2700 ppm with an inorganic fixed portion (in the form of NH 4 + -N) of more than 60%. The results of this study indicate an increasing proportion of the mineralogically fixed ammonium with increasing thermal maturity and storage up to catagenetic conditions. The isotopic composition of fixed-NH 4 is relatively homogeneous in the majority of the shales and ranges from +1 to +3.5&. In contrast, samples from the basin centre show a significant decrease in ammonium contents down to 460 ppm coupled with a shift in d 15 N up to +5.6& suggesting a release of nitrogen on a large scale. Calculation of nitrogen loss and isotopic fractionation indicate that more than 30% of nitrogen was released as ammonium probably as a consequence of fluid-rock interaction with highly saline brines.

Fluid inclusions have been studied in minerals infilling fissures (quartz, calcite, fluorite, anhydrite) hosted by Carboniferous and Permian strata from wells in the central and eastern part of the North German Basin in order to decipher the fluid and gas migration related to basin tectonics. The microthermometric data and the results of laser Raman spectroscopy reveal compelling evidence for multiple events of fluid migration. The fluid systems evolved from a H 2 O-NaCl±KCl type during early stage of basin subsidence to a H 2 ONaCl-CaCl 2 type during further burial. Locally, fluid inclusions are enriched in K, Cs, Li, B, Rb and other cations indicating intensive fluid-rock interaction of the saline brines with Lower Permian volcanic rocks or sediments. Fluid migration through Carboniferous sediments was often accompanied by the migration of gases. Aqueous fluid inclusions in quartz from fissures in Carboniferous sedimentary rocks are commonly associated with co-genetically trapped CH 4 -CO 2 inclusions. P-T conditions estimated, via isochore construction, yield pressure conditions between 620 and 1,650 bar and temperatures between 170 and 300°C during fluid entrapment. The migration of CH 4 -rich gases within the Carboniferous rocks can be related to the main stage of basin subsidence and stages of basin uplift. A different situation is recorded in fluid inclusions in fissure minerals hosted by Permian sandstones and carbonates: aqueous fluid inclusions in calcite, quartz, fluorite and anhydrite are always H 2 O-NaCl-CaCl 2 -rich and show homogenization temperatures between 120 and 180°C. Co-genetically trapped gas inclusions are generally less frequent. When present, they show variable N 2 -CH 4 compositions but contain no CO 2 . P-T reconstructions indicate low-pressure conditions during fluid entrapment, always below 500 bar. The entrapment of N 2 -CH 4 inclusions seems to be related to phases of tectonic uplift during the Upper Cretaceous. A potential source for nitrogen in the inclusions and reservoirs is C org -rich Carboniferous shales with high nitrogen content. Intensive interaction of brines with Carboniferous or even older shales is proposed from fluid inclusion data (enrichment in Li, Ba, Pb, Zn, Mg) and sulfur isotopic compositions of abundant anhydrite from fissures. The mainly light d 34 S values of the fissure anhydrites suggest that sulfate is either derived through oxidation and redeposition of biogenic sulfur or through mixing of SO 4 2À -rich formation waters with variable amounts of dissolved biogenic sulfide. An igneous source for nitrogen seems to be unlikely since these rocks have low total nitrogen content and, furthermore, even extremely altered volcanic rocks from the study area do not show a decrease in total nitrogen content.

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