HYPERTHERMOPHILIC ARCHAEA AND BACTERIA FROM HOT PETROLEUM RESERVOIRS

During the last decade, extremely thermophilic and hyperthermophilic archaea and bacteria had been isolated from different deep geothermally heated petroleum reservoirs (1). They belong to the genera Archaeoglobus, Thermococcus, Pyrococcus, Methanococcus, Methanobacterium, Thermotoga, Thermosipho, and Thermodesulfobacterium. Samples of produced fluids contain high concentrations of various viable cells, indicating the presence of complex microbial communities in situ. Due to their high growth temperatures, pressure tolerance, and extremely anaerobic mode of life, these reservoir organisms appear to be well adapted to their subterranean environment of heated porous rocks saturated with water and petroleum fluids. A great deal of the species are closely related to those, already known from submarine and terrestrial hydrothermal systems. By their energy-yielding reactions, the hot reservoir organisms are often anaerobic respirers of organic material like crude oil components or of hydrogen gas. The latter may be formed by fermentation (e. g. by the Thermococcales and Thermotogales). Oxidized compounds like sulphate, thiosulphate, sulphite, elemental sulphur, carbon dioxide, nitrate and ferric iron may be used as electron acceptors, leading to the formation of hydrogen sulphide, methane, ammonia, and magnetite, respectively. As an overall consequence, biodegradation by communities of anaerobic hyperthermophiles within deep reservoirs may lead to hydrogen-depletion of petroleum. However, in addition to the presence of suitable organisms, the rate of biodegradation may depend mainly on the availability of anaerobic electron acceptors. Similar to the organisms, these may enter reservoirs through natural routes such as faults and oil seeps or by artificial sea water injection during the oil recovery process. Sulphidogens may, therefore be strongly involved into reservoir souring, even at temperatures of 100° C.

References: (1) K. O. Stetter & R. Huber: Microbial Biosystems: New Frontiers, Proceedings of the 8th. International Symposium on Microbial Ecology (C.R. Bett et al. eds.). Atlantic Canada Society for Microbial Ecology, Halifax Canada, pp. 369-375 (2000).