Origin of quartz and its implications for different phase fluids in lacustrine shales from the Qingshankou Formation, southern Songliao Basin, China

Abstract

Quartz, an important mineral that occurs in lacustrine shales, can be of different types and origins. Yet, only few studies have investigated how quartz from different sources contribute to phase fluids, such as adsorbed and free fluids present within shale matrix pores. This study integrates various experimental techniques, including X-ray diffraction analysis, geochemical analysis, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and high-frequency two-dimensional nuclear magnetic resonance analysis to quantitatively characterize the different phase fluids associated with various types of quartz occurring in the Qingshankou Formation shale. The characterization results indicated four distinct quartz types: silt-sized quartz, microcrystalline quartz linked with clay minerals, organism-skeletal quartz, and quartz aggregates comprising multiple crystals. The Fe–Al–Mn ternary diagram and SiO2 versus Zr scatter plot indicate that terrigenous clastic material is the primary source of SiO2 in shale quartz. Smectite-to-illite transition plays a pivotal role in authigenic quartz formation. Based on quartz concentration data obtained using major element analysis, we estimated the ranges of terrigenous clastic quartz, diagenetic quartz formed by the smectite-to-illite transition, and biogenic quartz concentrations to be from 55.49% to 92.03% (average 84.70%), 6.67%–20.30% (average 13.12%), and 0%–24.21% (average 2.17%), respectively. The solid organic matter in shale is affected by both terrigenous detrital quartz and authigenic quartz transformed from the clay in the basin. Authigenic quartz, formed by the smectite-to-illite transition, considerably enhances the development of solid organic matter in the Qingshankou Formation. By contrast, an abundance of terrigenous detrital quartz can lead to a dilution of the solid organic matter. Thus, this study provides the first direct evidence that along with a high value of bio-organic matter yield factor, the source of quartz plays a significant role in controlling organic matter abundance.