| To date, most petroleum microbiological work has centered on waterflooded reservoirs that offer a cooled, oxygen-free, saline environment, which meets the environmental requirements of many different groups of bacteria.;Most bacteria have a natural tendency to grow attached to rock surfaces rather than free-floating in the liquid phase. In a petroleum reservoir, bacteria may attach to the rock; start to grow and then produce exopolymers-sugars-that help them attach to each other and rock surfaces. Such growth is termed as biofilm and offers the advantages of protection from biocides while encouraging the bacteria to interact to best use nutrients and other resources.;Petroleum reservoirs constitute a group of unique terrestrial sites, because they present an usual combination of extreme environmental conditions including temperature, pressure, and salinity. Attention has been paid recently to the microbial ecology of petroleum reservoirs where anaerobes have always been considered as the dominant microorganisms. (Fardeau et al, 2004). Petroleum reservoirs harbor a rich and diverse community of microorganisms including (i) fermentative, (ii) sulfate-, thiosulfate, and sulfur-reducing, and (iii) methanogenic. (Salinas et al, 2004 a). Since the beginning of commercial oil production, almost 140 years ago, petroleum engineers have faced problems caused by microorganisms.;The possibility of living organisms to survive or thrive in oil field environments depends on the physical characteristics and chemical composition of the ecosystem. Temperature is the main limiting factor for microbial growth in oil reservoirs. Since temperature increases with depth at a mean rate of 3 °C per 100 m (but regional geothermal gradients may be significantly different), deep oil reservoirs which attain an in situ temperature exceeding 130-150 °C cannot sustain bacterial growth(Magot, 2005). This temperature range is considered the highest theoretical limit for growth due to the thermal instability of biological molecules (Stetter et al, 1993). Different types of data suggest the presence of indigenous bacteria in oil fields could be limited to a thresh hold temperature between 80-90 °C. Philippi noted that in situ oil degradation was never observed in reservoirs whose temperature exceeded 82 °C (Phillipi, 1977). Analysis of a set of 87 water samples collected from North American oil reservoirs showed that fatty acid concentrations were maximum at a temperature of 80 °C in the reservoir (Bartha 1991). This indicated that maximum biodegradation occurs below 80 °C, and that thermal decarboxylation occurs above this temperature. In a microbiological study, hyperthermophilic bacteria could not be isolated from 100 oil field water samples whose reservoir temperature were higher than 82 °C (Bernard et al, 1992). Hyperthermophilic microorganisms growing at temperatures as high as 103 °C have been isolated from some reservoirs, but the authors suggest that they represented exogenous bacteria resulting from sea water injections (Stetter et al, 1993). Salinity and pH formation water can also limit bacterial activity. The salinity ranges from almost fresh water to salt saturated water and pH generally from 5-8. However, the pH measured at atmospheric pressure doesnot necessarily reflects the actual in situ pH, as it is influenced by dissolution of gasses under high pressure. The in situ pH is usually in the range 3-7. This physical characteristic has to be taken into consideration when designing culture media, or interpreting the potential indigenous nature of bacteria recovered from deep surface samples. The availability of electron donors and acceptors governs the type of bacterial metabolic activities within oil field environments. The oil industry undertakes routine chemical analyses of oil environments chemical analyses of oil field hence a wide body of data are available; nevertheless critical data (e.g., nitrogen and phosphorous availability) necessary for understanding microbial metabolism is not routinely performed thereby thwarting our ability to understand microbial processes in situ. Since oil fields are deep subterranean environments and are generally absent: in particular oxygen, nitrate and ferric iron.;Stratal waters (water retained in the pores of rock) generally contain sulfate at various concentrations and carbonate, factors which have led to the assumption that the major metabolic processes occurring in such ecosystems are sulfate reduction, methanogensis, acetogensis, and fermentation.;The potential electron donors include CO2, H2, of geochemical or bacterial, and numerous organic molecules, organic acids are present, but not in all oil reservoirs, and concentrations higher than 20 mM have been recorded (Bartha 1991). Connan showed that in vitro anaerobic biodegradation of crude oil by an indigenous bacterial community reproduced bioderadation effects which have been recorded under natural conditions (Connan et al, 1996). The presence of potential electron donors for anaerobic metabolism. Their presence could explain the observation that diverse groups of strict anaerobes can grow with crude oil as a sole carbon and energy source without any modification of alkanes or light aromatic compounds. (Abstract shortened by UMI.). |
