THE ISOTOPIC COMPOSITIONS OF MOLECULAR NITROGEN: IMPLICATIONS ON THEIR ORIGINS IN NATURAL GAS ACCUMULATIONS

Abstract

The isotopic compositions of N2 (, ‰, atm) imply the geochemical origins of molecular nitrogen in natural gas accumulations, but the irregular variation of 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‰≤15Nn2 ≤−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‰<15Nn2<+1‰ imply that N2 may be from the deep crust or mantle. N2 with and N2/Ar=38–84 suggest that the atmosphere is the origin. Values of +1‰≤15Nn2<+4‰ characterize N2 from ammonium clay minerals in shales and mudrocks during metamorphism. is the feature of N2 from saltpeter in evaporites. Values of +4‰<15Nn2≤+18‰ indicate that N2 maybe derived from post-mature sedimentary organic matter (Ro>2.0%).