超深层海相页岩型氦源岩特征与生氦潜力OA

[Objective]Large-scale,effective helium source rocks serve as a material basis for the formation of a helium-rich gas field.In the Weiyuan area,Sichuan Basin,the helium-rich gas reservoirs within the Dengying Formation exhib-it helium source rocks consisting of the pre-Sinian basement granites and the shales of the Cambrian Qiongzhusi Forma-tion.Compared to the basement granites,studies on the shales of the Qiongzhusi Formation remain limited,including those on the occurrence characteristics of uranium(U)-and thorium(Th)-rich minerals and helium generation potential.[Methods]This study investigated the shales of the Qiongzhusi Formation,Weiyuan area,Sichuan Basin.By integrat-ing field emission scanning electron microscopy(FE-SEM),energy-dispersive X-ray spectroscopy(EDS),and trace ele-ment analysis,this study examined the characteristics and helium generation potential of shales as helium source rocks in the Qiongzhusi Formation.[Results and Conclusions]The shales of the Qiongzhusi Formation in the study area exhibit significant U enrichment but a low Th abundance.The occurrence forms of U and Th are primarily governed by redox conditions.The shales of the Qiongzhusi Formation were deposited in a highly reducing environment.Under such a sedi-mentary setting,U primarily occurs within organic matter,clay minerals(e.g.,montmorillonite and chlorite),phosphorus-bearing minerals,and iron-bearing minerals(siderite)through the processes of reduction-precipitation,adsorption,and complexation.In contrast,Th predominantly occurs within stable minerals such as monazites and zircons via isomorph-ous substitution.The shales of the Qiongzhusi Formation show an average helium generation intensity of 2.41 ×10-2 m3/(kg.a),higher than that of the pre-Sinian basement granites(1.82× 10-21 m3/(kg·a)).However,the shales exhibit a high hydrocarbon generation intensity of approximately(2～200)×108 m3/km2.As a result,the enormous volumes of hy-drocarbon gases significantly dilute helium,leading to low helium abundance(volumetric fraction:0.018％)within the shale gas reservoirs and weakening the helium supply efficiency of the shales to the gas reservoirs within the Dengying Formation.In contrast,the extensive-scale basement granites effectively supply helium to the gas reservoirs through dis-solution-exsolution and transport along faults.Consequently,a dual-source helium supply pattern consisting of granites predominantly and shales secondarily is formed.