天然气中N_2来源及其地球化学特征分析

天然气中N2的来源很多，其地球化学特征也不相同：①大气源N2：N2／Ar≤84，且δ15NN2≈0‰（ATM）；②地壳越深部和上地幔来源的原生N2∶δ15NN2≈-2‰～＋1‰．且伴生Ar的40Ar／36Ar＞2000和He的3He／4He＞10－6；③微生物反硝化作用生成的N2∶δ15NN2＜－10‰和地下水中NO3-及NO2-浓度异常高；④未成熟沉积有机质经微生物氨化作用形成的N2∶δ15NN2＜－10‰，伴生CK4的δ13C＜－55‰（PDB）；⑤成熟（包括高成熟）沉积有机质经热氨化作用形成的N2∶δ15NN2值≈-10‰～－1‰，且伴生CH4的δ13C≈－55‰～－30‰；⑥过成熟沉积有机质裂解产生的N2∶δ15NN2≈＋5‰～＋20‰；⑦沉积岩中无机及在高温变质作用下释放出的N2∶δ15NN2≈＋1‰～＋3．5‰，N2／Ar》84。

ANALYSIS ON ORIGINS OF MOLECULAR NITROGEN IN NATURAL GASES AND THEIR GEOCHEMICAL FEATURES

There are multiple potential sources for molecular nitrogen (N2 ) in natural gases. The origins of N2 are identified with the different nitrogen isotopic composition and other geochemical features. (1 )Atmospheric N2: N2/Ar≤84, and δ 15N≈0‰(ATM). (2) Primordial N2 from supper-deep crust and upper-mantle: δ15N≈－2% ～ ＋1‰, 40Ar/36Ar > 2000 and 3He/4He > 10 -6 for associated Ar and He, and the positive correlationship between N2 and He contents. (3) N2 from denitrification by micrcorganisms: δ15N < - 10‰, and abnormal high concentrations of NO3- and NO2- in subsurface waters. (4) N2 from bacteriogenic ammonia during immature stage of sedimentary organic matter:δ 15N≈-20‰～10‰, δ13C< - 55‰ (PDB) for associated CH4. (5) Thermo-pyrogenic N2 from sedimentary organic matter during post-mature stage: δ15N≈＋5‰ ～ ＋20‰, δ13C≈- 30‰ ～ －20‰ for　associated CH4. (6) Released N2 from inorganic fixed nitrogen in sedimentary rock during metamorphism: δ 15N≈＋1‰ ≈ ＋ 3. 5‰, and N2/Ar>>84．