Molecular Nitrogen in Natural Gas Accumulations: Generation from Sedimentary Organic Matter at High Temperatures

Abstract

The occurrence of natural gas accumulations with high percentages (up to 100%) of molecular nitrogen in various hydrocarbon provinces represents a largely unresolved problem and a serious exploration risk. In this context, a geochemical and basin modeling study was performed to evaluate the potential of sedimentary organic matter to generate molecular nitrogen. The masses of nitrogen present in coals--if converted into molecular nitrogen--are sufficient to fill commercial gas reservoirs. A calculation for gas accumulations in northern Germany, where percentages of molecular nitrogen range from less than 5 to greater than 90%, reveals that the molecular nitrogen generated in underlying coal-bearing strata is sufficient to account for the nitrogen gas even in the largest fi lds. In addition, much of the total nitrogen in clay-rich rock types, such as shales and mudstones, is fixed in sedimentary organic matter and may add to the nitrogen generation capacity of the coals. A kinetic interpretation of the nitrogen and methane generation characteristics of humic coals during laboratory pyrolysis indicates that nitrogen is generated from organic matter in sedimentary basins at higher temperatures than methane. Minimum temperatures for preferential nitrogen generation are in excess of 300°C for extremely low heating rates or isothermal conditions lasting over 300 m.y. Thus, nitrogen-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 gener ted during the late Paleozoic and Mesozoic were lost to the atmosphere. The present gas composition mainly reflects the latest Cenozoic gas generation. This Cenozoic gas is nitrogen rich where the Carboniferous source rock sequences are highly mature and deeply buried.