Origin of quartz in the lower Cambrian Niutitang Formation in south Hubei Province, upper Yangtze platform

Brittle minerals, especially quartz, play an important role in the quality of shale reservoir. The precipitation of authigenic quartz caused a significant change in the rock stiffness and porosity. However, the formation mechanism of authigenic quartz in shale is still unclear. Lower Cambrian Niutitang shale from Guchengcun outcrop has been examined with respect to quartz cement. Four different types of quartz have been identified using SEM/ CL/EDS. Type 1 occurs as angular to sub-rounded silt-size quartz grains with uniform luminescence, indicative of detrital origin. Type 2 appears as relatively large quartz aggregates approximately 30-150 mu m in size, which is composed of small quartz grains (1-3 mu m) and abundant in siliceous shale. The petrological evidence and consideration of potential diagenetic process suggest that Type 2 quartz was formed by local re-crystallization of biogenic silica in syndiagenesis and preferentially precipitated in the axial cavities of sponge spicules. Type 3 occurs as quartz overgrowths that are essentially non-luminescent at the periphery of high luminescence detrital quartz grains. This type quartz is present in most investigated samples. Type 4 represents microcrystalline quartz embedded in the illitized clay matrix as discrete, short chains or small clusters/nests and they are most commonly observed in argillaceous shale. The petrological evidence and CL responses indicate that both Type 3 and Type 4 quartz are derived from precipitation of silica released during the smectite to illite dissolution-precipitation reaction. S-I transition can be driven by microbial metabolic processes in shallow buried diagenesis or by heat and pressure in late diagenesis. Some of Type 4 quartz may be originated from low-temperature microbial pathway in shallow buried diagenesis, while Type 3 quartz and the remaining Type 4 quartz were formed in late diagenesis. Siliceous shale contains less abundant detrital quartz than argillaceous shale, indicating that siliceous shale was deposited in deeper water and far from terrigenous input than argillaceous shale. Although identifying the exact cause for diagenetic silica deposition in shale is not a trivial matter, its recognition is important for the reconstruction of the depositional history of shale sequences and understanding the role of diagenesis in shale reservoirs development, moreover, it also has great potential for enhancing prediction of mechanical rock properties in shales.