Abstract

The Upper Devonian (Frasnian) Grosmont Formation, a large, pervasively dolomitized carbonate platform in northeastern Alberta, Canada, subcrops beneath the Lower Cretaceous Mannville Group and hosts a giant heavy-oil reservoir and several associated dry gas fields. Paleokarst is indicated by problems during drilling and heavy-oil production tests, including bit drops, lost circulation and loss of injected steam. Paleokarst features recognized in core include fractures, breccias, paleosols, dissolution cavities, several types of cavity-filling sediments, as well as extensive dissolution and calcitization of matrix dolomites. Unconsolidated intervals and “open” caves can also be identified from well logs. Reservoir quality was greatly enhanced by karstification and porosities of up to 40% and permeabilities greater than 1,000 md (up to 30,000 md) are common. However, the Grosmont shale breaks, shaly aquitards used to contain steam during reservoir stimulation were negatively affected by breaching near karst zones.

Reconstructions of the paleokarst distribution relative to the Cretaceous Lower Mannville datum show that much 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 that karstification was likely driven by carbonic acid dissolution.