The Upper Devonian (Frasnian) Grosmont Formation is a large, pervasively dolomitized carbonate platform in northeastern Alberta, Canada, that subcrops beneath the giant Athabasca tar sands deposits and contains giant reserves of bitumen and several associated dry gas fields. Paleokarst is initially indicated by bit drops, lost circulation and loss of injected steam during drilling and bitumen production tests. Paleokarst features recognized in core include fractures, breccias, paleosols, meter-size dissolution cavities, several types of cavity-filling sediments, as well as extensive dissolution and calcitization of matrix dolomites. Unconsolidated intervals and caves infilled by bitumen and unconsolidated sediments can also be recognized on wireline logs as excursions of the caliper log, off-scale neutron-density porosity readings, and severe cycle skipping of the acoustic log.

Depending on the location, reservoir quality was either enhanced or degraded by karstification. Benefits include-porosity values up to 40% and permeabilities greater than 1,000 md (matrix dissolution). Permeability values of up to 30,000 md are observed in severely fractured intervals. Detrimental reservoir characteristics include erosion, porosity and permeability reduction by cementation, and seal ineffectiveness.

Reconstruction of the paleokarst distribution relative to a Lower Cretaceous datum shows that most of the Grosmont paleokarst is developed along well-defined, traceable karst levels. These karst levels are interpreted to represent paleo-water table surfaces parallel to Cretaceous paleo-elevation levels. This interpretation suggests karstification by carbonic acid dissolution and a likely timing of this karstification event between the Jurassic (basin tilt in response to Columbia orogeny) and the early Cretaceous (final burial of carbonate highlands). During this period cave and valley formation probably occurred at the time of relative sea-level low stands prior to deposition of Lower Cretaceous valley fill successions.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah