天然气N2组成地球化学示踪指标研究---以柴达木盆地为例

Molecular nitrogen is one of most common non-hydrocarbon components in natural gas reservoirs, and is an important natural gas geochemical tracer, but, it is often overlapped when it is used to divide the genetic types of natural gas using N2 content or δ15N. In order to solve this problem, Key Laboratory of Petroleum Resources Research, CAS, has developed a gas chromatography technology with the detectors of FPD, FID and MSD and Chromatography, mass spectrometry workstation, which has independent intellectual property rights and the national invention patent. The technology can test 15 non hydrocarbon molecules in natural gas and the content of molecules with different mass number and mass number abundance ratios, which makes the N2 tracing indexes increasing from 2 to 21, and their geochemical tracing Information will be greatly increased, but the technology is lack of data collection and summarization and the basis for theory and application. The project is to test N2 component of biogenic gas, oil type gas and coal gas in the Qaidam basin to improve the non hydrocarbon gas molecular testing technology, to reveal formation mechanism of N2, to summarize the N2 distribution characteristics of the different genetic types of the natural gases, to establish discriminating models and charts of N2 of the different genetic types of the natural gases. This program also can expand the research and application of the other non hydrocarbon molecules in both organic genetic gas and abiogenic gas and provide a new method for natural gas sources and correlation method.

氮气是天然气中常见的非烃组分，也是天然气地球化学示踪的重要指标，其中最主要的两个参数是N2含量和δ15N，当应用其进行天然气成因类型划分时，经常出现重叠。针对这一问题，中科院油气资源重点实验室研发了获得国家发明专利的配置有FPD、FID、MSD检测器和色谱、质谱工作站的气相色谱分析技术。该技术可测试天然气中15个非烃分子及其不同质量数分子含量和质量数丰度比，也使得备选的N2示踪指标从2个增加到21个，预计其中蕴含的天然气成因信息也大量增加，但截止目前，本技术应用缺乏数据积累、总结和理论基础。本项目以此技术为依托，以柴达木盆地生物气、油型气和煤型气典型气田为例，开展天然气N2组成测试，揭示天然气N2组成形成机理，总结不同成因类型天然气N2组成分布特征，建立不同成因类型天然气N2组成判别模式和图版，为拓展其它非烃分子在有机与无机成因气中研究和应用奠定基础，也为天然气气源追索和对比提供新的方法。

柴达木盆地存在油型气、煤型气和生物气三种不同类型的天然气，应用天然气组分和碳同位素特征可以轻易区分生物气，而油型气和煤型气存在混合现象不易区分，需借助氮气地球化学特征进行判识。氮气是天然气中最常见的非烃组分，也是天然气地球化学示踪的重要指标，其中最主要的两个参数是N2含量和δ15N。通过组分和稀有气体同位素特征可以确定天然气中的氮气来自于烃源岩有机质热解，柴西油气主要来自古近系—新近系烃源岩，柴北缘油气主要来自侏罗系烃源岩。天然气碳同位素和轻烃特征均表明柴北缘油气成熟度高于柴西油气成熟度，成熟度与氮同位素成正相关性，因此柴北缘天然气的氮同位素值比柴西地区天然气氮同位素重。统计发现柴西油型气氮同位素小于-1，柴北缘煤型气氮同位素大于0，结合氮同位素特征可以对柴北缘煤型气和柴西油型气进行有效划分，为柴达木盆地内天然气成因类型判识提供新的参考标准。