Concepts and Models for the Formation of N₂-rich Natural Gas Reservoirs

Anke Jurisch¹, Bernhard M. Krooss¹, Ralf Littke¹, Birgit Plessen², Volker Lüders², and Peer Hoth^3
1Institute of Geology and Geochemistry of Petroleum and Coal, Lochnerstr. 4-20 RWTH Aachen University, D-52056 Aachen, Germany
²GFZ Potsdam, Telegrafenberg, D-14473 Potsdam, Germany
³BGR Berlin; Wilhelmstr. 25-30, D-13593 Berlin, Germany

Palaeozoic sedimentary sequences are considered the most likely sources of molecular nitrogen (N₂) occurring in high percentages in natural gases of the Central European Basin (CEB). The highest nitrogen contents are found within Rotliegend reservoirs of the North East German Basin (NEGB) where thick, highly mature Palaeozoic sedimentary sequences are present. The formation and accumulation mechanisms and the geological conditions determining the regional distribution of these nitrogen-rich gases are only marginally understood.

Current concepts consider both sedimentary organic matter and ammonium-bearing minerals as primary nitrogen sources. Several studies have been conducted to explore the contents, isotopic composition (d¹⁵N) and the speciation of nitrogen in organic and inorganic constituents of Carboniferous sequences.

Open-system non-isothermal pyrolysis of shales and isolated kerogen has revealed the presence of inorganic nitrogen species with relatively low thermal stability (decomposition between 450 and 700°C) in marine Namurian A shales. Inorganic nitrogen components in the paralic Namurian B facies show a higher thermal stability range (decomposition range 600 – 800°C) while nitrogen in kerogen and coals is fixed in very refractory compounds decomposing in the 700 – 1200°C temperature range.

Total nitrogen contents of Namurian shales from four deep wells (4400 - 7000 m) in NE Germany ranged from ~500 to ~2700 ppm. The d¹⁵N values of fixed NH₄+ ranged from +1 to +3.5‰ while the isotopic composition of the thermally liberated nitrogen was consistently lighter by 3-5‰.Low nitrogen contents (as low as 460 ppm) and high d¹⁵N values (up to +5.6‰) in one well in the basin centre suggest a large-scale release of nitrogen associated with isotopic fractionation.

The nature of the N₂liberation process (thermally or chemically controlled, single- or multi-step) remains to be investigated in more detail.

Predictive modeling of N₂-rich natural gas generation on the basin scale still remains a major challenge. Some key issues that need to be further constrained by a combination of laboratory tests and field studies are:
1 identification of depositional conditions producing facies types with high nitrogen contents
2 thermodynamic and kinetic controls on nitrogen release from different inorganic and organic species and facies types
3 analysis of thermal regimes that nitrogen-prone sediment layers have undergone in the context of basin evolution

 

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands