Multiple Controls on Petroleum Biodegradation and Impact on Oil Quality

Abstract

Biodegradation of oils in nature is important in reservoirs cooler than approximately 80° C. Oils from shallower, cooler reservoirs tend to be progressively more biodegraded than those in deeper, hotter reservoirs. Increasing levels of biodegradation generally cause a decline in oil quality, diminishing the producibility and value of the oil as API gravity and distillate yields decrease; and viscosity, sulfur, asphaltene, metals, vacuum residua, and total acid number increase. For a specific hydrocarbon system (similar source type and level of maturity), general trends exist for oil quality parameters versus present-day reservoir temperature < 80° C. However, additional controls on biodegradation may have significant effects, making pre-drill prediction of oil quality difficult. It has long been observed that fresh, oxygenated waters in contact with reservoir oil can cause extensive aerobic biodegradation. More recently it has been recognized that anaerobic sulfate-reducing and fermenting bacteria can also degrade petroleum. Highly saline formation waters may inhibit bacterial degradation and effectively shield oils from oil-quality deterioration. The timing of hydrocarbon charge(s) and the post-charge temperature history of the reservoir can have major effects on oil quality. Currently charging reservoirs may overwhelm the ability of bacteria to degrade the oil, resulting in better-than-anticipated oil quality. Fresh charge to reservoirs containing previously degraded oil will upgrade oil quality. Calibrated methods of oil quality risking, based on a detailed evaluation of reservoir charge and temperature history and local controls on biodegradation, need to be developed on a play and prospect basis.