Shale Gas Reservoir Systems: Insights from North of the Border

Ross, Daniel John Kerridge¹, R. Marc Bustin¹ (1) University of British Columbia, Vancouver, BC

Organic-rich Devonian and Jurassic strata are considered to be excellent candidates for gas shales in Western Canada. These strata have TOC contents ranging between 1-38 wt%, thermal maturities into the dry-gas window, and thicknesses in places over 1700 m. The ratio of total organic carbon (TOC) to gas sorption varies markedly from unit to unit depending on pore-structure, thermal maturation, moisture and inorganic composition. For low maturity shales, surface areas and micropore volumes are low, ranging from 0.23–0.64 cc/100g CO2 monolayer volume and 1–9 m2/g BET surface area, despite TOC values up to 38 wt%. The increased sorption of methane with increasing TOC, independent of microporosity and surface area, implies gas is primarily stored through solution in the bituminite fraction. Thermally mature shales have greater micropore volumes (0.3–1.2 cc/100g) and surface areas (5–31 m2/g) associated with the organics, resulting in more gas adsorbed per wt% TOC. Structural transformation of the bituminite during maturation-induced diagenesis plays a key role in the creation of adsorption sites.

The inorganic component is also important to gas capacities. Carbonate-rich samples often have lower organic carbon contents and porosity hence potentially lower sorbed and free-gas capacities. Highly mature Devonian shales are both silica and TOC-rich (up to 85% quartz and 5 wt% TOC) and as such, deemed excellent potential shale gas reservoirs because they are both brittle (fractures or fracable) and have high sorbed gas capacities. However the negative correlation between silica and porosity in certain regions implies potential frac-zones may not have economic gas capacities.