Origins and significance of siliceous minerals in the shale reservoirs of the Wufeng-Longmaxi Formation in the Zheng'an area, northern Guizhou

Siliceous minerals, highly enriched in the organic shales of the early Paleozoic in South China, play a significant role in the development of shale reservoirs and volume fracturing. This study takes the shales of the Wufeng-Longmaxi Formation in the Zheng'an area of northern Guizhou as the research subject, utilizing a variety of methods and technologies, including rock and thin section observation, x-ray diffraction analysis, scanning electron microscopy (SEM) analysis as well as major and trace element analysis. Therethrough, we have identified the structure of different types of siliceous minerals, quantitatively measured their content, and deeply explored their influence on the development of porosity in the shales and fracturing reconstruction. Results show that siliceous minerals, enriched in shale reservoirs of the research area, are mostly composed of quartz of elliptical and irregular shape. The minerals vary in grain size and degree of crystallization, revealing their origins from two sources of terrigenous and biogenic respectively, in which terrigenous silica accounts for 30.1%, while biogenic silica accounts for 41.4%. Comparatively, siliceous minerals content in the shales of the Wufeng-Longmaxi Formation in the research area is higher than that in the Jiaoshiba of central Chongqing. The increase is predominantly due to a high amount of terrigenous, while the content of biological quartz varies scarcely. Biogenic silica forms a rigid structure during sedimentation and diagenesis, which facilitates the preservation and later fracturing reconstruction of the shale reservoirs. In contrast, the presence of terrigenous silica has the opposite effect. Thus, although the overall content of siliceous minerals in the shale of the research area is high, the conditions for forming optimal reservoir physical properties and subsequent fracturing reconstruction are less favorable than those in the Jiaoshiba area of central Chongqing.