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Using Cathodoluminescence Imaging to Identify Forms of Quartz in the Middle Devonian Marcellus Formation, Pennsylvania, USA

Abstract

Quartz is found in various forms in the Marcellus Formation including detrital clastic grains, replacements/fillings of algal cysts (tasmanites?), overgrowths around detrital quartz grains and quartz-replaced algal cysts, and dispersed microcrystalline quartz cement. In this study, light microscopy and Scanning Electron Microscope / Cathodoluminescence (SEM/CL) imaging were used to identify modes of occurrence and the spatial distribution of quartz. Different forms of quartz cannot be differentiated readily through bulk mineralogical analyses or elemental logs. Advances in SEM/CL imaging enable study of the rocks at the micrometer scale which aids identification of different phases of quartz in fine-grained rocks. CL color and textural properties (shape, organic matter inclusions, cement inclusions etc.) were used to differentiate various modes of quartz. Quartz in mudrocks is commonly thought to be an extrabasinal detrital component that was brought to the basin by currents or winds. For depositional system interpretations, it is important to identify whether quartz grains are detrital or generated in-situ by biogenic silica precipitation in algal cysts. Also, the mode of occurrence of quartz in mudrock can affect mechanical properties. For example, microcrystalline quartz cement (chert) dispersed in the matrix binds the sediment and impacts the ability of the rock to initiate and propagate hydraulic fractures. Authigenic quartz in the form of overgrowths is commonly observed in the Marcellus but mostly in minor amounts. Therefore, it is much less effective at binding the rock. Dispersed microcrystalline quartz cement is most abundant at the base of Marcellus close to the contact with Onondaga limestone. The source of the microcrystalline silica is thought to be biogenic and mostly likely from dissolution of radiolaria. The Tioga ash bed is present in some localities only a couple of feet below the occurrence of microcrystalline quartz cement, and silica from volcanic glass is an additional potential source of silica for quartz cements. The abundance of detrital grains increases in upper intervals of Marcellus. Discrimination of the various types of quartz and their spatial distribution in fine-grained sedimentary systems such as the Marcellus has two fold importance: 1) it contributes to assessment of reservoir quality and mechanical properties, and 2) it helps quantification of clastic detrital input into the sediment.