MOLECULAR NITROGEN IN NATURAL-GAS ACCUMULATIONS - GENERATION FROM SEDIMENTARY ORGANIC-MATTER AT HIGH-TEMPERATURES

The occurrence of natural gas accumulations with high percentages (up to 100%) of molecular nitrogen in various hydrocarbon provinces repres ents a largely unresolved problem and a serious exploration risk. In t his context, a geochemical and basin modeling study was performed to e valuate the potential of sedimentary organic matter to generate molecu lar nitrogen. The masses of nitrogen present in coals-if converted int o molecular nitrogen-are sufficient to fill commercial gas reservoirs. A calculation for gas accumulations in northern Germany, where percen tages of molecular nitrogen range from less than 5 to greater than 90% , reveals that the molecular nitrogen generated in underlying coal-bea ring strata is sufficient to account for the nitrogen gas even in the largest fields. In addition, much of the total nitrogen in clay-rich r ock types, such as shales and mudstones, is fixed in sedimentary organ ic matter and may add to the nitrogen generation capacity of the coals . A kinetic interpretation of the nitrogen and methane generation char acteristics of humic coals during laboratory pyrolysis indicates that nitrogen is generated from organic matter in sedimentary basins at hig her temperatures than methane. Minimum temperatures for preferential n itrogen generation are in excess of 300 degrees C for extremely low he ating rates or isothermal conditions lasting over 300 m.y. Thus, nitro gen-rich gases are mainly formed in the final stage of gas generation, when sedimentary rocks grade into metamorphic rocks. Applications of the kinetic parameters for the generation of nitrogen and methane from coaly organic matter in Carboniferous sedimentary rocks are presented for part of the Northwest German basin. In this area, immense volumes of the methane-rich gas generated during the late Paleozoic and Mesoz oic were lost to the atmosphere. The present gas composition mainly re flects the latest Cenozoic gas generation. This Cenozoic gas is nitrog en rich where the Carboniferous source rock sequences are highly matur e and deeply buried.