Paleoenvironment Evolutionary Characteristics of Niutitang Shale in Western Hubei, Middle Yangtze, China

Abstract

The black shale developed in the first section of the Niutitang formation (ϵ ₁ n ¹) is one of the most important shale gas reservoirs in western Hubei, and its geological characteristics have been sufficiently studied by many predecessors. However, there are still three aspects that need further research: the origin of silicon, the discrimination of the euxinic sulfuretted and the anoxic ferruginous conditions, and the main controlling factors of organic matter enrichment. Based on geochemical data from well ZD1 located in the city of Yichang in western Hubei, first, the geochemical characteristics of ϵ ₁ n ¹ are analyzed, then the provenance, depositional site, and paleoenvironment evolution are discussed, and finally, the main controlling factor of organic matter enrichment is revealed. The results show that ϵ ₁ n ¹ can be divided into two units, organic-rich shales (ORS) and organic-lean shales (OLS), which have average total organic carbon contents of 4.21 and 0.84%, respectively. Additionally, the ORS is characterized by high contents of SiO ₂, U, V, Ni, Zn, and Cu and left-inclining types of rare earth element distribution curves. ϵ ₁ n ¹ is located in a passive continental margin with a material source mainly from mixed felsic and mafic rocks. Compared with the OLS, the content of biological quartz is much greater, and the terrigenous input is less in the ORS. The paleoclimate is cold and humid with low salinity in the ORS, whereas it is hot and dry with high salinity in the OLS. ϵ ₁ n ¹ is deposited in a semistagnant basin, and the ORS shows a relatively lower stagnant degree with euxinic to anoxic conditions and moderate to high paleoproductivity, while the OLS shows a high stagnant degree with suboxic to oxic conditions and lower paleoproductivity. The redox conditions are the main controlling factors affecting organic matter enrichment. The environmental evolution model with three stages shows that there is a good causal relationship between redox conditions, paleoproductivity, and sea level fluctuation. The black carbonaceous siliceous in the lower part of the ORS with a thickness of approximately 40 m is the most favorable layer, which will provide a theoretical basis for further shale gas exploration of ϵ ₁ n ¹ in the western Hubei.