Genesis of Quartz in Wufeng and Longmaxi Shales in the Sichuan Basin and Its Influence on Pore Structure

Abstract

AbstractIn order to clarify the influence of genetic types of quartz on the pore types and development degree in shale, thin section examination, X-ray diffraction analysis, scanning electron microscopy (SEM) determination with cathodoluminescence, geochemical analyses, and mercury intrusion methods were combined to investigate of Wufeng and Longmaxi shales in the Sichuan Basin. The types of quartz include detrital, biogenic, and formed though clay mineral transformation. Detrital quartz is characterized by larger angular-subangular particle size, generally between 10–20 μm. Biogenic quartz is mainly composed of nanometer quartz (chaledony) formed by in-situ transformation of biological radiolarian shells (opal-A, size between 10–300 μm) and microcrystalline quartz (0.5–4 μm) formed by dissolution and reprecipitation of siliceous biological shells, often filling interpartical pores in the form of microcrystalline aggregates. The quartz formed through clay mineral transformation is 1–3 μm in size with euhedral crystal within the flake clay minerals. Different types of quartz have different effects on shale pores structure. There are many brittle minerals in silty shale, which is conducive to the preservation of primary pores. It mainly develops interpartical pores and organic pores mainly with pore sizes ranging from 2.8 nm to 10 nm. A large amount of biogenic quartz is developed in the siliceous shale. Although the primary pores are filled with biogenic quartz, which can restrain compaction and preserve the internal pores as a rigid framework providing an effective space for hydrocarbon filling during the oil window. The pores with pore sizes ranging from 5 nm to 50 nm are mainly composed of organic matter pores. Argillaceous shale is mainly composed of clay minerals with almost no authigenic quartz developed. Therefore, clay mineral-related pores are mainly developed with pore size distribution mainly in the range of 2.8–5 nm.KeywordQuartz typesPore structureWufeng and Longmaxi shalesThe Sichuan Basin