There is a major concern that the existence of thief zones, such as top water and/or a gas cap overlying the oil sand deposit, has a detrimental effect on the oil recovery in the application of 1 the steam-assisted gravity drainage (SAGD) process. The objective of this numerical study is to investigate SAGD performance in the Athabasca oil sands in the presence of a top water zone. The reservoir model, STARS, developed by the Computer! Modelling Group (CMG) Ltd., has been previously validated based on a 3D SAGD laboratory experiment with top water that:, was conducted at the Alberta Research Council (ARC). It is 1 believed that the numerical simulation captured the major mechanism of oil movement from the pay zone into the top water 1 zone, as was observed in the experiment. In the field-scale simulation, SAGD performance in the; presence of confined and non-confined top water zones was; investigated. The operating strategies under the conditions of non-depleted top water/non-depleted pay zones and depleted top water/non-depleted pay zones were considered. Numerical findings indicated that: (1) there is a detrimental effect of a top water zone on SAGD performance; (2) plugging of a top water zone with oil was not observed in this study for a top water thickness of 8 in; and, (3) operating conditions that lead to a higher pressure difference between the steam chamber and the top water, I either by depletion of the top water zone pressure or a higher steam injection pressure, results in a more detrimental effect on the SAGD performance.