Biological life, primarily in the form of bacteria, can thrive in subsurface oil reservoirs. Under a specific range of conditions these organisms can metabolize compounds found in the oil, permanently altering its composition.  This process is known as biodegradation, and it has the ability to dramatically reduce the yield of oil reserves. The aim of this article is to briefly summarize the conditions under which biodegradation can occur, the effects of this biodegradation, and finally how rapidly it is occurring.
Degradation can occur in both aerobic and anaerobic conditions. In the subsurface this typically occurs under anaerobic conditions. Degradation is mediated by sulfate reducing bacteria when sulfur is present.  Methanogenic bacteria is largely responsible when sulfates are low.  While this degradation can occur in anoxic conditions it is important to note that several nutrients such as N, P, and K are required.  Water is often the source of these, supplying them in dissolved form. 
One limiting condition to the process is temperature. It has shown that biodegradation only occurs within deposits that have not exceeded 80 °C. [3,4] It has been surmised that the bacteria involved in this process do not survive past this temperature point. [3,4] Beyond this, deposits that have ever exceeded this temperature, even if at present they are considerably lower in temperature, are seen to not be a sight of biodegradation.  The consensus is that a form of pasteurization occurs, whereby the bacteria have all been eliminated, and thus the sample will not degrade. 
Biodegradation metabolizes a variety of compounds found in oil reserves. This activity can significantly alter the properties of the deposit and in turn dramatically alter the productivity and value of the deposit.  Biodegradation is known to raise viscosity, reduce the API gravity, increase asphaltene concentrations, increase sulfur concentration, raise acidity, and produce compounds such as phenols as a byproduct. 
Rates of biodegradation are largely limited by the concentration of nutrients essential for the process, and not by the size of the carbon deposit. There is also a corollary between temperature and rate, with the most rapid degradation occurring in deposits below 40 °C. Rates of 10-3 to 10-4 kg petroleum per m2 of water contact per year have been suggested. 
© Michael Anderson. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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