The isotopic compositions of molecular nitrogen: implications on their origins in natural gas accumulations

The isotopic compositions of N2 (δ15N, ‰, atm) imply the geochemical origins of molecular nitrogen in natural gas accumulations, but the irregular variation of δ15NN2 for most petroliferous basins puzzles scientists. We believe that this is due to multiple origins of N2 which are often mixed together and leads to the irregularities in gas pools. The four petroliferous basins, the West Siberian basins in Russia, the Yinggehai basin of China, the Californian Great Valley basin of USA and the Mid-European basin, have significantly different isotope characteristics of N2, which may provide insight into the isotope geochemical implications of N2 in natural gas accumulations. N2 with −19‰⩽δ15NN2⩽−10‰ may be from sedimentary organic matter at the immature and the early mature stage (Ro⩽0.6%). Values of −10‰<15NN2⩽−2‰ may indicate that N2 originates from mature (including high mature) sedimentary organic matter (Ro≈0.6–2.0%). Values of −2‰<δ15NN2<+1‰ imply that N2 may be from the deep crust or mantle. N2 with δ15NN2=0‰ and N2/Ar=38–84 suggest that the atmosphere is the origin. Values of +1‰⩽δ15NN2<+4‰ characterize N2 from ammonium clay minerals in shales and mudrocks during metamorphism. δ15NN2=+4‰ is the feature of N2 from saltpeter in evaporites. Values of +4‰<δ15NN2⩽+18‰ indicate that N2 maybe derived from post-mature sedimentary organic matter (Ro>2.0%).