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.