Brittleness in the Devonian Horn River shale, British Columbia, Canada

The Middle and Upper Devonian Horn River Group shale in northeast British Columbia is recognized as a major source of natural gas in Canada. This shale unit is approximately 200-500 m thick and consists of three stratigraphic units in ascending order: Evie Member, Otter Park Member and Muskwa Formation. In this study, Uniaxial Compressive Strength (UCS) tests were conducted on core plugs across stratigraphic units of the Horn River Group. Shear and compressional wave velocities are also measured for the core plugs to establish a relationship between static and dynamic elastic parameters. Brittleness is measured using three independent methods: (1) a hand-held hardness tester is used to measure mechanical hardness along a core; (2) the Elemental Capture Spectroscopy (ECS) log is used to calculate a composition-based brittleness in the same well; (3) Density and sonic logs along with laboratory experiments are used to calculate a brittleness coefficient based on elastic (static and dynamic) parameters. In order to compare brittleness and rock composition, a detailed lithofacies analysis was also conducted. Three major lithofacies were observed in the Horn River Group, including massive mudstones, pyritic mudstones and laminated mudstones. The Muskwa Formation and Evie Member are dominantly represented by massive and pyritic mudstones while the vast majority of the Otter Park Member is represented by laminated mudstones. The three independent measurements of brittleness show similar trends of higher brittleness in Muskwa Formation and Evie Member compared to Otter Park. This indicates that all three methods are suitable in identifying brittle zones in shale formations. The results show a clear relationship between lithofacies and brittleness. Massive mudstones and pyritic mudstones lithofacies show relatively higher brittleness compared to laminated mudstones lithofacies. Additionally, brittleness is influenced by the mineralogy of the rock. Clay content is observed to have the most dominant (inverse) relationship with brittleness coefficients, followed by quartz content. The analysis is further confirmed by calculating the fracability index for the Horn River Group.