Based on temperatures measured in observation wells of the UTF Phase B project, the behaviour of the rising steam chamber when it reached impermeable layers was studied using a numerical thermal simulator. Results indicated that steam could rise by detouring the impermeable layers when they exist within the channel sands. The extension of these impermeable layers is generally less than 10 m in diameter. Most of these impermeable layers became permeable after one to one and a half years of heating with steam. When steam reached shaly sands in the upper part of the reservoir, steam rise was terminated. However, in some instances, it rose slowly after one to one and a half years of heating. Steam generally stopped to rise at few metres from the bottom of the point bar sands. However, a significant amount of bitumen could be produced from these sands. In the Hangingstone reservoir, Japan Canada Oil Sands Limited (JACOS) began initial steam circulation in April 1999 and the regular SAGD operation in July 1999. The early oil production volumes were close to our expectations. An extended understanding of the rising mechanism of the steam chamber is achieved by a comparison of the performances obtained in the Hangingstone and UTF reservoirs. Markedly different characteristics between each were detected, which can be caused by the variance in geomechanical behaviour of the tar sands.