青海共和盆地自生自储型氦气的初步发现

Preliminary discovery of self-generating and self-preserving helium gas in Gonghe Basin

为查明共和盆地潜在氦气源岩(干热岩)稀有气体、元素地球化学和矿物学特征,笔者在共和盆地采集了与干热岩相关的花岗岩进行了稀有气体和放射性铀钍测试、扫描电镜及能谱分析,发现花岗岩中氦气含量达到49×10^(-6)~851×10^(-6),每吨岩石中氦气含量达到0.27798~4.76992 m^(3),局部区段氦气含量更高。计算3 He/4 He同位素值在7.30×10^(-10)~2.5×10^(-8)之间,确定该区氦气为放射性成因。共和盆地花岗岩体中常见铀钍独立矿物及分散矿物,铀含量在0.74×10^(-6)~38.1×10^(-6),钍含量在4.7×10^(-6)~42.2×10^(-6)。估算研究区氦生成量约269×10^(8)m^(3)。在此基础上,进一步开展了花岗岩氦气储层储集特征研究,岩体中存在低密度、较高孔隙度和微裂缝发育区段,测井数据也显示花岗岩体中储集特征和脆性良好的区段,是氦气赋存的空间。花岗岩中的富铀矿物对氦气的封闭温度(27~250℃)与干热岩相关的花岗岩温度(80~236℃)基本一致,具备封存条件,估算岩体中封闭氦量为4.03×10^(8)~46.8×10^(8)m^(3)。岩体中浅部、深部和压裂返排液中均有氦气,深部高含量氦气存在,压裂后返排液中氦气聚集明显,初步证明了自生自储型氦气藏的存在。

In order to find out the noble gas content,element geochemistry and mineralogy of potential helium source rocks(hot dry rocks)in Gonghe Basin,the granites associated with hot and dry rocks in Gonghe Basin were collected for rare gas and radioactive uranium-thorium testing,scanning electron microscopy and energy spectrum analysis.The results show that the helium content in the granite is 49×10^(-6)~851×10^(-6),and the helium content per ton of rock is 0.27798~4.76992 m^(3).The helium content in local section is higher.The isotope values of 3 He/4 He are in the range of 7.30×10^(-10)~2.5×10^(-8),which indicates that the helium in this area is of radioactive origin.Independent and dispersed minerals of uranium and thorium are commonly found in granite bodies in the Gonghe Basin,with uranium content ranging from 0.74×10^(-6)to 38.1×10^(-6),with thorium content ranging from 4.7×10^(-6)to 42.2×10^(-6).Estimated helium generation in the study area is approximately 269×10^(8)m^(3).On this basis,further research was conducted on the reservoir characteristics of granite helium gas reservoirs.There are sections with low density,high porosity,and microcrack development in the rock mass.Logging data also shows that the storage characteristics and brittleness of the granite rock mass are good,which is the space for helium gas occurrence.The sealing temperature of uranium rich minerals in granite for helium gas(27~250℃)is basically the same as that of granite related to dry hot rocks(80~236℃),which meets the sealing conditions.It is estimated that the sealed helium amount in the rock mass is 4.03×10^(8)~46.8×10^(8)m^(3).Helium content is present in the shallow and deep parts of the rock mass,as well as in the fracturing flowback fluid.High helium content is present in the deep part,and helium accumulation is evident in the flowback fluid after fracturing,which preliminarily proves the existence of self-generated and self-preserved helium gas accumulations.