Geochemical and Geological Characteristics of the Upper Ordovician–Lower Silurian Shales in the Upper Yangtze Basin, South China: Implication for the Shale Gas Exploration oleh Yuan-Yuan Sun, Jian-Fei Yan, Yu-Peng Men, Qian Yu, Wei Liu, Xiao-Wei Chen, Jian-Wei Kang, Hai-Quan Zhang, Yuefeng Shen, Jun Liu