Sedimentary transformation of the Wufeng-Longmaxi Formation and its geologic significances of shale gas in Northeast Yunnan

The organic shale of the Wufeng-Longmaxi Formation in the upper Yangtze area is the main target sequences for marine shale gas exploration. It is also an important window to explore the event of the Late Ordovician mass extinction has been the focus of geological experts and scholars. The sedimentary response to the sea level changing and paleoclimate succession from the Wufeng to the Longmaxi period is still unclear, and the macroscopic description in sedimentology and its deposition kinetics analysis have not been reported. In this paper, based on the core description of the Yundaye well 1 in northeast Yunnan, the sedimentary sequences, sedimentary structures, mineral compositions, organic carbon isotope responses to the sedimentary transforming processes of the Wufeng-Longmaxi Formation were studied and compared with the adjacent wells. It was found that the sedimentary responses of the paleoclimate and paleo-environmental succession from the late Ordovician to early Silurian were clear. The conversion process from the Hirnantian cool event and large-scale sea level descent to the carbonaceous graptolite shale in the deep-water shelf facies of the Rhuddanian are continuous, during which three relatively transient stages, shallow water turbulence, shallow water laminar flow and deep water laminar flow, have been experienced. The transgression process is clear and the geological record is complete. At the same time, it is concluded that this transformation process has an important influence on the physical properties and gas-bearing properties of the organic-rich shale in the Wufeng-Longmaxi Formation, and it is an important basis for further understanding the differences between different layers of organic-rich shale in the Wufeng-Longmaxi formation.