Geological controls on matrix permeability of Devonian Gas Shales in the Horn River and Liard basins, northeastern British Columbia, Canada

Controls of matrix permeability are investigated for Devonian Gas Shales from the Horn River and Liard basins in northeastern British Columbia, Canada. Mineralogy is varied with high carbonate, high quartz and moderate quartz, carbonate and clay rich strata. Quartz content varies between 2 and 73%, carbonate varies between 1 and 93% and clay varies between 3 and 33%. The TOC content ranges between 0.3 and 6 wt.% and porosity varies between about 1 and 7%. For Horn River basin samples, quartz is mainly biogenic in origin derived from radiolarians. TOC content increases with the quartz content suggesting the TOC and quartz both are derived from siliceous phytoplankton. A positive relationship between porosity and quartz content is due to the positive relationship between quartz and TOC. Matrix permeability parallel to bedding varies between 7.5E-02 and 7.1E-07 mD at an effective stress of 15 MPa. Variation in permeability is due to a complex combination of factors that includes origin and distribution of minerals, pore-size distribution and fabric. Mercury intrusion capillary curves indicate that the higher matrix permeability values (>2E-03 mD) occurs in samples that contain interconnected pore apertures greater than 16 pm even when these samples may contain less macropores than low permeability samples. The fabric of high permeability samples can be either isotropic or anisotropic; however permeability of anisotropic samples is more sensitive to changes in effective stress than isotropic samples. More highly anisotropic samples contain moderate amounts of quartz, carbonate and in some, clay. High permeability samples that contain a more balanced ratio between micro-, meso- and macroporosity would not only have faster flow rates but also greater access to sorbed gas within the microporosity compared to samples that lack mesopores. Several Muskwa samples compared to Evie and Besa River samples contain higher quartz, moderate clay and high TOC content coupled with high permeability, less sensitivity to effective stress and balanced ratios between micro-, meso- and macroporosity would be a lower exploration risk due a greater propensity to fracture, the ability to produce and store hydrocarbons due to higher TOC contents and greater communication between macropores and micropores in the organic and clay fractions. (C) 2012 Elsevier B.V. All rights reserved.