Characteristics and helium generation potential of ultra-deep marine shales as helium source rocks

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 exhibit helium source rocks consisting of the pre-Sinian basement granites and the shales of the Cambrian Qiongzhusi Formation. 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 integrating field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), and trace element 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 sedimentary 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 isomorphous substitution. The shales of the Qiongzhusi Formation show an average helium generation intensity of 2.41×10⁻²¹ 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 hydrocarbon 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 dissolution-exsolution and transport along faults. Consequently, a dual-source helium supply pattern consisting of granites predominantly and shales secondarily is formed.