Ediacaran, Cambrian, Ordovician, Silurian and Permian shales of the Upper Yangtze Platform, South China: Deposition, thermal maturity and shale gas potential

For many years, the Yangtze Platform has been successfully explored and exploited for petroleum. The Lower Silurian Longmaxi Shale proved to contain commercial quantities of gas, but several other formations might also have a high potential for shale-hosted gas exploration. This paper comprises of geochemical, petrographical, petrophysical and mineralogical information on the major Proterozoic and Paleozoic gas shales in the Upper Yangtze area, which provides insights on the depositional environment, the thermal maturity of organic matter, shale gas storage capacity and the fracability of these rocks. The total organic carbon (TOC) content varies from 0.1 to 22.5% with an average value of 2.7% and most of these shales were deposited in an oxygen-depleted marine environment. Equivalent vitrinite reflectance (VRr) values range from 2.20 to 4.25%, indicating that all samples are in the gas generation window and lost a large amount of primary organic carbon. Porosity varies between 1.5 and 13.6% with an average value of 5.8%, while the excess sorption capacity ranges from 0.10 to 0.22 mmol/g rock. Minerology data show that quartz is the predominant mineral except for the Upper Permian Longtan Formation, in which clay minerals account for 29 to 84%. New data were combined with those from other publications and compared to information on other, well-studied shale-hosted gas systems.