Devonian-Mississippian strata in the northwestern region of the Western Canada sedimentary basin (WCSB) were investigated for shale gas potential. In the subsurface, thermally mature strata of the Besa River, Horn River, Muskwa, and Fort Simpson formations attain thicknesses of more than 1 km (0.6 mi), encompassing an area of approximately 125,000 km(2) (48,300 mi(2)) and represent an enormous potential gas resource. Total gas capacity estimates range between 60 and 600 bcf/section. Of particular exploration interest are shales and mudrocks of the Horn River Formation (including the laterally equivalent lower Besa River mudrocks), Muskwa Formation, and upper Besa River Formation, which yield total organic carbon (TO C) contents of up to 5.7 wt.%. Fort Simpson shales seldom have TOC contents above 1 wt.%. Horn River and Muskwa formations have excellent shale gas potential in a region between longitudes 122 degrees W and 123 degrees W and latitudes 59 degrees N and 60 degrees N (National Topographic System [NTS] 94008 to 94015). In this area, which covers an areal extent of 6250 km(2) (2404 mi(2)), average TO C contents are higher (>3 wt.% as determined by wire-line-log calibrations), and have a stratal thickness of more than 200 m (65 6 ft). Gas capacities are estimated to be between 100 and 240 bcf/section and possibly greater than 400 tcf gas in place. A substantial percentage of the gas capacity is free gas caused by high reservoir temperatures and pressures. Muskwa shales have adsorbed gas capacities ranging between 0.3 and 0.5 cm(3)/g (9.6-16 scf/t) at reservoir temperatures of 60-80 degrees C (140-176 degrees F), whereas Besa River mudrocks and shales have low adsorbed gas capacities of less than 0.01 cm(3)/g (0.32 scf/t; Liard Basin region) because reservoir temperatures exceed 130 degrees C (266 degrees F). Potential free gas capacities range from 1.2 to 9.5 cm(3)/g (38.4 to 304 scf/t) when total pore volumes (0.4-6.9%) are saturated with gas. The mineralogy has a major influence on total gas capacity. Carbonate-rich samples, indicative of adjacent carbonate platform and embayment successions, commonly have lower organic carbon content and porosity and corresponding lower gas capacity (<1% TOC and <1% porosity). Seaward of the carbonate Slave Point edge, Muskwa and lower Besa River mudrocks can be both silica and TOC rich (up to 92% quartz and 5 wt.% TOC) and most favorable for shale gas reservoir exploration because of possible fracture enhancement of the brittle organic- and siliceous-rich facies. However, an inverse relation between silica and porosity in some regions implies that zones with the best propensity for fracture completion may not provide optimal gas capacity, and a balance between favorable reservoir characteristics needs to be sought.