Formation of Nitrogen-rich Gas Accumulations in the Central European Basin. I. The Significance of Fixed Ammonium in Palaeozic Rocks

Plessen, Birgit¹, Peer Hoth², Volker Lüders³ (1) GeoForschungsZenrum Potsdam, Potsdam, Germany (2) Bundesanstalt für Geowissenschaften und Rohstoffe, Berlin, Germany (3) GeoForschungsZentrum Potsdam, Potsdam, Germany

The Central European Basin represents a key area for the investigation of nitrogen accumulation in natural gas reservoirs. Especially in its eastern part, the so called North German Basin (NGB), natural gases of Rotliegend reservoirs are nitrogen-rich with up to 90 % N2.

Our studies focus on the fixation of nitrogen in form of ammonium in clay minerals, the deep burial of this ammonium during subsidence, the release of nitrogen and related migration processes. Measurements of Ntotal, NH4+fixed, TOC and corresponding stable isotopes, Vr, R, main and trace elements were performed on Palaeozoic sedimentary and volcanic rocks and on separated illites.

The principal source of nitrogen in sediments is organic material. Biological and thermal degradation during diagenesis results in inorganic nitrogen compounds such as ammonium which can be substituted for K+ on the interlayer sites of clay-minerals. More than 2000 ppm nitrogen was detected in studied potential Carboniferous source rocks with an inorganic portion of up to 90 %. This nitrogen is mainly fixed as ammonium in illites, indicating that illites may act as efficient nitrogen storage sites during burial.

Ammonium content and nitrogen isotopic composition of Upper Carboniferous sediments from the eastern part of the NGB indicate a preferred release of nitrogen by brine-induced K-NH4 exchange reactions. The migration of such ammonium-enriched brines is reflected by the presence of abundant authigenic NH4-bearing illites in the Rotliegend rocks. Due to the high oxidation potential of these rocks a significant part of ammonium could have been oxidized to N2 and stored in the gas reservoirs.