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Quartz Cement in Mudrocks: Review of Occurrence and Habit in 18 Mudrock Units

Abstract

Microquartz cementation is a relatively rare phenomenon across global mudrocks. Because cementation has significant implications for the evolution of porosity, permeability, and mechanical rock properties, a predictive model for the occurrence of microquartz cement is of interest for reservoir quality assessment. A comparison of 18 mostly marine mudrock units, some in unconventional reservoir rocks and others not, ranging in age from Pleistocene to Silurian, reveals that microquartz cement is found primarily in mudrocks that contained significant biogenic opal (sarl, argillaceous sarl, and calcareous sarl) at the time of deposition. Where microquartz cement occurs it tends to be relatively abundant, typically composing around 30-40 percent of the rock volume. The observed volume of microquartz cement provides an important constraint on the timing of cement emplacement, indicating that precipitation occurred at burial depths generally less than 1.5 km, where primary depositional porosity was still significant. In mudrocks that have reached or exceeded the temperature sufficient for illitization of smectite (~80° C; a potential source for Si) and which also lack allochems of biogenic opal (tarl), significant authigenic microquartz is not observed. Careful inspection of such tarls using cathodoluminescence reveals sparsely distributed, micrometer-scale overgrowths on detrital quartz that are volumetrically trivial. This suggests that conditions favorable for quartz precipitation existed in these mudrocks but that limitations of nucleation and growth inhibited emplacement of significant quartz cement. Theoretically, prediction of microquartz cement can be best achieved by an understanding of basin-scale stratigraphic controls on the distribution of abundant allochems of biogenic opal, whereas thermal history is of lesser importance.