鄂尔多斯盆地杭锦旗地区东胜气田氦气成因来源及富集规律

Source and enrichment regularity of helium in Dongsheng Gas Field of Hangjinqi area,Ordos Basin

近期勘探发现,鄂尔多斯盆地杭锦旗地区东胜气田上古生界多口井天然气中氦气含量达到0.1%以上,具有较好的工业开发价值。基于天然气地球化学特征的系统分析,发现氦气含量分布介于0.045%~0.487%之间,氦气以微量组分赋存于以烃类气体为主的烃类—富氦气藏中。杭锦旗地区上古生界煤系烃源岩中U、Th元素含量高于基底岩石,基底岩石中石英砂岩—石英岩U、Th元素含量稍高于片麻岩—花岗片麻岩;研究发现片麻岩—花岗片麻岩解析氦气丰度含量要高于石英砂岩—石英岩,煤系烃源岩解析氦气丰度远低于基底岩石,这主要与其生成的大规模烷烃气的稀释以及地质时代年轻有关,说明烃源岩作为潜在氦源岩所生成的氦气含量较低,无法形成具有工业价值的含氦气藏。东胜气田上古生界天然气伴生氦气^(3)He/^(4)He值介于(1.83~6.25)×10^(-8)之间,系统对比研究发现其来源于基底发育的太古界—中元古界变质岩—花岗岩系,氦气所赋存的天然气为典型煤型气,来源于石炭系—二叠系煤系烃源岩,与伴生氦气具有不同的来源,二者具有异源同储成藏特征,伴生氦气在运移、聚集与成藏过程中与上古生界常规天然气藏的储、盖、圈及运聚组合具有良好的时空配置关系。东胜气田氦气富集主要受基底变质岩—花岗岩系岩相发育和深大断裂展布等双重因素控制,主要分布在二级断裂的沟通氦源岩断裂与四级断裂的输导体系交会处和太古界—中元古界基底岩相发育区,断裂带的展布控制了油气及其伴生氦气藏的空间分布。

Recent exploration has found that helium contents are higher in the Upper Paleozoic natural gas in Dongsheng Gas Field of Hangjinqi area in Ordos Basin,being more than 0.1%in many gas wells,which have good values for industrial development.Based on the systematic analysis of natural gas geochemical characteristics,it is found that hydrocarbon gases dominate the Upper Paleozoic natural gas in Dongsheng Gas Field,the helium contents vary from 0.045%to 0.487%,and helium occurs in a small fraction in a hydrocarbon-rich helium reservoir dominated by hydrocarbon gases.The contents of U and Th in the Upper Paleozoic coal measure source rocks are higher than those in the basement rocks,and the U and Th contents of quartz sandstone and quartzite are slightly higher than those in gneiss-granitic gneiss rocks in basement rocks in Hangjinqi area.It is found that the helium abundance of resolved gas in gneiss-granitic gneiss rock is higher than that in quartz sandstone-quartzite,and the helium abundance of resolved gas in coal-measure source rocks is much lower than that in basement rocks,which is mainly due to the dilution of large-scale hydrocarbon gas generated and the short geological time.The content of helium generated by the hydrocarbon source rocks is low,so it cannot form the industrial value of helium reservoirs.The ^(3)He/^(4)He ratios of the Upper Paleozoic natural gas of Dongsheng Gas Field vary from 1.83×10^(-8) to 6.25×10^(-8),and it is found that helium originated from the Archaean-Middle Proterozoic metamorphic rock-granite series with basement.The natural gas occurred with helium is typical coaltype gas,which comes from Carboniferous-Permian coal-measure source rocks,and has different sources from the associated helium gas.The hydrocarbon gas and the associated helium have the characteristics of different sources and the same reservoir.In the process of migration,accumulation and reservoir formation,the associated helium gas has a good space-time configuration with the reservoir,cap,trap and migration-accumulation combination of the Upper Paleozoic conventional natural gas reservoir.The helium enrichment in Dongsheng Gas Field is mainly controlled by the lithofacies development of basement metamorphic rock-granite series and the distribution of deep and large faults.Helium mainly distributes at the intersection of the second-order fault communicating helium source rock and the fourth-order fault being the transport system and the Archaean-Middle Proterozoic basement lithofacies development area.The distribution of the fault zone controls the spatial distribution of natural gas and its associated helium gas reservoirs.