THE DEPOSITIONAL MECHANISM AND DISTRIBUTION PATTERN OF THE ORDOVICIAN-SILURIAN WUFENG AND LONGMAXI ORGANIC-RICH SHALES, SOUTH CHINA

The Ordovician-Silurian organic-rich shales in the Wufeng and Longmaxi formations are widespread in South China and are the most important shale gas producers in China. The shales display varying total organic carbon content ranging from 0.2% to 6% and are composed of black siliceous mudstone, shelly limestone, siliceous clay-rich mudstone, gray silty-shaly interlaminated mudstone, gray argillaceous mudstone, and calcareous or dolomitic mudstone. The difference among these lithofacies, in terms of mineral composition and sedimentary characteristics, are the fundamental driving factors for the heterogeneities that displayed in the fluid-shale interaction of diagenesis, petrophysical properties of the source rock, and geochemical properties of the hydrocarbon. Therefore, it is of great importance to study the depositional mechanisms of different lithofacies and their distribution pattern through time and space that could provide guidance for future profitable shale gas exploration. From a comprehensive, basinal-scale perspective, a detailed stratigraphic correlation of the Wufeng and Longmaxi formations were established across the Upper Yangtze platform. The Wufeng Formation and the Guanyinqiao Bed were interpreted to be a condensed section with four cycles of sea-level fluctuation. The Longmaxi Formation consists of three members. The Long-1 Member was interpreted to be a 3^rd order depositional sequence that can be subdivided into a transgressive systems tract, an early highstand systems tract and a late highstand systems tract. The deposition of black siliceous mudstone of high total organic carbon in the Wufeng Formation and the Long-1 transgressive systems tract was likely influenced by volcanic activity. The Guanyinqiao shelly limestone or calcareous mudstone was associated with the Ordovician-Silurian transition when glaciation and mass extinction took place. The gray silty-shaly interlaminated mudstone of moderate total organic carbon in the Long-1 early highstand systems tract was the product of bottom flow intrusion. The deposition of gray argillaceous mudstone and calcareous or dolomitic mudstone with low total organic carbon in the Long-1 late highstand systems tract resulted from enhanced terrigenous input from peripheral uplifts. The distribution pattern of different lithofacies varies greatly in the Upper Yangtze area, and this difference is a result of dissimilar sediment provenances and variable hydraulic restriction levels that were controlled by basin geometry.