Geochemical characteristics and enrichment factors of helium-bearing natural gas in the Ordos Basin

The Ordos Basin is renowned for its rich natural gas resources, which exhibit a high helium content in some areas. Therefore, there is an urgent need to thoroughly investigate the geochemical characteristics and enrichment factors of helium-bearing natural gas. Based on the experiments, analysis, and tests of the compositions and isotopes of natural gas samples, along with geological conditions, we examine the distribution, geochemical characteristics, and controlling factors of helium-bearing natural gas in the Ordos Basin. Key findings are as follows: (1)The natural gas in the basin is dominated by hydrocarbon gases, with a helium content ranging from 0.016 % to 0.487 %(average:0.060 %); (2)The carbon isotope values of methane in the helium-bearing natural gas within the basin range from -53.88 ‰ to -29.23 ‰,and those of methane, ethane, propane, and butane indicate an organic origin of the hydrocarbon gases; (3)The helium-bearing natural gas in the Ordos Basin exhibits ³He/⁴He ratios ranging from 20.10×10^-9 to 120.00×10^-9, with an average of 42.00×10^-9, and R/R_a ratios varying between 0.014 and 0.085, with an average of 0.030. These ratios represent crust-derived helium characteristics, which are unaffected by the genetic type and maturity of naturalgas; (4)The natural gas with a high helium content is primarily distributed in the Carboniferous-Permian strata of the Upper Paleozoic of Dongsheng gas field (north of the basin), Qingyang gas field (southwest of the basin) and Huanglong gas field (southeast of the basin). This distribution is closely associated with factors like ancient and modern tectonic locations,basement faults, and the intensities of helium and hydrocarbon generation. Based on the geochemical characteristics of helium-bearing natural gas, we identify helium-rich-to-moderate, low-helium, and helium-deficient areas in the Ordos Basin.