The geomechanical properties of a shale reservoir are essential both to the development of natural fractures and to the formation’s response to hydraulic fracture stimulation. We evaluate the rock mechanical properties of the Middle and Upper Devonian Horn River Group shale, including the Evie and Otter Park Members and the Muskwa Formation, applying core hardness measurements and log-derived Young’s modulus, Poisson’s ratio, and brittleness and relating these to shale composition and texture. Clay content is the most significant factor controlling the brittleness of shale rocks. The effect of quartz content on rock mechanical properties depends on the type of quartz present in the rock: authigenic quartz is positively correlated with brittleness, but detrital quartz has little or no effect. Factor analysis indicates that carbonate increases brittleness, although no obvious correlation between total organic carbon content and brittleness was observed. Depositional facies are related to geomechanical properties, because each facies has a distinctive composition. Massive mudstone and pyritic mudstone are relatively brittle because of abundant authigenic quartz cement. Laminated mudstone and bioturbated mudstone are relatively ductile, because most detrital carbonate and quartz grains are set in a matrix of clay minerals. Brittleness in the Horn River Group shale shows both geographic and stratigraphic variability. Increasing brittleness in the northwestern part of the basin largely results from greater distance from the clastic sediment source. The Otter Park Member represents a period of major relative sea-level fall and is relatively ductile because of its high clay content, whereas the underlying Evie Member and the overlying Muskwa Formation are relatively brittle because of high carbonate and biogenic silica content, respectively.