鄂尔多斯盆地含氦天然气地球化学特征与富集影响因素

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

鄂尔多斯盆地天然气资源丰富,部分地区天然气中的氦气含量较高,亟需深入研究含氦天然气地球化学特征与富集影响因素。通过采集天然气样品并进行组分和同位素等实验分析测试,结合地质条件,研究了鄂尔多斯盆地含氦天然气的分布、地球化学特征和影响因素,研究结果表明:(1)鄂尔多斯盆地天然气主要为烃类气体,氦气含量为0.016%~0.487%,平均值为0.060%;(2)鄂尔多斯盆地含氦天然气甲烷碳同位素值分布在-53.88‰~-29.23‰,甲烷、乙烷、丙烷和丁烷碳同位素表明烃类气为有机成因;(3)鄂尔多斯盆地含氦天然气3He/4He含量比值为20.10×10^(-9)~120.00×10^(-9),平均值为42.00×10^(-9),R/Ra值为0.014~0.085,平均值为0.030,为壳源氦的特征,不受天然气成因类型和成熟度等因素影响;(4)氦气含量较高的天然气主要分布在鄂尔多斯盆地北部东胜气田、西南部庆阳气田和东南部黄龙气田的上古生界石炭系-二叠系,其分布与古今构造位置、基底断裂、生氦强度及生烃强度相对强弱等因素密切相关。根据含氦天然气地球化学特征评价出富氦-中氦区、低氦区及贫氦区。

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/Ra 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 natural gas;(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.Key words:geochemistry,helium source rock,helium,natural gas,Ordos Basin gas;(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.