Abstract:

In order to study the geochemical characteristics and enrichment patterns of the organic-rich black fine-grained rock series in the Middle Ordovician Yanxi Formation in the central and southern Hunan， samples from Ningyuan Mianhuaping (MHP)， Shuangpai Hejiadong (HJD)， Quanzhou Shazitang (SZT) and other sections are collected and analyzed for TOC, major elements,  trace elements and rare earth elements. The results show that the organic geochemical index of the Middle Ordovician Yanxi Formation in the central and southern Hunan is good, and the SiO2 content is high, which belongs to the high-quality source rock;The organic matter abundance of the black shale is relatively high， the average w（TOC）is 2.12%， which is generally higher than 2%， and the brittle mineral content is more than 50%， so it has good gas generating capacity. The large number of siliceous layers developed in the Yanxi Formation are mainly of sedimentary origin. The redox parameter U/Th,V/(V+Ni) and other indicators indicate that the overall water body is an oxygen-limited reductive environment， and the productivity of the Yanxi Formation was high； The terrigenous material input， redox environment， relative changes of sea level, and paleoproductivity affect the enrichment of organic matter together. The enrichment of organic matter in the shale of Yanxi Formation is mainly controlled by the anoxic environment, which belongs to the enrichment model of “deep-water retention basins”.

Key words: central and southern Hunan, Yanxi Formation, black shale, geochemical, organic matter