SHALE CHARACTERISTICS OF THE UPPER MOST ORDOVICIAN AND THE LOWER SILURIAN IN UPPER YANGTZI PLATFORM, SOUTH CHINA

The realistic shale reservoirs in Upper Yangtzi platform occur in Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation. The upper most Ordovician Kuanyinqiao and cotemporary Formation during Hirnantian characterized by marked changes in paleontology species, paleoclimate, sea level and deposition environment in the boundary between the Ordovician and Silurian, records the key transition from transgression to regression due to the Late Ordovician short glaciation that has been discussed by previous researchers. In this study, a range of textures of Kuanyinqiao Fm. and adjacent rocks represented by geologic sections from both outcrop and the subsurface are described with multiple disciplines. Two contrasting lithofacies, transported external assemblages and endogenous sources, are identified from Kuanyinqiao of South China. The former is mainly driven by turbidity current. Many variable features of the black shales are also exhibited, which involve in the original mineral composition, grain size, cementation degree, chemical diagenetic behaviour. They can significantly affect shaly matrix pore scale and rock mechanical properties, hydrocarbon migration and accumulation as well, especially in bearing-radiolarian brittle interval of hot shales. It is exactly that spatial and temporal distribution of sedimentation and diagenetic evolution of the rocks interpret mass transfer in sedimentary basins and have an impact upon shale reservoir properties and gas production potential.