Origins of N₂ in the Dongtai Depression of the Subei Basin

Abstract

Identification of N 2 origins can help to reduce exploration risks and to improve the understanding of gas generation and accumulation. N 2 from the mantle and organic matter in basins cannot be unraveled by using δ 15 N ratios alone, due to overlapping δ 15 N signatures. In this paper, we comprehensively studied N 2 , Ar,He, δ 13 C and their relationships of natural gases in the Dongtai Depression of the Subei Basin. Nitrogen contents of the gases in this depression range from 0.2 to 85.9%. Some samples have the air N 2 /Ar ratio but others are excess relative to this ratio. δ 15 N data of all the samples except one constitute two populations in the histogram. All these features reflect at least two origins of N 2 . One of these populations has δ 15 N values around 0‰ with N 2 / 3 He ratios of 1.0×10 7 to 9.5×10 9 , whereas the other population has negative δ 15 N values from -7.3 to -2.2 ‰ with N 2 / 3 He of 1.1×10 7 to 9.4×10 9 . It was obtained that the nitrogen of the former population is mainly atmospheric in origin. For the latter population, a considerable amount of nitrogen from organic matter in sedimentary rocks was identified, which coexist with the nitrogen from the air and mantle. In the Dongtai Depression of the Subei Basin, therefore, atmospheric, organic and mantle origins provided remarkable contributions to the natural gases.

Stable Isotope Evaluation of Geothermal Gases from the Kızıldere and Tekke Hamam Geothermal Fields, Western Anatolia, Turkey

“…The stable isotopic composition of nitrogen δ 15 N (‰ vs. Air) has been used as a tracer for the delineation of the possible non-atmospheric nitrogen components present within terrestrial fluids issuing from various tectonic settings [94,95]. Nitrogen derived from (i) sediments (inorganic) of continental crust and/or subducted oceanic slab have δ 15 N values around +7‰, [55], (ii) mantle derived N 2 , including unaltered ocean crust and N 2 degassed from the upper mantle exhibit δ 15 N values close to −5‰, [89], (iii) nitrogen related to organic sources of different degrees of maturity have values in the negative range, reaching −19‰ [96].…”

“…Nitrogen derived from (i) sediments (inorganic) of continental crust and/or subducted oceanic slab have δ 15 N values around +7‰, [55], (ii) mantle derived N 2 , including unaltered ocean crust and N 2 degassed from the upper mantle exhibit δ 15 N values close to −5‰, [89], (iii) nitrogen related to organic sources of different degrees of maturity have values in the negative range, reaching −19‰ [96]. Since the isotopic characteristics of different nitrogen reservoirs within the subsurface coincide at some level, it is the general trend to compare the δ 15 N isotopic values with those of the elemental ratios, such as N 2 / 36 Ar [54,55] and N 2 / 3 He [95] for the delineation of the possible nitrogen sources.…”

Origins and Fates of CO2 in the Dongying Depression of the Bohai Bay Basin

“…Mantle-derived geofluids have been found in a series of rift basins in East China (Jin et al 2002a;Sun et al, 2009;Zhang et al, 2009;2010). The Dongying Depression is one of these rift basins, located in the southeastern part of the Bohai Bay Basin.…”