Geological Controls on Gas Capacity of Shales: Example from the Lower Cretaceous Buckinghorse Formation, Northeastern British Columbia, Canada

Chalmers, Gareth R.L.¹, R. Marc Bustin¹ (1) University of British Columbia, Vancouver, BC

The geological controls on the methane sorption capacity of the Lower Cretaceous Buckinghorse Formation and stratigraphically equivalent strata have been investigated. Over 200 samples were analyzed for methane sorption capacities which range between 0.04 to 1.89 cm3/g at 6 MPa (3.2 to 60.4 scf/ton at 870 PSIA). The total organic carbon (TOC) content is between 0.5 and 17 wt% and equilibrium moisture contents between 1.5 and 11 wt%. The strata range in maturity from immature to over mature with respect to the oil window (Tmax between 416 and 476°C). Both illite (4-56 vol.% ) and quartz (12-83 vol.%) dominate the mineralogy.

Although TOC content is the most significant control on methane sorption capacity (R2 = 0.64), there are still large variations in methane capacity at similar TOC values. Variations are due to secondary controls which includes moisture content, maturity, kerogen type and mineralogy. High methane capacities are associated with lower maturities because these samples have higher TOC contents compared to higher maturity samples. A positive relationship exists between illite and sorption capacity (R2 = 0.33) because illite contains both micro- and mesoporosity. The relationship is weak because moisture restricts access to pores and has a negative effect on the sorption capacity. The illite content increases with maturity by the process of illitization (R2 = 0.72) resulting in increased microporosity. This process compensates for the loss of micropores with a reduction in TOC content by thermal maturation.