Petroleum formation occurs by various hydrocarbons combining with certain minerals such as sulphur under extreme pressure.When small sea plants and animals die they will sink, they will then lie
on the sea bed where they will decompose and mix with sand and silt.
During the decomposition process tiny bacteria will clean the remains of
certain chemicals such as phosphorus, nitrogen and oxygen.This leaves the remains consisting of mainly carbon and hydrogen. At the
bottom of the ocean there is insufficient oxygen for the corpse to
decompose entirely. What we are left with is the raw materials for the formation of petroleum.
The partially decomposed remains will form a large, gelatinous mass,
which will then slowly become covered by multiple layers of sand, silt
and mud. This burying process takes millions of years, with layers
piling up one atop another.
As the depth of the sediment build up increases the weight of the
sand and silt pressing down on the mass will compress it into a layer
which is much thinner than the original.
Finally, when the depth of the buried decomposing layer reaches
somewhere around 10,000 feet the natural heat of the earth and the
intense pressure will combine to act upon the mass. The end result, over
time, is the formation of petroleum.