Sheng Yang, University of Calgary; Nicholas B. Harris and Tian Dong, University of Alberta; and Wei Wu and Zhangxin Chen, University of Calgary Summary This paper documents the formation of natural fractures in the Horn River Group, a major Canadian shale gas play, and addresses relationships between natural-fracture development and rock-mechanical properties derived from cores and well logs. Most natural fractures in the Horn River Shale are narrow vertical fractures, sealed with carbonate minerals. In this study, the formation of observed fractures is primarily determined by a lithology type, mineral composition, and rock-mechanical properties at the timing of fracturing. Brittleness is an important geomechanical property controlling the formation of fractures, because brittle shale is more easily fractured than ductile shale, and fractures in brittle shale tend to persist when the fracturing pressure is released. In this study, a hardness value measured by a commercial hardness tester is found to be a good proxy for the brittleness of shale layers. On the basis of a statistical analysis, the threshold values of both hardness and brittleness are estimated to predict the distribution of natural fractures, assuming that the mechanical properties of the host rock were relatively stable from at least the time at which fractures formed. Hardness values are shown to be more reliable than brittleness. Introduction Researchers have conducted many studies on different aspects of natural fractures and fractured reservoirs (Lorenz et al. 1991; Laubach et al. 2004) [e.g., the characterization and effects of fractures in the carbonate and siliciclastic reservoirs of the Middle East (Ameen et al. 2009, 2010, 2014)]. In recent years, natural fractures in shale reservoirs have become a key focus of research (Curtis 2002; Kresse et al. 2011; Gale et al. 2014) [e.g., the comprehensive study of natural fractures in the Qusaiba shale conducted to evaluate shale reservoir proceptivity by Ameen (2016)].