Figure 2. One of possible effects of reservoir compartmentalization on bitumen quality and composition in the Athabasca Oil Sands Deposit (AOSD). Sedimentological compartmentalization is evident from gamma ray (GR) log and core images (first two columns on the left). In core image black is bitumen saturated sand and grey is impermeable mud layer that separates the reservoir into two compartments. Third column shows mz 192 with peaks of 4 phenantrene isomers from 5 samples collected at depths of 7.4, 27.1, 41.9, 49.4 and 57.4 meters. Changes in ratio of abundance between 3MP and 2MP as well as 1MP and 9 MP are obvious and are typical of biodegradation, since 1MP and 3MP are more susceptible to microbial degradation than isomers 2MP and 9MP. Viscosity and API gravity measurements (last two columns on the right) show not only vertical gradients within the each of the compartments but also increased bitumen quality at the top of the lower compartment. Note, viscosity is measured at 20°C which is significantly higher than anticipated reservoir temperature of 4°C inferring that projected viscosities will be even greater and even more pronounce the gradient. Increased quality may be because of late charge of more mature petroleum, salty water at the base, or other factors mitigating the intensity of the biodegradation.