Influence of quartz types on rock fabrics and bulk physical properties in organic-rich mudstone: A review

Abstract

Quartz is one of the most volumetric abundant minerals and can occur in a wide variety of forms, such as detrital grains and authigenic cements, in organic-rich mudstones. Despite the wide variation of quartz types, uncertainties remain regarding the detailed mechanism and manner through which the quartz types determine rock fabrics and bulk physical properties (i.e., porosity, permeability, and rock strength) in organic-rich mudstones.This study reviewed quartz types that are commonly observed in organic-rich mudstones and provided a complete guideline to discriminate quartz types at the centimeter to micron scale based on conventional optical microscopy, scanning electron microscopy (SEM)-based energy-dispersive spectroscopy (EDS) elemental mapping (to determine mineralogy), and SEM-based cathodoluminescence (CL) imaging techniques (to determine quartz types). The authors suggest that the quartz types and rock fabric cannot be fully assessed either by bulk geochemical or mineralogical analysis (e.g., X-ray diffraction, X-ray fluorescence, etc.), whereas a combined SEM-based EDS and SEM-based CL observation is an indispensable and reliable way to determine the quartz forms and rock fabric in mudstones.Furthermore, unlike most quartz types that commonly exhibit strongly localized spatial distribution patterns in organic-rich mudstones, the matrix-dispersed microquartz cement can widely distribute throughout the rock matrix and occur in large volume fractions. These matrix-dispersed microquartz cements are usually sourced by biogenic silica and can potentially determine rock fabrics during the early diagenetic stage, which will further influence the physical properties of rocks. The matrix-dispersed microquartz cement can form a rigid touching framework to suppress the collapse of larger micron-scale intergranular pores, shelter intervening clay-size particles from alignment during mechanical compactions and preserve micron-scale to nanoscale intergranular pores or intragranular pores within clay aggregates, and preserve nanoscale organic matter-hosted pores. In general, three types of rock fabric can be identified in organic-rich mudstones based on the occurrence of matrix-dispersed microquartz cement: cement-poor mudstone (microquartz cement generally <10 vol%), cement-bearing mudstone (microquartz cement generally ranges from 10 vol% to 20 vol%), and cement-rich mudstone (microquartz cement generally higher than 20 vol%). Specifically, increasing matrix-dispersed microquartz content will linearly increase bulk porosity and permeability and enhance rock strength. This general principle has a wide application for silica-bearing organic-rich mudstone successions worldwide.