Investigation of the fate of nitrogen in Palaeozoic shales of the Central European Basin

https://doi.org/10.1016/j.gexplo.2005.11.075Get rights and content

Abstract

Nitrogen geochemistry of Upper Carboniferous shales from the Central European Basin (CEB) was investigated by elemental analysis, stable isotope mass spectrometry and non-isothermal pyrolysis. Total N-contents of Namurian shales from four deep wells (4400–7000 m) in NE Germany ranged between 520 and 2680 ppm. Up to 90% of this nitrogen occurs as ammonium in minerals with δ15N values between + 1‰ and + 3.5‰. Low nitrogen contents (down to 460 ppm) and high δ15N values (up to + 5.6‰) in one well in the basin centre suggest a large-scale release of nitrogen associated with isotopic fractionation. Pyrolytic liberation of N2 from pelagic Namurian A shales of NW and NE Germany occurred at significantly lower temperatures than from paralic Namurian B shales and terrestrial Westphalian samples. On-line isotope analysis of N2 liberated between 400 and 1200 °C indicates the presence of precursor pools with different thermal stability and nitrogen isotopic composition.

Introduction

The Central European Basin (CEB) extends from the United Kingdom across the Netherlands and northern Germany into Poland. Natural gas is produced from Carboniferous, Permian, and Triassic reservoirs and the major source rocks are Carboniferous coal-bearing strata of Westphalian age and marine black shales of Namurian and Viséan age. While gas composition is hydrocarbon-dominated in the United Kingdom and the Netherlands, gas in most areas of NE Germany has high concentrations of molecular nitrogen (e.g. Gras and Clayton, 1998). Both, the sources and the migration and accumulation processes of N2 are still poorly understood. Possible origins for the high nitrogen contents in natural gas reservoirs include sedimentary organic matter, ammonium fixed in minerals of sedimentary, magmatic or metamorphic rocks and the deep crust and mantle (e.g. Müller et al., 1973, Krooss et al., 1995, Gerling et al., 1997, Zhu et al., 2000, Boyd, 2001, Hoth et al., 2002). The highest nitrogen contents were found within the Rotliegend reservoirs in the North East German Basin (NEGB) where thick, highly mature Palaeozoic sedimentary sequences are present. The natural gases of the NGB show δ15N values ranging from + 3‰ to 11.2‰ in Rotliegend Reservoirs (Müller et al., 1976).

Various studies have been conducted to explore the nitrogen cycle and the speciation of nitrogen in organic and inorganic constituents of sedimentary matter (e.g. Mingram and Bräuer, 2001, Orberger et al., 2005, Krooss et al., 2005, Mingram et al., 2005). In the present study, Palaeozoic shale samples from different locations in the CEB have been analysed with respect to their nitrogen contents and the isotopic composition of the fixed nitrogen. The results were compared with mineralogical and thermal maturity data in order to identify systematic trends indicating the extent and conditions of nitrogen liberation. Non-isothermal open-system pyrolysis experiments with on-line analysis of the gaseous products (N2, CH4, CO2, CO) were conducted on the bulk shale samples and kerogen concentrates to explore the thermal stability of inorganic and organic nitrogen species. For selected bulk samples, the δ15N values of the pyrolytically liberated N2 were determined on-line as a function of temperature.

Section snippets

Analytical methods

Total carbon and nitrogen contents were measured using a Heraeus varioEL-CHN elemental analyzer. For extraction of fixed-NH4+, organic material was chemically removed from approximately 2 g of bulk sample material using a solution of 10 ml 0.1 N KCl and 10 ml NaOCl (10% active Cl) as described by Williams et al. (1995). 500 mg of the dried sample were digested in 40% HF using polypropylene bottles. After raising the pH by adding 20% KOH, the solution was distilled using a Kjeldahl apparatus,

Results

Recent analyses on Upper Carboniferous shales of different facies and maturity in the CEB suggest relatively homogeneous nitrogen contents and low variations in the isotopic composition (Mingram et al., 2005). Total nitrogen (Ntot) contents ranging from 520 to 2680 ppm were measured for grey to greyish black Upper Carboniferous (Namurian A, B, Westphalian A, B and C) shales from four deep wells (4400 to 7000 m) in NE Germany (between Schwerin and the Altmark region; cf. Fig. 1 and Table 1 in

Concluding remarks

The results presented here were obtained by a complementary approach of elemental analysis and pyrolysis experiments combined with stable isotope measurements. Evidence was found for significant nitrogen storage as NH4+ during diagenesis in Carboniferous shales of the CEB. Fixed NH4+ contents apparently did not change significantly during burial. This suggests that fixed ammonium is stable under deep burial up to catagenetic p-T conditions. The low fixed-NH4+–N contents and high δ15N values of

Acknowledgements

We are indebted to the following colleagues who have contributed to the research on nitrogen in Palaeozoic systems in various projects over recent years: Lothar Friberg, Yves Gensterblum, Jan Hollenstein, Peer Hoth, Ralf Littke, Volker Lüders, and Dirk Prinz.

Financial support by Deutsche Forschungsgemeinschaft (DFG) within the framework of the priority programme SPP 1135 “Dynamics of Sedimentary Systems under Varying Stress Conditions as Exemplified by the Central European Basin System” is

References (14)

There are more references available in the full text version of this article.

Cited by (23)

  • Evaluation of redox versus thermal maturity controls on δ<sup>15</sup>N in organic rich shales: A case study of the woodford shale, anadarko basin, Oklahoma, USA

    2015, Organic Geochemistry
    Citation Excerpt :

    Alteration of the δ15N signal also appears to depend on whether NH4+ is lost from the sediments or is retained through reincorporation into biomass or adsorption into the sediments and measured along with the bulk N (Freudenthal et al., 2001; Junium and Arthur, 2007; Robinson et al., 2012). The effect of catagenesis on the alteration of N concentrations and δ15Nbulk is not well constrained (e.g. Stiehl and Lehmann, 1980; Williams et al., 1995; Barth et al., 1996; Krooss et al., 2005, 2006; Boudou et al., 2008; Jurisch and Krooss, 2008; Li et al., 2009). For example, Boudou et al. (2008) finds there is no set increase in δ15N with increasing thermal rank, and therefore the observed loss of N must take place without isotopic fractionation.

View all citing articles on Scopus
1

Fax: +49 331 288 1436.

View full text