渭河盆地地热水伴生天然气成因及氦气富集机理

Genesis and helium enrichment mechanism of geothermal water-associated gas in Weihe Basin

渭河盆地地热水伴生气中氦气含量异常高,引起了国内学者的普遍关注,而伴生气中天然气成因及氦气富集机理是核心科学问题。采集渭河盆地典型井地热水伴生气样品,系统开展天然气组分、碳氢同位素组成及稀有气体氦、氩同位素比值分析。结果表明,渭河盆地地热水伴生天然气主体为煤成气,碳氢同位素组成与鄂尔多斯盆地石炭系—二叠系煤成气相似,主要来自上古生界煤系烃源岩;部分为生物气,来自新近系张家坡组泥岩的贡献。伴生气中稀有气体氦同位素比值显示,渭河盆地氦气主要为壳源氦,主要来自基岩中U、Th的放射性衰变。渭河盆地地热水中氦气与天然气可能是同源或异源,地热水伴生气中氦气百分含量虽高,但天然气和氦气的总体含量很低,现阶段以地热水为载体的氦气资源仍然不具有工业价值。按照天然气资源的勘探思路,在渭河盆地寻找天然气藏,可能是实现氦气规模建产的有效手段。

The content of helium in geothermal water-associated gas in Weihe Basin is extremely high,which has attracted the attention of domestic scholars. The source and enrichment mechanism of helium becomes a focused scientific issues in recent years. The samples of geothermal water-associated gas from typical wells in Weihe Basin were collected,and the chemical composition,carbon and hydrogen isotopic compositions,helium and argon isotopic composition of these samples were systematically analyzed. The results show that most of the geothermal water-associated gases are coal-formed gas,the carbon and hydrogen isotopic compositions of the geothermal water-associated gas are similar to those of Carboniferous and Permian coal-formed gas in Ordos Basin,which were mainly from the contribution of source rock of coal series in Upper Paleozoic. The rest of the associated gas of geothermal wells is biogas,which is contributed by mudstone of Zhangjiapo Formation. The helium isotope ratios in geothermal water-associated gas show that the helium is mainly crustal source helium. It mainly originated from the U,Th radioactive decay of the basement granite. Although the helium content is high,the total content of natural gas and helium in geothermal water is very low. At present,the helium resource with geothermal water as the carrier still has no industrial value. According to the idea of natural gas exploration,looking for small and medium-sized natural gas reservoirs in Weihe Basin may be an effective means to break through the bottleneck of helium resource exploration in Weihe Basin.