GENERATION OF NITROGEN AND METHANE FROM SEDIMENTARY ORGANIC-MATTER - IMPLICATIONS ON THE DYNAMICS OF NATURAL-GAS ACCUMULATIONS

Nitrogen (N-2) contents of natural gases in Rotliegend and Buntsandste in reservoirs of the North German basin regionally approach 100%. A re view is given of the various hypotheses (primordial origin, volcanic o r magmatic origin, radiogenic origin, atmospheric origin, organic orig in, inorganic nitrogen in sedimentary rocks) presented to account for nitrogen anomalies in this area and other parts of the world. The obje ctive of the present study was to investigate sedimentary organic matt er, in particular coals, as a potential source of molecular nitrogen i n the subsurface. Comparison of reservoir sizes and gas generation pot entials indicates that Carboniferous coal measures, which are consider ed as the source of the natural gas in the North German basin, can rea dily account for the nitrogen quantities found in present-day reservoi rs. Laboratory pyrolysis experiments were carried out to investigate t he 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 me thane and nitrogen generation rates from coals were calculated for geo logic heating rates. Gas containing more than 50% nitrogen is generate d under these conditions at temperatures in excess of 300 degrees C. N itrogen-rich gases are thus formed only in the final stage of gas gene ration after methane generation has practically ceased. It is conclude d that the amounts of gas encountered in nitrogen-rich gas accumulatio ns represent only a small fraction (possibly < 1%) of the total gas ge neration potential of this area while the bulk of the generated gas ha s escaped to the atmosphere. The present-day composition of the reserv oir gases reflects the composition of only the most recently generated gas (on a geologic time scale).