The migration of an oil into a trap is governed by its buoyancy, the capillary pressure, and the hydrodynamic forces. For heavy oils the buoyancy is low; therefore, they can only saturate high-permeability zones, which are also preferentially swept by steam in a steam-drive recovery operation.

However, the nature of the heavy oils as well as their geological history must be taken into account.

Several case histories are reviewed.

If biodegradation and water washing occur after the primary migration of a mature light oil into a trap possessing poor reservoir characteristics, recovery will be difficult. The Likouala oil field and its counterpart, the Emeraude oil field, both in the Congo Republic, exemplify this case.

If water washing increases and partially sweeps the reservoir, recovery will be impossible; this is the case in some parts of the Warner reservoir (Tri-State area, United States).

If a mature conventional oil migrates through an open hydrogeological system, it undergoes biodegradation before entering into the trap. Its low buoyancy allows saturation only of the high-permeability, high-porosity zones of the reservoir. The less permeable areas remain water saturated. Recovery by steam drive should be more efficient. Some areas of the North Poso Creek oil field exemplify this case.

The most attractive heavy oils from the point of view of steam drive recovery are immature to marginally mature oils that migrate, unaffected by biodegradation, into the trap. Heavy, mature oils generated through the combined effect of biodegradation and water washing prior to the accumulation in the trap also deserve attention.