Geochemical characteristics of the Upper Paleozoic coal series shale in the Linxing area, Ordos Basin, China: implications for paleoenvironment, provenance, and tectonic setting

The geochemical characteristics of shales can reflect their formation environment and reservoir quality, which are significant for further exploration of shale gas and optimization of favorable areas. Therefore, we have determined the contents of trace and rare earth elements (REEs) in 27 shale samples collected from the Shanxi and Taiyuan formations in the Linxing area, which is located on the northeastern margin of the Ordos Basin. The results show that the Shanxi formation shale is enriched in Pb, Bi, U, Li, Rb, Cd, In, Cs, and Cu; in the Taiyuan formation shale, these elements include Li, U, Pb, Bi, Rb, Cd. In, Cs, and Zn. The content of heavy REEs (HREEs) is lower than that of light REEs (LREEs), and Eu is in a depleted state. The ratios among elements indicate that the Shanxi and Taiyuan formations developed in an anaerobic and weakly reduced environment. However, weak oxidation and oxygen-rich environments also existed during the same period. From the Taiyuan to Shanxi formation, the sedimentary environment transitions from brackish water to continental freshwater. The climate during the Taiyuan period changed from dry to humid, and the dry and humid climates alternated during the Shanxi period. The sedimentary debris that makes up the shale is mainly composed of ancient sedimentary rocks, granites, and a small amount of alkaline basalt sourced from passive continental margin and continental island arc environments. The magma intrusion in the Linxing area has no effect on the geochemical compositions of these shale.