Oil (or crude) is a liquid
fuel kept in the sedimentary layers of the Earth. Both oil and gas have
their origin in the algae and plankton remains that settled in calm, shallow
waters (bays or a deltas) or deep waters. These remains, together with sand
and slime, formed the sapropel, a dark mixture of bad smelling decaying
organic matter. Little by little these sediments were covered by new ones
which put great pressure and increased the temperature. These layers
compacted till they formed the oil mother rock.
The oil seldom stays in the mother rock because of its lower density.
Sometimes it migrates to the surface, where a part evaporates and the rest
form asphalts and bitumens. It can also be kept in porous rocks called oil