A combined geochemical, geological and microbiological analysis of an actively biodegrading 24.5. m thick oil column in a Canadian heavy oil reservoir has been carried out. The reservoir properties associated with the cored vertical well are characterised by a 15.75. m thick oil column and an 8.75. m zone of steadily decreasing oil saturation below the oil column, referred to as the oil-water transition zone (OWTZ), grading down into a thin water leg. The oil column exhibits systematic gradients in oil physical properties and hydrocarbon composition and shows variations in biodegradation level throughout the reservoir consistent with the notion that the biodegradation of oil is focussed in a bioreactor zone at the base of the oil column. Through the oil column, the dead oil viscosity measured at 20 °C ranged from 50,000. cP (0.05. McP) at the top of the oil column to 1.4. McP at the oil-OWTZ contact, and continued to increase to 10.5. McP within the OWTZ. The saturated and aromatic hydrocarbons are characterised by systematically decreasing bulk fraction and component concentrations down through the oil column. Different compound classes decreased to levels below their detection limit at different depths within the OWTZ, defining a likely bioreactor extent of over 5. m in depth with, for example, n-alkanes being reduced to their detection limit concentration at the bottom of the oil column/top of the OWTZ, while branched isoprenoid alkanes were not completely degraded until well into the OWTZ.Core samples from the oil column and the lower part of the OWTZ were estimated to contain ca. 104-105 bacterial cells/g, based on qPCR of bacterial 16S rRNA genes, while samples from a narrow interval in the OWTZ immediately below the oil column contained on the order of 106-107cells/g of sediment. Interestingly, these latter numbers are typical of those observed in active deep subsurface biosphere systems with the notion that microbial activity and abundance in the deep subsurface is elevated at geochemical interfaces. The numbers of organisms are not constant throughout the OWTZ. The highest bacterial abundance and geochemical gradients of, for example, methylphenanthrene biodegradation define a zone near the oil-water contact as likely the most active in terms of biodegradation. The largest bacterial abundances in the upper part of the OWTZ are in line with the trend of bacterial abundance with depth that has emerged from extensive analysis of microbial cells in deep subsurface sediments, implying that in terms of deep biosphere cell abundance, oil reservoirs are similar to other deep subsurface microbial environments. This is puzzling, given the atypical abundance of organic carbon in petroleum reservoirs and may imply a common large scale control on microbial abundance and activity in the deep biosphere, including in oilfields.