Origins of N2 in the Dongtai Depression of the Subei Basin

Abstract

Identification of N2 origins can help to reduce exploration risks and to improve the understanding of gas generation and accumulation. N2 from the mantle and organic matter in basins cannot be unraveled by using θ15 N ratios alone, due to overlapping θ15 N signatures. In this paper, we comprehensively studied N2, Ar, θ15 N, N2/3He, 3He/4He, θ13 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 N2/Ar ratio but others are excess relative to this ratio. θ15N data of all the samples except one constitute two populations in the histogram. All these features reflect at least two origins of N2. One of these populations has θ15N values around 0‰ with N2/3He ratios of 1.0×107 to 9.5×109, whereas the other population has negative θ15N values from −7.3 to −2.2 ‰ with N2/3He of 1.1×107 to 9.4×109. 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.