The increase in global energy demand has led to a rise in interest in the use of heavy oil. However, the widespread use of petroleum hydrocarbons has resulted in considerable environmental issues. In this study, we aimed to observe the biodegradability of the heavy oil using microbial consortia derived from contaminated soil and water. We investigated the biodegradability of heavy oil under aerobic and anaerobic conditions over three weeks, and the residual percentage of heavy oil was estimated using gas chromatography analysis. Under aerobic conditions, soil microbial communities demonstrated a higher biodegradation rate of heavy oil than water microbiota. On the other hand, in anaerobic conditions, soil and water samples showed reduced biodegradation rates, and some major hydrocarbon peaks persisted, suggesting the presence of recalcitrant fractions such as PAHs (polycyclic aromatic hydrocarbons). However, this study also examined initial bacterial counts (colony-forming units, CFU) to assess its effect on the biodegradability of heavy oil. Although we did not observe a statistically significant correlation between initial CFU/ml and heavy oil biodegradation, we observed high levels of heavy oil biodegradation when the initial CFU/ml was high. In conclusion, this study is significant in the context of the potential to observe the degradability of heavy oil in natural environments under aerobic and anaerobic conditions, as well as the influence of microbial communities.