Grain-scale petrographic evidence for distinguishing detrital and authigenic quartz in shale: How much of a role do they play for reservoir property and mechanical characteristic?

Abstract

Quartz grains and their microstructural characteristics are essential both to the formation of interparticle porosity and to the influencing reservoir brittleness of siliceous shale. Here we analyze quartz forms, origins, and textures and evaluate their impacts on reservoir property and mechanical behavior's response to hydraulic fracturing. Scanning/transmission electron microscopy imaging reveals the presence of both detrital and authigenic quartz with the content from 13.6 to 88.3%. Detrital quartz occurs as randomly distributed asperities that are characterized by an irregular ellipsoidal shape. At least four distinct authigenic forms are distinguished: (1) euhedral microquartz with well crystallization and similar size, which are derived from smectite-to-illite transformation and fracture fill, (2) nanospheres, which are product of biogenic silica and intraparticle cement, (3) grain-rimming syntaxial overgrowths, which come from pressure dissolution and cementation, and (4) replacement of skeletal grain, which may be of in situ authigenic origins. Reservoir property is controlled by quartz microstructures. The authigenic quartz grains have significant implications for mechanical fracture property. Two mechanisms of grain cushioning and grain rolling effect have been invoked to explain the common perception that authigenic quartz is positively correlated with reservoir brittleness while detrital quartz has little or no effect.