There are a few things required for oil to form, including the quick burial of large amounts of hydrogen-rich organic matter in a low-oxygen environment. These conditions are most often met in shallow seas near continental shelves, where periodic nutrient-rich upwellings from the deep sea cause blooms of plankton and algae. These temporary blooms soon burn themselves out, dying and falling to the oxygen-poor seabed as marine snow. If they’re quickly buried, they may eventually form oil or gas. Land life, on the other hand, is more likely to form peat and eventually coal.
