ISSN 1009-6248CN 61-1149/P 双月刊

主管单位:中国地质调查局

主办单位:中国地质调查局西安地质调查中心
中国地质学会

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    花岗岩在氦气成藏中的双重作用:氦源与储集

    Dual Contribution of Granites in Helium Accumulation: Source and Reservoir

    • 摘要: 氦气是重要战略性稀有气体资源,寻找富氦天然气藏是解决氦气资源短缺的关键。目前工业利用的氦资源主要是壳源氦气,有效氦源岩是富氦天然气藏形成的基础,储层是氦气聚集的必须条件。资料调研表明已知富氦天然气藏与花岗岩关系密切,为研究花岗岩生氦、排氦及储氦能力,利用花岗岩铀钍含量与赋存状态、分段加热释氦、氩离子抛光扫描电镜等实验分析数据,结合前人成果综合研究表明:①渭河盆地南缘(北秦岭)花岗岩和银额盆地蒙额地1井花岗岩均富铀、钍,生氦能力较强。②花岗岩中的铀、钍有铀钍独立矿物和铀钍类质同象矿物等2种赋存状态。③温度是控制花岗岩中氦气释放的首要因素,不同矿物对氦气的封闭温度差异较大,晶质铀矿中4He封闭温度最低,为27℃~76℃,而磁铁矿封闭温度最高,可达250℃,即花岗岩在温度大于250℃时对4He无封闭能力,27℃~250℃具有部分封闭能力,27℃以下具有完全封闭能力。④花岗岩纳米级孔隙大小优于页岩(页岩气储层),微裂缝发育,具有氦气运移和储存能力。可见,花岗岩不仅是有效氦源岩,也可以是氦气储集岩。与在烃源岩中找页岩气类似,可在花岗岩等氦源岩中寻找氦气聚集。

       

      Abstract: As a strategically important noble gas resource, the exploration of helium in natural gas reservoirs is the key to solving the shortage of helium resources. Industrial use of helium resources is currently dominated by crust-derived helium. Effective helium source rocks are the foundation for the generation of helium-rich natural gas reservoirs, whereas reservoirs are necessary for the accumulation of helium. Previous researches have shown that the known helium-rich natural gas reservoirs are strongly associated with granites. To understand the helium generation, release and storage capacity in granites, this research analyzes the concentration and occurrence states of uranium(U) and thorium(Th), stepwise heating, argon ion polishing technique and scanning electron microscope, combined with previous results, it has been shown that:①The granites in the southern margin of the Weihe basin(North Qinling) and from the Meng'erdi 1 well in the Yingen-Ejinaqi basin are both abundant in uranium and thorium, resulting in good helium production capacity. ②U and Th in granites have two states of occurrence, including, U-Th independent minerals and U-Th isomorphic minerals. ③Temperature is the primary factor controlling the release of helium from granites. The closure temperature for helium differs widely among minerals. Uraninite has the lowest closure temperature for 4He at 27℃~76℃, while magnetite has the highest temperature and reaches 250℃. In other words, granite cannot block 4He inside above 250℃, 27℃~250℃ suggests a partial closure range, and below 27℃ has complete closure capacity. ④ Granite has better nanoscale pore size than shale(shale gas reservoirs), and develops microfractures, therefore it has a good helium transport and storage capacity. In summary, granites are not only effective helium source rocks, but can also be helium reservoirs. Just like exploring shale gas in hydrocarbon source rocks, it is also possible to find helium accumulations in helium source rocks such as granites.

       

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