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Possible Origins for Low Thermal Maturity, High-Nitrogen Natural Gases


Some areas in the western USA have high-nitrogen natural gases in settings with low thermal maturity, such as western Kansas, SE New Mexico, and SE Utah. Published models for high-nitrogen gas generation require high thermal maturity. This presentation evaluates sources and mechanisms for forming high-nitrogen natural gas at relatively low thermal maturity. Both organic and inorganic components of sediment contain nitrogen. During early diagenesis, organic matter may generate microbial gases with up to about 20% N2. Planktonic organic matter N/C drops precipitously during diagenesis (possible N2 generation) but remains stable during late diagenesis and catagenesis (no N2 generation). Sedimentary rocks typically contain about 0.1 to 1 mg N/g rk after removal of organic matter. Much of this nitrogen is ammonium in clays. Ammonium in clays comes from organic matter during early diagenesis and catagenesis. Clays can generate N2 where ammonium is released from clays and oxidizes in the pore water. Ammonium in illite increases with depth, so smectite illitization is not likely to generate N2. However, kaolinization or chloritization of smectite could generate N2. A nitrogen-rich gas phase will form at shallow depth after minor N2 generation. For example, pure N2 gas is formed after generation of only 0.01 mg N/g rk in rocks with 5% porosity saturated with 200,000 ppm NaCl pore water at 1 km depth. Low pressure, high salinity, and low porosity favor gas-phase formation. A dispersed, lean, nitrogen source could therefore form high N2 gas at shallow burial. A lean nitrogen source rock can generate large quantities of high N2 gas where N2 source rock volume is large. For example, release of 10% of the nitrogen in a rock with 1 mg N2/g rk releases about 70 BCF N2 gas from a rock unit 100 m thick over the area the size of a USA township (36 mi2; 94 km2). Nitrogen generation is dwarfed by methane generation from thermally mature petroleum source rocks. High-nitrogen gases are expected only where hydrocarbon gases cannot dilute the nitrogen. Many of the western USA high-nitrogen gases are associated with high salinity, low porosity, and low thermal maturity consistent with a lean nitrogen source. An inorganic nitrogen source is more probable due to low organic carbon, but a late diagenetic organic source is possible where planktonic organic carbon is abundant.