The favourable behaviour of heavy oil reservoirs under solution-gas drive has intrigued the oil industry for a long time. Many mechanistic models have been proposed to explain this behaviour. This paper investigates one of the theories that attributes the low producing GOR and high recovery of heavy oil to low gas mobility. A simulation study was carried out on a commercial black oil simulator to match the "typical" production data of heavy oil fields in Lindbergh and Frog Lake. Sensitivity studies were carried out to investigate the effect of gas relative permeability on oil recovery. The effect of sand production, which is an integral part of heavy oil production in Western Canada, was also investigated. A parameter was used in the simulator to account for the increased permeability due to sand production. The results indicate that the field performance can be matched by using low gas mobility and incorporating improved permeability due to sand production. Neither of them alone was sufficient for matching the field performance. The use of a low gas mobility was successful to explain high pressure-gradients in the field, by acting as a pressure maintenance mechanism, and leading to a high recovery under solution-gas drive.