Today, we can create methane gas from decaying organic matter
through bacterial fermentation. Fermentation is a chemical
transformation by bacteria that chemically alters different substances.
It is likely that the environment for petroleum production is without
oxygen, as sediment covers the organic matter. This is called a reducing
environment. It has been shown in the laboratory that reducing
bacteria tend to convert organic matter into a petroleum like substance.
Reducing bacteria are very slow, and maybe that is why geologic time
plays an important part. Petroleum also has sulfur as part of its
component, and a by-product of reducing bacterial is sulfur. This may be
another clue about how oil is produced.