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

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• 作者：Quanyou Liu^a ; ^{qyouliu@sohu.com} ; Jin Zhijun^a ; Chen Jianfa^b ; Bernhard M. Krooss^c ; Shengfei Qin^d
• 关键词：Tarim Basin ; natural gas ; nitrogen molecule ; stable isotopes ; N2 origins
• 刊名：Journal of Petroleum Science and Engineering
• 出版年：2012
• 出版时间：January, 2012
• 年：2012
• 卷：81
• 期：Complete
• 页码：112-121
• 全文大小：1408 K

文摘

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 (¦Ä¹³C₂ > ?#xA0;26? ¦Ä¹⁵N > + 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 (¦Ä¹³C₂ = ?#xA0;36?to ?#xA0;29? ¦Ä¹⁵N = ?#xA0;6.0¡ë? 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 (¦Ä¹³C₂ < ?#xA0;34? and widely varying nitrogen isotope compositions (?#xA0;15?#xA0;< ¦Ä¹⁵N < + 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.