This study investigates the effect of clay type on the performance variations of steam-assisted gravity drainage (SAGD). Two SAGD experiments at identical experimental conditions were conducted. The reservoir rock for the first experiment (SAGD1) is prepared with a sand (85 wt%) and kaolinite (15 wt%) mixture, and the second experiment (SAGD2) is prepared with a sand (85 wt%), kaolinite (13.5 wt%), and illite (1.5 wt%) mixture. The effectiveness of the steam-chamber growth did not change with the clay type; however, 15-wt% reduction in oil recovery was observed for SAGD2. The possible reasons were investigated with the analyses on the produced-water, the produced-oil, and the spent-rock samples. Contact-angle, particle-size, zeta-potential, and interfacial-tension measurements were carried out on the samples. The mineralogical changes on spent-rock samples were determined by X-ray diffraction (XRD) and scanning-electron-microscope (SEM) analyses. The contact-angle measurements on the spent-rock samples displayed the higher oil-wetness for SAGD2 than SAGD1. However, the water-wetness of illite is known to be higher than that of kaolinite. This unexpected result is explained by the interaction of illite and the asphaltenes from SAGD2. The particle-size measurements, along with the SEM images, on post-mortem samples reveal that illite containing clay exhibits cementation behaviour and, hence, reduces the permeability of the rock. According to the experimental results, we developed hypotheses to understand the bitumen/illite and bitumen/kaolinite interactions for SAGD. Because of the high water-wetness of illite, illite particles first interact with water. This interaction results in cementation and forms illite lumps with sand. Then, illite lumps continue to interact more vigorously with the polar molecules (water, asphaltenes, and resins). Clay migration and production occur in both clay types; however, while kaolinite is produced in the water phase, illite-containing clay as a result of its interaction with asphaltenes is produced in the oil phase.