Origin of nitrogen molecules in natural gas and implications for the high risk of N2 exploration in Tarim Basin, NW China

Abstract

We have detected the N2 content in the natural gas from Tarim Basin by measuring the chemical and stable isotope composition of 107 natural gas samples, and found the N2 contents vary widely. Based on the carbon isotopic composition of gas hydrocarbons, we have classified the natural gases into two genetic types: coal-type and oil-type gases, and the latter have consistently higher nitrogen contents than the former. Furthermore, according to the N2 content, stable carbon and nitrogen isotope composition, three groups of N2 sources have been identified: (I) Coal-type gas in Cenozoic reservoirs has low N2 contents (N2<5%), less depleted carbon and nitrogen isotope compositions (δ13C2>−26‰, δ15N>+5.0‰). These nitrogen molecules likely come from ammonium montmorillonites and thermal degradation of aromatic and heterocyclic structures of anthracite; (II) Oil-type gas with moderate N2 contents (5%<N2<20%) and relatively depleted carbon isotopes (δ13C2=−36‰ to −29‰, δ15N=−6.0‰–+6.0‰), distributed in the gas fields of Yakela, Hetianhe, Lungu, Jiefagnqu, Jilake, etc. and preserved in Paleozoic and Mesozoic, with N2 likely originating from ammonium clays, evaporate and thermal decomposition of organic matter at the high- and over-mature stage; (III) Oil-type gas with high N2 content (N2>20%), depleted carbon (δ13C2<−34‰) and widely varying nitrogen isotope compositions (−15‰<δ15N<+15‰), located in Paleozoic and Mesozoic intervals such as Tazhong, Donghetang, Hade and Lunnan gas fields, etc. This high concentration of N2 is mainly derived from Cambria–Ordovician shale carbonates at mature and highly mature stages, and the N2 contents tend to increase with increasing thermal stress. In the end, we have provided a broad correlation between N2 content and 3He/4He ratio and helium concentration, which indicates that a deep N2 gas source may be relevant.