Dual Contribution of Granites in Helium Accumulation: Source and Reservoir

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.