Characterization of quartz in the Wufeng Formation in northwest Hunan Province, south China and its implications for reservoir quality

Abstract

Abstract 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.