Quartz types, formation mechanism, and its effect on shale oil and gas enrichment: A review

Abstract

Quartz is one of the most common minerals in the Earth's crust, and its deposition and cycling are ubiquitous and crucial in energy and environmental sciences. Due to the existence of multiple types of quartz and diverse mechanisms that result in their formation, this variation is expected to significantly impact shale deposition, diagenesis, and reservoir properties. Moreover, it plays a crucial role in the enrichment, development, and production of shale oil and gas plays. Considering their importance, this study systematically summarizes observation and various research methods, such as optical and scanning electron microscopy (SEM), cathodoluminescence (CL), energy dispersive spectrometry (EDS), X-ray fluorescence (XRF), quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN), fluid inclusion, which supports our understanding of the mineral diagenesis and generation of both detrital and authigenic origins. Parent rock type, transport distance, and depositional environment are known factors that control the grain size, sorting, roundness and types of detrital quartz. The authigenic quartz contains biogenic, hydrothermal origins and clay mineral transformation while fluid source, diagenetic mechanisms, and growth space control the formation time and crystal size of them. In addition, quartz controls the total organic carbon content, reservoir quality, fracturing ability, organic matter preservation and reservoir enrichment, etc. Notably, the microquartz cement derived from biosiliceous allochems (namely biogenic quartz) has a noticeable positive correlation with total organic carbon content and is formed during the early diagenetic stages which together with the detrital quartz form a rigid framework favorable to primary pores, ultimately forming high-quality marine shale reservoirs. Furthermore, the diagenesis of biogenic quartz also enhances the mechanical properties and fracturing potential of shale reservoirs. The biogenic quartz content and the thickness of shale intervals determine the potential and development of marine shale oil/gas reservoirs. However, in transitional and lacustrine shales, quartz is predominantly detrital in origin and negatively correlated with organic carbon content. In these two types of shales, detrital quartz and quartz formed during the transformation of clay minerals are known to play a positive role in the formation of shale reservoirs and hydrocarbon enrichment. Considering all of these factors, this study investigates different types and contents of quartz in typical shale oil and gas reservoirs worldwide and explains how they have influenced shale oil and gas enrichment and reservoir productivity.