Characteristics and helium-generating potential of helium source rocks in the Ordos Basin

Helium sources act as the primary element in generating helium resources. Potential helium source rocks encompass basement metamorphic rocks and various sedimentary rocks, such as mudstone, argillaceous dolomite, coal, and bauxite, in basins. Based on field geological surveys, gravity-magnetic data interpretation, core description, and major-and trace-element analyses, we investigate five suites of potential helium source rocks of two categories in the Ordos Basin and their helium-generating potential. The results show that two categories of potential helium source rocks, namely basement metamorphic rocks and sedimentary rocks, are developed in the studied basin. The helium source rocks of the basement type are found in the Archean land block and the superimposed Paleoproterozoic strata, dominated by high-grade metamorphic gneiss, granulite, marble, migmatite, and granitic gneiss, with average abundances of U and Th of 3.15×10^-6 and 12.38×10^-6, respectively, and a helium-generating intensity of 0.735×10^-6 cm³/(a·g). On the other hand, the helium source rocks of the sedimentary type are primarily found in Changchengian metasedimentary rocks of the Mesoproterozoic strata and the Paleozoic sedimentary rocks. The Changchengian black slate is widely seen in the northern and southwestern Ordos Basin, and exhibits average abundances of U and Th of 2.36×10^-6 and 8.28×10^-6, respectively, and a helium-generating intensity of 0.522×10^-6 cm³/(a·g). The argillaceous dolomite in the Lower Ordovician Majiagou Formation of the Lower Paleozoic strata, found in the central and eastern Ordos Basin, displays average abundances of U and Th of 1.71×10^-6 and 9.80×10^-6, respectively, and a helium-generating intensity of 0.487×10^-6 cm³/(a·g). The Carboniferous-Permian mudstones and coals of the Upper Paleozoic strata are extensively distributed throughout the Ordos Basin. The Taiyuan Formation mudstones, among others, show average abundances of U and Th of 9.69×10^-6 and 22.68×10^-6, respectively, and a helium-generating intensity of 1.82×10^-6 cm³/(a·g), while its coals exhibit average abundances of U and Th of 16.12×10^-6 and 44.13×10^-6, respectively, and a helium-generating intensity of 3.21×10^-6 cm³/(a·g). The Carboniferous bauxite of the Upper Paleozoic strata, primarily occurring in the eastern and southwestern Ordos Basin, demonstrates average abundances of U and Th of 7.14×10^-6 and 38.57×10^-6, respectively, and a helium-generating intensity of 1.97×10^-6 cm³/(a·g). Various helium source rocks are discovered in the southwestern Ordos Basin, suggesting a multi-source helium supply. Overall, this study lays the foundation for helium resource exploration in the Ordos Basin.