International Geologiical Congress - Oslo 2008

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SES-05 Dynamics of complex intracontinental basins

 

The role of basinal brines in the release and migration of nitrogen in the NGB

 

Birgit Plessen, GeoForschungsZentrum (Germany)
Volker Lueders, GeoForschungsZentrum (Germany)
Peer Hoth, BGR (Germany)
 

 

Preliminary studies on the distribution of nitrogen in Paleozoic sediments from the Central European Basin (CEB) show that especially the eastern part of the North German Basin represents a key area for the investigation of nitrogen accumulation in natural gas reservoirs. In this part of the basin, very nitrogen-rich natural gases are preferably stored in Rotliegend reservoirs, where coal-bearing, hydrocarbon source rocks of Upper Carboniferous age are almost absent in shallow marine or deltaic shales. Our studies focus on Ammonium (NH4+) as a major source of nitrogen in sedimentary basins. The behavior of NH4+ during diagenesis, deep burial and fluid/rock interaction during basinal brine events could be responsible for the nitrogen accumulations in the NGB.
The principal source of nitrogen in sediments is the breakdown of organic material during diagenesis that results in inorganic nitrogen compounds such as ammonium. NH4+ can be adsorbed on mineral surfaces or be substituted for K+ on the interlayer sites in clay-minerals. The studied Paleozoic shales from deep bore holes and outcrops of the CEB show total nitrogen contents up to 3600 ppm where up to 90 % of this nitrogen can be fixed in form of NH4+ in illites. This characterise the high nitrogen storage potential of the shales and the stability during deep burial.
Ammonium either can be released from silicates by thermal decomposition (>400°C), cation exchange reactions or oxidation. VR values between 2.5 and 5.5 Rm% exclude processes of thermal nitrogen release from these rocks and oxidation processes seem to be negligible. Instead, carbonate displacement, chloritization in whole rocks as well as Sr and NH4+ depletion and K enrichment in illites indicate brine-induced K-NH4 exchange reactions as a preferred ammonium release process in Upper Carboniferous shales (Mingram et al. 2005). This experimental well studied exchange reaction by Pöter et al. (2004) offers a possibility for nitrogen release under diagenetic to catagenetic conditions. Intensive interaction of brines with Palaeozoic shales is further evidenced by the composition of fluid inclusions (Lders et al. 2005) and the formation fluids. K-Ar dating on NH4+-bearing illites indicate that illite was formed not only during burial diagenesis but also during brine migration events associated with NH4+ or K+ exchange processes during stages of subsidence and inversion in the NGB.

 

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