天然气中氮的地球化学研究进展

ADVANCES IN NITROGEN GEOCHEMISTRY OF NATURAL GAS

综合分析了国内外大量文献,指出了天然气中氮的地球化学研究进展。天然气中氮的主要来源有地球深部氮的运移和沉积物中有机与无机成因氮的释放。不同热解实验结果研究表明,氮气的生成既可以在低温也可在高温,但形成机理各不相同:低温主要是以大量蛋白质发生水解作用而产生氨基酸,氨基酸很不稳定,在粘土矿物的作用下生成NH3,而NH3被进一步氧化生成氮;而高温阶段主要以煤岩芳构化重新组合形成吡咯与吡啶释放出氮。不同环境下形成的氮,其1δ5N值是不同的,陆相N2的1δ5N低于+3‰,海相N2的1δ5N则高于+4‰。不同成因形成的氮,其同位素组份也各不一样,一般有机质经微生物作用生成N2的1δ5N通常小于-10‰,有机质热解形成N2的1δ5N>-10‰,N2/A r值远大于84;沉积岩中含氮矿物高温变质作用生成N2的1δ5N为+1.0‰^+3.5‰,N2/A r>84;地壳深部或上地幔来源N2的1δ5N值主要集中在-20‰^+1‰之间,40A r/36A r>2000,3H e/4H e为>1.39×1-0 6。

Molecular nitrogen encountered in varying amounts （up to 〉90%） in natural gas reservoirs may be derived from two potential sources, thermal liberation from the organic and/or inorganic constituents of sediments （crustal source） and accumulation of upward migrating deep （mantle） nitrogen. Different types of pyrolysis experiments show that nitrogen is produced not only at low temperatures, but also at high temperature levels. A variety of different release mechanisms are involved. It is well known that proteins are hydrolysed to amino acids during the early, diagenetic stage of maturation （R0〈0. 6 %）. However, amino acids are generally unstable and are preferentially transformed to NH3 under the catalysis of clay minerals which then may be further oxidized to N2, nitrate or nitrite. The bulk of nitrogen in coals at higher ranks occurs in condensed aromatic units as pyridinic and pyrrolic structures of high thermal stability. Natural gases from different depositional environments are characterized by nitrogen isotope ratios. The δ^15N values are usually lower than ＋3‰ in continental deposits whereas they are higher ＋4‰ in marine deposits. Different nitrogen isotope compositions are indicative for specific origins. Generally δ^15N vaues of biogenic nitrogen are below -10whereas for nitrogen from pyrolysis experiments a]SN values are higher than -10‰. The N2/Ar of gases produced by microbial processes and thermal pyrolysis is often higher than 84. The δ^15N of N2 resulting from high temperature metamorphism of N-containing minerals ranges from ＋1.0‰ to ＋3.5‰, with N2/Ar〉84. The δ^15N composition of nitrogen from the upper mantle and/ or the deep crust is between -20‰ and ＋1‰, with ^40Ar/^36Ar〉2000 and ^3He/^4He〉1. 39×10^16.