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Quartz is one of the most important and common minerals in marine shales. This is especially true for the Wufeng-Longmaxi and Niutitang Formations in south China. However, the type, origin and formation mechanism of the quartz in these marine shales are as yet unclear. Thirteen samples of Ordovician shale from the Wufeng Formation in northwest Hunan Province were collected for this study. The quartz within the Wufeng shale was characterized through a series of experiments employing scanning electron microscopy with cathodoluminescence, nitrogen physisorption, X-ray diffraction, and geochemical analysis. The results were then used to assess the implications of the quartz component for reservoir quality. Three quartz types were identified based on scanning electron microscopy and cathodoluminescence imaging. Type I quartz has a relatively large grain size and exhibits bright luminescence, indicative of a terrigenous origin, whereas Type II and Type III quartz is authigenic and has dull or no luminescence. Type II quartz has small grain sizes and may be mainly biogenic in origin. This type of quartz may have formed during early diagenesis, with the silica being sourced from the dissolution of siliceous organisms. Type III quartz generally appears as dark rims on Type I quartz grains; the silica may have been derived from clay mineral alteration, with quartz-formation occurring during middle or late diagenesis. The authigenic quartz (Types II and III quartz) likely formed as a cement and would be expected to generate brittle behavior. The shale with a high content of authigenic quartz has a high Young's modulus and low Poisson's ratio. The biogenic authigenic quartz (Type II quartz) was pore-filling and formed during shallow burial. It would thus have been able to resist further compaction and provide a rigid framework within which the primary pores in the shale could be preserved, controlling the distribution of organic matter and the development of organic pores. The biogenic authigenic quartz has positive relationships with porosity and pore volume and specific surface area. Shale that was high in biogenic quartz had high porosity and adsorption capacity. The characterization of quartz in marine shales has significance for shale gas exploration and development and provides a new angle for evaluating the fracability and storage capacity of shale gas reservoirs. • Three different types of quartz were identified based on SEM/SEM-CL. • The origin and silica source and formation time of authigenic quartz were discussed. • Different types of quartz have different effects on shale gas reservoir quality. [ABSTRACT FROM AUTHOR]

Understanding the differences among shale pore systems is crucial to exploring the enrichment mechanism of shale gas. In this study, we found that radiolarian micro-fossils are widely distributed in the overmature marine organic-rich Wufeng-Lungmachi formations surrounding the Sichuan Basin, South China. This enables an investigation on the full-scale pore structure characteristics of the two formations by using a combination of qualitative methods (field emission scanning electron microscopy (FE-SEM)) and quantitative methods (low-pressure N 2 and CO 2 adsorption and high-pressure mercury intrusion porosimetry (MIP)). The results show that micropores (accounting for 75.81%) and mesopores (accounting for 63.56%) are mainly responsible for the total surface area and the total pore volume, respectively. Furthermore, organic matter significantly contributes to the shale surface area, and the intercrystalline pores of clay minerals make an important contribution to the pore volume of shale. Abundant biological pores can be discovered in radiolarian fossils, and these biological pores belong to mesopores and macropores. The investigation of gas adsorption shows that radiolarian-poor shale reflects the presence of slit-shaped pores, while radiolarian-rich shale indicates the presence of ink-bottle-shaped pores. In addition, despite similar TOC contents, clay minerals and brittle minerals, radiolarian-rich layers have larger pore volumes than radiolarian-poor layers. The results calculated using the threshold method with ImageJ software show that the average surface porosities of the organic matter, mineral matrix, and radiolarians are 15.69%, 1.84%, and 5.34%, respectively. Given that the quantitative calculation results in a greater contribution by the pores developed in radiolarians, the biological pores developed in radiolarians are beneficial for the enrichment of free gas. • Influences from radiolarian fossils on pore structure of gas shales were evaluated. • Pore structure differences between radiolarian rich and poor shales were analyzed. • A novel method of narrow neck-ink bottle-shaped pores in radiolarian was proposed. • Effects of pores in radiolarian fossils on gas storage in shales were analyzed. [ABSTRACT FROM AUTHOR]