Origins of N_2 in the Dongtai Depression of the Subei Basin

【摘要】：正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 usingδ~(15)N ratios alone,due to overlappingδ~(15) N signatures.In this paper,we comprehensively studied N_2,Ar,δ~(15)N,N_(2/3)He,~3He/~4He,8℃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 hasδ~(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 negativeδ~(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.