The bacterial communities of production water containing reservoir microorganism were 30% lower than groundwater and injection water despite that all of them have rich biodiversity. After thirty days of oxygen treatment the n-alkanes degradation were 92.5% in injected water while 44.3% in groundwater and 92.4% in production water. The selected PAHs biodegradation rate in production water was lower as compared to the injection water but significantly higher than groundwater.