Brittleness in unconventional reservoirs is mainly controlled by mineralogy, and it increases with quartz and dolomite content, whereas an increase in the clay content represents an increase in ductility. To generate regional brittleness maps, we have correlated the mineralogy-based brittleness index to elastic parameters measured from well logs. This correlation can then be used to predict the brittleness from surface seismic elastic parameter estimates of and . We applied the workflow to a 3D seismic survey acquired in an area where more than 400 wells were drilled and hydraulically fractured prior to seismic acquisition. Combining and into a single 3D volume allowed the combination of both attributes into a single 3D volume, which can be converted to brittleness using a template based on the well log and core data. Neither of these seismic estimates were direct measures of reservoir completion quality. We, therefore, used production logs and extracted surface seismic estimates at microseismic event locations to analyze the completion effectiveness along several horizontal wellbores in the reservoir. We defined four petrotypes in and space depending on their brittleness and gas saturation, and we found that most of the microseismic events fell into the zone described as brittle in the - crossplots. These observations supported the well-known idea that regardless of where the well was perforated, microseismic events appeared to preferentially grow toward the more brittle areas, suggesting the growth of hydraulic fractures into the brittle petrotype.