Geological
Controls on Gas Capacity of Shales: Example from the Lower Cretaceous
Buckinghorse Formation,
Chalmers, Gareth R.L.1, R.
Marc Bustin1 (1)
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.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California