Distribution and Diagenesis of Hondo Evaporites In the Grosmont Heavy Oil Carbonate Reservoir, Alberta, Canada

The Hondo Formation is an evaporitic unit within the Grosmont shelf complex, which is the world's largest heavy oil deposit hosted in carbonates, with an estimated 318 billion barrels of bitumen. At present, the Grosmont reservoir is not under production. Reservoir evaluation includes constraining the spatial distribution of the evaporites, and their nature of deposition (marine-lagoonal, lacustrine, sill, sabkha).

The Grosmont reservoir is located in east-central Alberta near the updip limit of the Alberta Foreland Basin at depths of 350m to about 1100m. Bitumen is contained in the Grosmont Formation and the overlying Upper Ireton and Nisku Formations. The Grosmont consists of four upward-shallowing cycles, and the overlying strata form one or more cycles. The Hondo Formation is an evaporitic sub-unit that replaces these strata in parts of the complex. However, the areal distribution of these strata and their depositional nature are enigmatic.

Detailed petrographic analyses of cores from 19 wells and cuttings form 61 wells reveals the following. ‘Primary' evaporites are present in 3 cores, and evaporites occur in cuttings of 22 more wells. The longest continuous evaporite core section is 25m long and contains five lithofacies: laminated anhydrite in dolomudstone, chicken wire anhydrite with algal mats, enterolithic anhydrite, nodular anhydrite, and green argillaceous dolostone. Most of these evaporites appear to be marine-subaqueous (‘primary'). Additionally, diagenetic anhydrite is filling fractures and molds, both within the Hondo and in the adjacent strata.

The spatial distribution and petrography of these rocks suggests that the Hondo represents deposition in a series of relatively small (one to three townships) lagoons on the much larger Grosmont shelf (about 20x15 townships). In the eastern part of the study area, the Hondo appears to be dissolved and replaced by solution-collapse breccias. The significance of these findings for heavy-oil trapping and reservoir exploitation are under further investigation.