Generation of nitrogen and methane from sedimentary organic matter: Implications on the dynamics of natural gas accumulations

Abstract

Nitrogen (N 2) contents of natural gases in Rotliegend and Buntsandstein reservoirs of the North German basin regionally approach 100%. A review is given of the various hypotheses (primordial origin, volcanic or magmatic origin, radiogenic origin, atmospheric origin, organic origin, inorganic nitrogen in sedimentary rocks) presented to account for nitrogen anomalies in this area and other parts of the world. The objective of the present study was to investigate sedimentary organic matter, in particular coals, as a potential source of molecular nitrogen in the subsurface. Comparison of reservoir sizes and gas generation potentials indicates that Carboniferous coal measures, which are considered as the source of the natural gas in the North German basin, can readily account for the nitrogen quantities found in present-day reservoirs. Laboratory pyrolysis experiments were carried out to investigate the kinetics of generation of methane and molecular nitrogen from coals of different type and rank. Under experimental conditions nitrogen is formed at higher temperatures than methane supporting the concept of a ‘fractional generation’ of methane and nitrogen in natural systems. Based on the kinetic parameters derived from laboratory experiments methane and nitrogen generation rates from coals were calculated for geologic heating rates. Gas containing more than 50% nitrogen is generated under these conditions at temperatures in excess of 300°C. Nitrogen-rich gases are thus formed only in the final stage of gas generation after methane generation has practically ceased. It is concluded that the amounts of gas encountered in nitrogen-rich gas accumulations represent only a small fraction (possibly < 1 %) of the total gas generation potential of this area while the bulk of the generated gas has escaped to the atmosphere. The present-day composition of the reservoir gases reflects the composition of only the most recently generated gas (on a geologic time scale).