An extra 600 billion barrels of oil could be extracted from existing oilfields by using microbes according to oil giant BP. The paper investigeates this statement. Proposed since the 1920s, the use of microbes to enhance oil recovery has remained practically unseen. However, the advances of modern biotechnology and the recent discovery and characterisation of indigenous microbes living in the oil reservoirs has now brought this technology to the attention of oil companies. The process is called microbially enhanced hydrocarbon recovery (MEHR), and it is being studied in a BP-funded programme at the Energy Biosciences Institute (EBI) at the University of California at Berkeley. The most basic method of oil recovery, called primary oil recovery, uses the natural pressure in the well to capture the oil. To increase the recovery rate, secondary recovery methods are used in which the oil wells are flooded with a medium water, gas or vapour allowing the modification of the physicochemical properties of the reservoirs. But the average recovery rate obtained by using both these classical methods is only around 20%.