This study focusses on the impact of wettability alteration of reservoir clays on the overall efficiency of Steam Assisted Gravity Drainage (SAGD). Samples from two SAGD experiments were investigatedSAGD1, consisting of kaolinite in the oil-sand packing and SAGD2, consisting of a mixture of kaolinite (90 wt%) and illite (10 wt%). The residual oil saturation from two different zones (inside steam chamber and steam chamber edge) from each SAGD experiment was determined from spent rock samples. Series of systematic optical microscopy analyses were carried out on clay-sand and clay-sand-asphaltene mixtures under steam and water exposure to represent the inside steam chamber zone and steam chamber edge, respectively. The higher residual oil saturation for SAGD2 was associated with the wettability alteration of illite in the reservoir at the steam chamber edge, leading to significant illite-asphaltene association. The pore-bridging property of illite was also observed, adversely affecting reservoir permeability. Kaolinite-asphaltene interactions in the presence of liquid water, on the other hand, were found to be temporary and not binding. Our findings suggest that wettability of clays plays an important role in determining the efficiency of SAGD process, controlled mainly by the polar asphaltene fractions in bitumen reservoirs with high asphaltene concentrations.