Geochemical and geological characteristics of the Lower Cambrian shales in the middle–upper Yangtze area of South China and their implication for the shale gas exploration

• TOC values in most analyzed samples from the middle–upper Yangtze area of South China are higher than 2.0%.
• Brittle mineral contents are generally above 40.0%.
• Porosity and permeability are relatively low in most samples and range 0.1%–21.9% and 0.001–2.540 mD respectively.
• Five areas are selected as favorable targets for shale gas exploration.

During the Early Cambrian, the middle–upper Yangtze area of South China evolved to an open shelf with anoxic bottom waters prevailing in most places. Accumulated black shales in this area, named Niutitang Formation (Qiongzhusi Formation in some places), thus become the prospective resources for shale gas exploration and development. Based on compiled petrological and organic geochemistry data, and other information obtained by relevant laboratory tests and field investigation, we documented regional distributional patterns of Lower Cambrian organic-rich shales in the middle–upper Yangtze area according to their lateral extent, thickness, total organic carbon, thermal maturity and burial depth. This study also features the general reservoir properties of dark shales in this area. Together these results highlight several most favorable areas including Youyang-Xiushan-Huayuan, Suiyang-Daozhen-Dejiang, Shiqian-Sinan-Yinjiang, Jiangkou-Tongren-Songtao and Huangping-Shibing for shale gas exploration.