Abstract
Natural gases in China are mainly coal-derived gas, with assistance from oil-typed gas. At present, many genetic identification methods, from hydrocarbon composition and isotope to light hydrocarbon and biomarker indexes, have been formed, but combined methods from non-hydrocarbon gases are lacking. Based on compositions and isotopes geochemical characteristics and the differences of non-hydrocarbon nitrogen gas in coal-derived gas and oil-typed gas, and combining the isotopic geochemical characteristics of non-hydrocarbon helium, the comprehensive identification methods of coal-derived gas and oil-typed gas for hydrocarbon gases according to the associated non-hydrocarbon gases of nitrogen and helium are established and the preliminary applications have been engaged. The main recognitions are as follows:1)Coal-derived gas generally has relatively lower nitrogen abundance, mainly distributed from 0 to 31.2% with main frequency from 0 to 2%. Oil-typed gas, on the other hand, usually has relatively higher nitrogen abundance, mainly distributed from 1.1 to 57.1% with main frequency from 2 to 16%. 2) Coal-derived gas generally has relatively heavier nitrogen isotope values, mainly distributed from -8 to 19.3‰ with main frequency from -8 to 8‰.Oil-typed gas usually has relatively lower nitrogen isotope values, mainly distributed from -10.6 to 4.6‰ with main frequency from -8 to 4‰.3)The geochemical characteristic differences of coal-derived gas and oil-typed gas are mainly due to the fact that the sapropel parent material is relatively rich in nitrogen element and rich in light δ14N, while the humic parent material is relatively poor in nitrogen element and rich in heavy δ15N. The differences on thermal maturity of source rocks, the redox conditions of source rock sedimentary environment, and the salinity of water body are also important effective factors.4)Differences on nitrogen abundances and isotopes in coal-derived gas and oil-typed gas have great significance in genetic identification. The genetic identification chart of R/Ra-δ15N for organic and inorganic nitrogen in natural gas and the comprehensive joint identification charts of R/Ra-δ15N-N2 of nitrogen and helium for coal-derived gas and oil-typed gas have been established, and are of great reference in investigating the origins and sources of natural gas and guiding natural gas exploration in China.
Highlights
Natural gas is an environmentally friendly, low-carbon, effective, and clean energy
From the histogram depicting nitrogen isotope distribution of coal-derived gas and oil-typed gas in China, it can be concluded that the value of the nitrogen isotope (δ15N) of 41 coal-derived gas samples is totally distributed from −8 to 19.3‰ with main frequency from −8 to 8‰, while the value of the nitrogen isotopes of 40 oil-typed gas samples is generally distributed from −10.6 to 4.6‰ with main frequency from −8 to 4‰(Figure 4)
The geochemical characteristic differences of coal-derived gas and oiltyped gas are mainly due to the fact that the sapropel parent material is relatively rich in nitrogen element and rich in light δ14N, while the humic parent material is relatively poor in nitrogen element and rich in heavy δ15N
Summary
Natural gas is an environmentally friendly, low-carbon, effective, and clean energy. It occupies an important position in the world’s energy structure and is one of three important sources and targets in future energy development (natural gas, unconventional oil and gas, and new energy). Three methods involving 28 comprehensive indicators and charts for identifying the origin of natural gas, especially for coal-derived gas and oil-typed gas, have been developed, such as carbon isotopes and compositions, light hydrocarbons, and biomarkers (Dai et al, 1988; Dai et al, 1992; Dai et al, 1993; Dai et al, 1994; Dai et al, 1995; Dai et al, 1996; Dai et al, 1999; Dai et al, 2005; Dai et al, 2008; Dai et al, 2009; Dai et al, 2014; Dai and Qi, 1989; Li Jian et al, 2001; Li Jian et al, 2005; Hu Guoyi et al, 2007). In order to check the reliability of the instrument in the long-term detection process, air standard and a natural gas sample are selected for nitrogen isotope repeatability test at a FIGURE 1 | Research areas of coal-derived gas and oil-typed gas on gas-bearing basins in China. Based on the experimental analysis and previous researcher’s data statistics, the geochemical characteristics of nitrogen component and isotope of coal-derived gas and oiltyped gas in China were systematically studied and discussed
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