Zhiwen, Li, and Guo Jianhua. “Prospect of Middle Ordovician Yanxi Formation Black Shale in South Hunan Province”. Journal of Mines, Metals and Fuels 67, no. 9 (October 20, 2022): 424–430. Accessed May 9, 2024. https://www.informaticsjournals.com/index.php/jmmf/article/view/31651.