Lloyd M. Wenger

Multiple Controls on Petroleum Biodegradation and Impact on Oil Quality

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 sulfatereducing 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.

The influence of pressure on petroleum generation and maturation as suggested by aqueous pyrolysis

Because fluid pressures are transient in sedimentary basins over geologic time, the effect of increasing fluid pressure on organic-matter metamorphism is difficult to determine, and conflicting opinions exist concerning its influence. Properly-performed aqueous-pyrolysis experiments can closely simulate hydrocarbon generation and maturation in nature, and thus offer an excellent way to study the influence of pressure. Such experiments, carried out on the Retort Phosphatic Shale Member of the Lower Permian Phosphoria Formation (type II-S organic matter) at different constant temperatures, demonstrated that increasing pressure significantly retards all aspects of organic matter metamorphism, including hydrocarbon generation, maturation and thermal destruction. This conclusion results from detailed quantitative and qualitative analyses of all products from hydrocarbon generation, from the C1 to C4 hydrocarbon gases to the asphaltenes, and also from analyses of the reacted rocks. We have documented that our aqueous-pyrolysis experiments closely simulated natural hydrocarbon generation and maturation. Thus the data taken as a function of pressure have relevance to the influence of normal and abnormal fluid pressures as related to: 1) depths and temperatures of mainstage hydrocarbon generation; 2) the thermal destruction of deposits of gas or light oil, or their preservation to unexpectedly high maturation ranks; and 3) the persistence of measurable to moderate concentrations of C15+ hydrocarbons in fine-grained rocks even to ultra-high maturation ranks.

Control of hydrocarbon seepage intensity on level of biodegradation in sea bottom sediments

Offshore surface geochemical surveys, which target the surface expression of potential migration pathways for sampling such as fault scarps or diapiric features, have become a commonly-applied approach in the petroleum industry. Results of such surveys help to reduce risk on key exploration play elements and are used to evaluate prospects and to predict hydrocarbon phase and expected properties. Based on geochemical surveys conducted by ExxonMobil in many basins worldwide, there is an interrelation of the seep intensity (concentration) and level of biodegradation. Results from offshore west Africa, where many active macroseeps show moderate-to-severe biodegradation, and a frontier basin offshore United Kingdom (Rockall Trough), where active microseeps show no evidence of biodegradation, are compared. The specific biochemical controls on the difference in biodegradation-proneness are not known, although it appears that a certain threshold of oil concentration is needed to sustain an active bacterial community, or to exceed clay-adsorption capacities that may protect microseeps from biodegradation. It is notable that the 25-norhopane series, often considered an indication of severe biodegradation in reservoir oils, has not been recognized in even ultra-severely biodegraded seeps. This suggests that different biodegradation pathways may be followed in marine surface seeps versus those in subsurface hydrocarbon accumulations, a likely scenario in light of the fact that physiologically diverse bacterial communities are prevalent under different physiochemical conditions.