A comparative discussion of the evidence for biogenic silica in Wufeng-Longmaxi siliceous shale reservoir in the Sichuan basin, China

The siliceous Wufeng-Longmaxi Formation is the main shale gas play in China. It contains a large amount of silica, which has been reported to be of biogenic origin. The previous evidences for a biogenic origin include the presence of siliceous skeletal debris resembling organisms, such as sponge spicules, various parameters, such as the ratios of Al/(Al + Fe + Mn) and Si/(Si + Fe + Al + Ca), and a number of different correlations, such as a positive relationship of silica content with total organic matter (TOC). However, these evidences, especially various parameters and correlations, become unpersuasive when comparing Wufeng-Longmaxi Formation siliceous shale with other shale in these parameters and correlations. A series of petrographic thin section and scanning electron microscope (SEM) observations, elemental analysis, TOC analysis and mineral composition analysis were conducted to verify the unpersuasiveness of the existing evidences, and then propose new evidences to demonstrate the abundance of biogenic silica in Wufeng-Longmaxi siliceous shale. It is shown that siliceous ellipsoids are a better mark of biogenic silica in Wufeng-Longmaxi siliceous shale than sponge spicule, radiolarian and foraminifera. Except for the Barium (Ba) concentration, most elemental composition data cannot be used to distinguish biogenic silica from silica with an airborne volcanic ash origin. Furthermore, the poor crystal morphology of the silica in Wufeng-Longmaxi siliceous shale was a clear difference from the airborne volcanic ash-derived silica in Lucaogou Formation and hydrothermal silica in Nutitang and Doushantuo formations. Hence, the crystal morphology can be considered as a distinguishing feature for biogenic silica.