28-06-01 A natural "palaeosterilisation" process may have preserved more oil for use as fossil fuel than was previously thought. Petroleum geologists have a rule of thumb that states that petroleum biodegradation over geological time scales ceases at temperatures above 80 degrees Celsius, so many shallow oil reservoirs around the world, where temperatures are relatively low, are assumed to contain degraded oil.

But a number of oil reservoirs in cool basins have now been found to contain undegraded oil, in situations where the basins have been uplifted from deeper, hotter regions of the Earth.
Heating during deep burial may have removed bacteria from these oil reserves, and the sterilized sediments have not been subsequently colonized by hydrocarbon-hungry bacteria.