天然气中分子氮成因及判识

ORIGINS OF MOLECULAR NITROGEN IN NATURAL GASES AND THEIR IDENTIFICATION

天然气中分子氮（Ｎ２）的成因复杂多样，其同位素组成和其它地球化学特征各不相同。（１）大气源的Ｎ２：ＣＮ２／ＣＡｒ≤８４，且δ１５ＮＮ２≈０‰（ＡＴＭ）；（２）地壳超深部和上地幔来源的原生Ｎ２：δ１５ＮＮ２≈－２‰～＋１‰，伴生Ａｒ的４０Ａｒ／３６Ａｒ＞２０００和Ｈｅ的３Ｈｅ／４Ｈｅ＞１０－６，且Ｎ２和Ｈｅ含量具有正相关性；（３）微生物反硝化作用生成的Ｎ２：δ１５ＮＮ２＜－１０‰和地下水中ＮＯ－３及ＮＯ－２浓度异常高；（４）沉积有机质在未成熟阶段经微生物氨化作用形成的Ｎ２：δ１５ＮＮ２＜－１０‰；伴生ＣＨ４的δ１３ＣＣＨ４＜－５５‰（ＰＤＢ）；（５）沉积有机质在成熟（包括高成熟）阶段经热氨化作用形成的Ｎ２：δ１５ＮＮ２≈－１０‰～－１‰，且伴生ＣＨ４的δ１３ＣＣＨ４≈－５５‰～－３０‰；（６）沉积有机质在过成熟阶段裂解产生的Ｎ２：δ１５ＮＮ２≈＋５‰～＋２０‰；（７）沉积岩中无机氮在高温变质作用下释放出的Ｎ２：δ１５ＮＮ２≈＋１‰～－３．５‰，ＣＮ２／ＣＡｒ８４．对这些潜在源的特征进行了综合论述。

There are multiple potential sources for molecular nitrogen (N 2) in natural gases. The origins of N 2 are identificated with the different nitrogen isotopic composition and other geochemical marks. (1) Atomspheric N 2: C N 2 / C Ar ≤84, and δ 15 N N 2 ≈0‰ (ATM); (2) Primordial N 2 from supper deep crust and upper mantle: δ 15 N N 2 ≈-2‰～+1‰, 40 Ar/ 36 Ar>2 000 and 3 He/ 4He>10 -6 for associated Ar and He, and the positive correlationship between contents of N 2 and He. (3) N 2 from denitrification by microoganisms: δ 15 N N 2 <-10‰, and abnormal high concentration of NO - 3 and NO - 2 in subsurface water; (4) N 2 from bacteriogenic ammonia during immature stage of sedimentary organic matter: δ 15 N N 2 ≈-20‰～-10‰, δ 13 C CH 4 <-55 (PDB) for associated CH 4. (5) N 2 from thermogenic ammonia during mature stage of sedimentary organic matter: δ 15 N N 2 ≈-10‰～-1‰, δ 15 C CH 4 ≈-55 ‰～-30‰ for associated CH 4. (6)Thermo pyrogenic N 2 from sedimentary organic matter during post mature stage: δ 15 N N 2 ≈+5‰～+20‰, δ 13 C CH 4 ≈-30‰～ -20‰ for associated CH 4. (7)Released N 2 from inorganic fixed nitrogen in sedimentary rock during metamorphism: δ 15 N N 2 ≈+1‰～+3.5‰, and C N 2 / C Ar 84.