Abstract:The black shale in the lower part of the Wufeng and Longmaxi formations in the Western Hubei Province has become a hot spot for the shale gas exploration in South China. In this paper, in order to determine the different silica sources in the Wufeng and Longmaxi formations and their geological significance for shale gas exploration, we investigate the origins of siliceous minerals in the black shale of the Wufeng and Longmaxi formations in the Yichang area by systematic collection and analytical test of samples, as well as the impact of different original silicon on organic matter accumulation and reservoir characteristics. The results show that the black shale of the Wufeng and Longmaxi formation in the study area has high content of siliceous minerals, the content of excess silicon ranges from 2.59%~88.31%, average of 27.91%, and most excess silicon develops in the deep-shelf environment corresponding to the Katian and Rhuddanian stage. Angular terrigenous quartz and siliceous fossil such as radiolarians and sponge spicules with serrated edges can be observed by microscopic examination. Element geochemistry analysis indicates the average content of TiO2 and Al2O3 in samples is only 0.63% and 12.41% respectively, and the relationship between SiO2 and TiO2, Al2O3 are negative. The ratio of Si/(Si+AL+Fe) and Al/( Al+Fe+Mn) is 0.81 and 0.77, and Eu/Eu* has clear negative anomaly. Excess silicon has not linear relationship with U/Th and the content of Ni. These data above demonstrate that terrigenous silicon and biosilicon are the main sources of siliceous minerals in the black shale of the Wufeng and Longmaxi formations, and hydrothermal silicon has little impaction on the origins of siliceous minerals. Deep-water anoxic and reducing environment should be favorable conditions for the development of biosilicon, but the biosilicon has not evident linear relationship with paleoproductivity and redox conditions, which can be demonstrated by the normal distribution curve relation between the high value of TOC and the contents of different original silicon. According to the normal distribution curves, it can be found that when the contents of Al2O3 and excess silicon are around 10% and 40% respectively, the TOC of shale will have highest value, which indicates the complicated relation between organic matter accumulation and siliceous mineral content. Furthermore, the development of siliceous mineral, especially the biosilicon with microcrystal occurrence can not only provide effective protection for reservoir space, but also promote the development of microfractures, which can greatly improve the reservoir capacity and flowing ability, as well as the reservoir fracturing reformation effect.