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Grieser and Bray (2007) and Rickman et al. These mineralogy-based BIs (MBIs) require core measurements or a lithology log such as the Elemental Capture Spectroscopy (ECS) data. (2007), Wang and Gale (2009), and Jin (2014) proposed brittleness index (BI) definitions based on the mineral composition of the rock, dividing the fractional content of the most brittle minerals – quartz (Jarvie et al., 2007), quartz and dolomite (Wang and Gale, 2009), or quartz-feldspar-mica and carbonate minerals (Jin, 2014) – by the sum of the all constituent minerals. As such, rock brittleness is an important measure for the completion quality of shale reservoirs. High completion-quality shales must be brittle enough to readily fail upon hydraulic fracturing and maintain fractures for proppant placement for higher production rates. Completion quality is also affected by fracture density, orientation and anisotropy of in-situ stresses, and strength properties (Glaser et al., 2014 Herwanger et al., 2015). Completion quality is the geomechanical conditions that depend on elastic properties of a rock such as Young's modulus, Poisson's ratio, and bulk modulus as well as mineralogy. Reservoir quality for a shale play, i.e., the ability to produce hydrocarbons economically after hydraulic fracture stimulation, is governed by porosity, hydrocarbon saturation, total organic carbon (TOC), and thermal maturity (Glaser et al., 2014). The two factors that determine the prospectivity of a shale play are reservoir quality and completion quality (Glaser et al., 2014). The future drilling should be focused on these areas to increase the economic viability of the field. The areas of the highest SPI have not been fully tested. The higher SPI corresponds to higher production rates in the Muskwa and Evie shales. We propose a shale prospectivity index (SPI), computed by the arithmetic average of the normalized probability of the most brittle petrotype and the normalized TOC. Therefore, the prospectivity of the shales in the study area can be represented by high brittleness and increased TOC. Increased TOC is correlated with high brittleness, contrasting with the commonly-held understanding. The relationship between TOC and P-wave and S-wave velocity ratio ( V P/ V S) at the wells allowed the conversion of the V P/ V S volume from prestack inversion to the TOC volume, which in turn was used to construct the TOC maps for the three shales. The probability maps of the most brittle petrotype for the three shales were generated from Bayesian classification, based on the λρ-μρ template. We used the λρ-μρ brittleness template, constructed from the mineralogy-based brittleness index (MBI) and elastic logs from two wells, to convert the λρ and μρ volumes from prestack seismic inversion to the volume for the brittleness petrotypes (most brittle, intermediate, and least brittle). We analyzed seismically-derived brittleness and TOC to investigate the prospectivity of the Horn River Group shale (the Muskwa, Otter Park, Evie shales) of a shale-gas field in the western Horn River Basin, British Columbia, Canada. Total organic carbon (TOC) is an important reservoir quality and brittleness is the most critical condition for completion quality. Prospective shale plays require a combination of good reservoir and completion qualities.