The origin of non-hydrocarbon gases in the North-German Basin: Sources, migration paths and relative timing of nitrogen release

The North German Basin forms part of the Mid European Basin where natural gas produced from Carboniferous, Permian, and Triassic reservoirs mainly sourced from coal-bearing strata and marine shales of Carboniferous age. The gas composition ranges from predominantly hydrocarbons in the United Kingdom and the Netherlands to predominantly nitrogen (up to 90 %) in most of the areas of northeastern Germany. Until now, the origin of the large nitrogen contents in the eastern reservoirs are discussed controversy.

The NGB is subdivided into several sub basins that are characterized by different intensities of subsidence. In the central part of the basin, Carboniferous sediments are buried to more than 7000 m depth. The goal of this project is to determine whether deep buried shales might be a major source of nitrogen in sedimentary basins and to explore the behavior of fixed nitrogen during diagenetic, hydrothermal, and metamorphic processes. This includes understanding of the interrelationship between the nitrogen content and sedimentary facies; the mobilization of nitrogen during diagenesis; and the migration from source(s) into the reservoirs. The methods comprise the determination of total and fixed nitrogen contents and δ¹⁵N isotopic compositions of rocks, minerals and organic compounds, and detailed studies of deep fluids and fluid inclusions (microthermometry, laser-Raman spectroscopy, anion/cation variation, ammonium, nitrogen and chlorine isotopic compositions). Our previous studies indicate that carbonaceous shales posses a large storage potential for nitrogen as NH₄⁺ in illites and are capable of releasing nitrogen during thermal and fluid/rock interaction events. Comparative studies on the behavior of nitrogen in different parts of the Central European Basin (CEB) show that the (East North German Basin) ENGB represents a key area for these processes. K-Ar dating on NH₄⁺-bearing illites indicates that illite was formed not only by burial diagenesis but also during multiple brine migration events associated with NH₄⁺ or K⁺ exchange processes in illites. Hydrothermal experiments on the (K, NH₄)-partitioning between fluids and illites will be conducted to verify these interactions, and their influence on isotopic fractionation. Fluid inclusion studies and chemical analyses of reservoir brines will be performed to characterize the composition of fluids that were involved in w/r interaction processes. All results will contribute to a detailed p-T, geochemical, and chronological description of fluid generation and migration during subsidence and inversion events of the NGB and will form the basis for modeling the NH₄⁺ or K⁺ exchange reactions in illites for which thermo-dynamic constants have to be derived. The newly developed thermo-dynamic data reaction-transport modeling will be performed to examine and to quantify timing of exchange processes and transport mechanisms.

Backer: DFG-Schwerpunktprogramm -SPP 1135- "Dynamik sedimentärer Systeme unter wechselnden Spannungsbedingungen am Beispiel des zentraleuropäischen Beckensystem“
Employee: Dr. Birgit Plessen, Dr. Volker Lüders, Petra Meier, Sylvia Pinkerneil, Rudi Naumann
Partner: Peer Hoth (BGR), Bernd Krooß (RWTH Aachen), D. Banks (University of Leeds, UK)