Multiple Controls on Petroleum Biodegradation and Impact on Oil Quality

Abstract

Summary 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; in addition, viscosity, sulfur, asphaltene, metals, vacuum residua, and total acid numbers increase. For a specific hydrocarbon system (similar source type and level of maturity), general trends exist for oil-quality parameters vs. present-day reservoir temperatures of <80°C. However, other controls on biodegradation may also have significant effects, making predrill prediction of oil quality difficult in some areas. 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 also can 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. Reservoirs undergoing current charging with hydrocarbons 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.

Distribution of acids and nitrogen-containing compounds in biodegraded oils of the Liaohe Basin by negative ion ESI FT-ICR MS

“…Hydrocarbons are preferentially destroyed during biodegradation. The selective consumption of saturate and aromatic hydrocarbons during biodegradation is well documented (Connan, 1984;Hunt, 1995;Wenger et al, 2002;Peters et al, 2005). In general, normal alkanes are preferentially consumed, followed by branched alkanes, monocyclic paraffinic and monoaromatic hydrocarbons, multi-ring naphthenic and polynuclear aromatic hydrocarbons and finally non-hydrocarbons (Fedorak and Westlake, 1984a,b;Huang et al, 2003).…”

Abyssivirga alkaniphila gen. nov., sp. nov., an alkane-degrading, anaerobic bacterium from a deep-sea hydrothermal vent system, and emended descriptions of Natranaerovirga pectinivora and Natranaerovirga hydrolytica

“…This, in combination with a reduction in the alkane content of the oil during biodegradation, reduces the ability of the oil to dissolve gas. Biodegradation thus reduces the gas : oil ratio of the trapped oil, and may result in the exsolution of a dry gas cap enriched in methane and CO 2 (Jones et al, 2008;Larter et al, 2005;Wenger et al, 2002). This process may offer a way to extract energy otherwise trapped in nonextractable oil (Gieg et al, 2008;Gray et al, 2009;Jones et al, 2008).…”

Gas chromatographic analysis of whole oil samples: implications for biodegradation in the Niger Delta

“…The Bs crude oils are produced from the onshore fields in the coastal swamp depobelt of the Niger Delta. Since shallow onshore reservoirs with hydrodynamic drive from meteoric water influx are the precursor for heavy biodegradation (Wenger et al 2002), it is suspected that influx of meteoric water could be the cause of this moderate to heavy biodegradation for the Bs crude. The GC fingerprint of the Ai crude shows that the light fractions have been removed up to nC 9 while the fraction of C 11 -C 13 has been reduced slightly ( Fig.…”

“…The order in which the hydrocarbon compounds are oxidized depends on a variety of factors, but in general small molecules up to C 20 are consumed before large ones, and within the same molecular weight range the order is usually nparaffins first, followed by iso-paraffins, naphthenes and aromatics; single-ring naphthenes and aromatics are attacked before iso-prenoids, steranes and triterpenes. Preferential consumption of the low molecular weight components causes the high density (low API gravity) of the unconsumed residue (Wenger et al 2002). Empirically, it has been noted that biodegraded oil accumulations occur in reservoirs that are at temperatures less than 80°C (Connan 1984;Barnard and Bastow 1991).…”