This paper examines the role played by geological history and by capillary pressure in the evolution of the unusual water saturations of the McMurray reservoir of the Athabasca Oil Sands Region (AOSR). The McMurray formation water and bitumen have been a dynamic system evolving through geological time in order to maintain equilibrium between buoyancy and capillary pressure. The main driving force was the change in hydrocarbon as well as formation water density so that they became similar in value.

These changes were likely caused by a combination of factors including hydrocarbon biodegradation, reduction in temperature and pressure through erosion of the overburden above the McMurray, and lowering of water salinity. The weakening of buoyant forces would enable capillary pressure to increase the average water saturation and to redistribute water and hydrocarbon so that water was preferentially moved to areas with a higher fines fraction.

The paper reviews pertinent geological history and discusses theoretically how water saturation should have changed as a result of changes in oil/bitumen and in formation water density. Field data from two sites with different levels of biodegradation as well as a conventional non-biodegraded analogue were analyzed in light of the theoretical expectations.

There will be water saturations above irreducible levels and hence some water mobility throughout much of the McMurray. There also is evidence that the irreducible water saturation in good quality McMurray reservoir sands at one site is ~ 10% or less. It is possible that the present McMurray irreducible water saturation levels are somewhat similar to what they were at the time of first oil charge approximately 80 million to 110 million years ago.

High-water saturation zones form when biogenic gas in depleted gas caps is replaced by water, as concluded by Fustic at al., (2013). As water is drawn into the depleted gas cap from the surrounding reservoir, water saturation can be lowered to nearly irreducible levels at the base of the reservoir below. Capillary pressure can also create transition zones below these high-water saturation zones. Therefore, thin depleted paleo-gas caps could create zones of elevated water saturation much thicker than the original gas caps. There can be significant heterogeneity in these high-water saturation zones across the McMurray Formation and at field scale.

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