Sequence Stratigraphy and Evolution Model of the Wufeng-Longmaxi Shale in the Upper Yangtze Area

Guo Xusheng

doi:10.3799/dqkx.2017.086

Abstract：Shale sequence division and composition remain highly controversial, and the existing research on the characteristics and genesis of sequence framework is inadequate.The Wufeng-Longmaxi Formation Shale is one of the most important horizons of shale gas exploration in the Yangtze region.On the basis of research on well-core, outcrop, conventional log, a spectral gamma-ray log, geochemical parameters and other information are used for quantitative sequence stratigraphy analysis of black shale in the upper Ordovician Wufeng Formation to the lower Silurian Longmaxi Formation.Three third-order sequences are divided based on the recognition of four third-order boundaries.The Wufeng Formation is equivalent to a third-order sequence and is subdivided into a transgressive system tract (TST) (black shale of lower Wufeng Formation) and a highstand system tract (HST) (Guanyinqiao member of upper Wufeng Formation).the first member of Longmaxi Formation is equivalent to a third-order sequence and is subdivided into a TST, an early highstand system tract (EHST) and a late highstand system tract (LHST);the second member and third member of Longmaxi Formation are combined to be one third-order sequence and is subdivided into a lowstand system tract (LST), a TST and a HST.Sequence development and sedimentary environment characteristics are analyzed within each system tract unit.TOC is correlated to V/Cr and EF-Ni differently within different system tract units, suggesting paleoproductivity and water redox condition in different system tract units have different effect on the enrichment of organic matter in the studied shale.Based on the changes of depositional processes and water column conditions, we develop a depositional model to interpret the evolution of depositional environments of the Wufeng-Longmaxi Formation.