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Silica diagenesis has been proved to significantly affect rock composition and fabric. However, it still needs to be clarified how silica diagenesis affects the mechanical properties of shales. To further investigate this impact, we collected shale samples from the Upper Ordovician Wufeng and the Lower Silurian Longmaxi formations, southeast Sichuan Basin. The total organic carbon (TOC) content, mineralogical composition, and major oxide concentrations were measured to characterize the shale composition. Thin-section observations and high-resolution imaging techniques were used to characterize the rock fabric and identify quartz types. The Young's modulus, Poisson's ratio, and the measured hardness were used to characterize the shale mechanical properties. Four major lithofacies were identified, namely, siliceous shale, siliceous-argillaceous mixed shale, silty mudstone, and argillaceous shale. Three types of authigenic quartz in the Wufeng-Longmaxi formations were observed, namely, quartz overgrowth, matrix-dispersed microquartz, and microquartz aggregates. Matrix-dispersed microquartz is typically mixed with clay matrix, while microquartz aggregates typically coexist with migrated organic matter. The combined analysis of Fe–Al–Mn ternary diagram, cross-plots of Si versus Al, Rb versus K 2 O, and TOC content versus excess silica demonstrate that the dominant silica source of quartz cementation is biogenic. The cross-plot of excess silica contents versus Young's modulus, Poisson's ratio, and hardness suggest that silica diagenesis significantly affects the mechanical properties of shale rocks. Microquartz aggregates greatly contribute to shale brittleness, while detrital quartz minimally affects shale brittleness if the detrital quartz was enclosed by ductile clay minerals. • Four quartz types were identified in the Wufeng-Longmaxi shales. • Silica cementation could significantly enhance shale rock mechanical properties. • The organic-rich siliceous shale typically manifests the best brittleness behavior. • The evolution patterns of silica diagenesis in different lithofacies were discussed. [ABSTRACT FROM AUTHOR]