Quartz types, silica sources and their implications for porosity evolution and rock mechanics in the Paleozoic Longmaxi Formation shale, Sichuan Basin

Abstract

Quartz is a common mineral in mudrocks, displays variable forms and can have multiple origins. Consequently, the form and source of quartz in mudrocks may have a distinct impact on porosity evolution and rock mechanics such as brittleness. In this study, 51 shale samples from both the Lower and Upper parts of the Longmaxi Formation in the Sichuan Basin of China were collected for geochemical, mineralogical and elemental analyses, microscopic observation, porosity characterization, as well as rock mechanics testing, in order to better understand the quartz types, the sources of the silica, and their impacts on porosity evolution and rock mechanical behavior. Optical microscopy and field emission-scanning electron microscopy (FE-SEM) with energy dispersive spectroscopy (EDS) were applied to describe the crystal morphology and investigate the types and silica sources of quartz and their associated pores. Images from both methods reveal several forms of quartz in the Longmaxi Shale, including terrigenous detrital quartz, organism skeletal quartz, quartz overgrowths, authigenic microquartz and quartz veins. Furthermore, the authigenic microquartz has three subcategories, namely clay matrix-dispersed, euhedral and amorphous. The distinguishing criteria include crystalline morphology, luminescence, grain size and forming stage (including the stage of deposition and different periods of the diagenesis progress). Geochemical proxies, such as an Al–Fe–Mn ternary diagram, negative correlations of Zr vs. SiO2, and positive correlations of TOC vs. excess Si, indicate a substantial contribution of biogenic silica to the total silica in the Lower Longmaxi Formation. All possible pores associated with quartz are systematically described and classified into 3 types and 8 subtypes, notably dissolution pores, organism skeletal cavity pores, crystal defect pores, pores between quartz grains, grains contact micro-fractures, shrinkage micro-fractures and quartz filling micro-fractures. This study shows that the origin of the quartz may have implications for porosity evolution and rock mechanics. Positive correlations of surface area vs. excess Si indicate that biogenically derived forms of microquartz (including euhedral microquartz and amorphous microquartz cements) play a constructive role for micro-scale pore development. The correlation between high brittleness index and excess Si in the Lower Longmaxi Formation suggests that biogenic silica is the major cause of the brittleness.