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

We have detected the N-2 content in the natural gas from Tarim Basin by measuring the chemical and stable isotope composition of 107 natural gas samples, and found the N-2 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 N-2 content, stable carbon and nitrogen isotope composition, three groups of N-2 sources have been identified: (I) Coal-type gas in Cenozoic reservoirs has low N-2 contents (N-2<5%), less depleted carbon and nitrogen isotope compositions (delta C-13(2)>-26 parts per thousand, delta N-15>+5.0 parts per thousand). These nitrogen molecules likely come from ammonium montmorillonites and thermal degradation of aromatic and heterocyclic structures of anthracite; (II) Oil-type gas with moderate N-2 contents (5%<20%) and relatively depleted carbon isotopes (delta C-13(2) = -36 parts per thousand to -29 parts per thousand, delta N-15 = -6.0 parts per thousand-+6.0 parts per thousand), distributed in the gas fields of Yakela, Hetianhe, Lungu, Jiefagnqu, Jilake, etc. and preserved in Paleozoic and Mesozoic, with N-2 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 N-2 content (N-2>20%), depleted carbon (delta C-13(2)<-34 parts per thousand) and widely varying nitrogen isotope compositions (-15 parts per thousand