Viscosity is directly related to the mobility and quality of hydrocarbons in reservoirs, and its distribution is commonly heterogeneous. Therefore, viscosity has a great impact on the exploitation of heavy oil and bitumen reservoirs. Previous studies showed that viscosity measurements are problematic and are often inconsistent mainly because of the challenges of sample preparation and lack of a standard procedure for measuring viscosity in the oil and gas industry. This study aims to improve the characterization of reservoir viscosity by understanding the rheological properties of bitumen using bitumen samples from the Upper Devonian Grosmont reservoir, Alberta, Canada. This study shows that Grosmont bitumen is essentially a non-Newtonian fluid, exhibiting a distinctive shear-thinning behavior at low temperatures of less than 40 C. With increasing temperature, however, the bitumen changes from a non-Newtonian fluid to a Newtonian fluid. At low temperatures, the viscosity variations can be divided into four stages. A standard viscosity called "zero-shear viscosity" is obtained from the viscosity variations and can be used throughout the reservoir as characteristic viscosity for Grosmont bitumen. The viscosity distribution in the Grosmont reservoir is complex and appears to be stratigraphically related. Bitumen samples from formation boundaries tend to have higher viscosities, suggesting more severe biodegradation at these locations. Possible causes for the observed patterns in the Grosmont include (1) oil-water contacts migrating up and down over time; (2) oil migration and/or biodegradation controlled by aquitards that divide the reservoir into hydrostratigraphically separated units; and (3) differences in microbial activity, that is, aerobic versus anaerobic, possibly controlled by the level of oxygenation over time.