The Upper Devonian Grosmont reservoir in Alberta, Canada, is the world’s largest heavy oil/bitumen reservoir hosted in carbonates, with an estimated 400 to 500 billion barrels of “Initial Oil In Place” at average depths of about 250 to 400 m. Our study is part of a more comprehensive effort to evaluate the Grosmont reservoir through geological, geophysical, and petrophysical methods in order to determine the most advantageous method(s) of exploitation.
The reservoir is a carbonate–evaporite system. The carbonates of the Grosmont were deposited during the Late Devonian on an extensive platform and/or a ramp in five or six cycles. Evaporites are interbedded with the carbonates at several stratigraphic levels. These evaporites, informally referred to as the “Hondo Formation,” have received scant attention or were ignored in most earlier studies. However, they may play a crucial role regarding the distribution of the most porous and/or permeable reservoir intervals via dissolution, as permeability barriers to compartmentalize the reservoir during or after hydrocarbon migration, and as a source of dissolved sulfate for microbial hydrocarbon degradation.
Most Hondo primary evaporites are anhydrite that formed subaqueously as well as displacively and/or replacively very close to the depositional surface. Secondary/diagenetic sulfates were formed from primary sulfates much later and under considerable burial. The locations of primary evaporite deposition were controlled by a shift from carbonate platform or ramp deposition over time. At present the primary sulfates occur in a number of relatively small areas of about 10 by 20 km to 20 by 30 km, with thicknesses of a few meters each. If these areas represent the depositional distribution, the primary evaporites were deposited in a series of large, shallow subaqueous ponds (salinas). Alternatively, the primary evaporites were deposited in a more extensive lagoon, and their present distribution represents the remnants after postdepositional, mainly karstic dissolution. The evaporites would have acted as intraformational flow barriers up until the time of dissolution, which may be a factor in the development of compositional differences of the bitumens contained at various stratigraphic levels. In the eastern part of the Grosmont reservoir the evaporites appear to be dissolved and replaced by solution-collapse breccias and bitumen-supported intervals of dolomite powder. In the western part of the reservoir the sulfates may form effective reservoir seals on the scale of the sizes of former brine ponds. However, it is likely that hydrocarbons bypassed them wherever the carbonates had sufficient permeability and/or where the marls were breached by faults and/or karstification.
