The terms oil and petroleum have been used interchangeably by industry
since the early 1900s. However, there are some who refer to petroleum as the
crude oil prior to any treatment/renery, with oil reserved as the denition
of “any of the various kinds of greasy, combustible substances obtained from
animal, vegetable, and mineral sources, and liquid at ordinary temperature.”
Both terms are employed interchangeably in this chapter as a matter of con-
venience. The typical petroleum reservoirs are mostly sandstone or lime-
stone formation containing oil. The viscosity of the oil in these formations
may be as thin as gasoline or as thick as tar. It may be almost clear or black.
Petroleum, created by the decay of biological materials similar to those creat-
ing coal, is called a nonrenewable energy source because it takes millions of
years to form. At present, full-scale technologies are not available to make oil
in large quantities.
Unlike coal, in which the stored energy actually takes the form of rock, oil
is an entirely separate substance from the rock in which it was formed. Oil,
in its liquid form, is very mobile and tends to move upward as the heat in the
Earth’s crust causes it to expand. Rising oil frequently penetrates surround-
ing rock formations and continues its upward journey until it meets a layer
of rock that is so tightly compressed that passage is not possible. Such a bar-
rier might be produced by movements in the Earth’s crust that have caused
hard, impenetrable rock to come against the porous layers through which oil
is moving. When this happens, the oil becomes trapped in a formation that
is dened today as an oileld.
Water has also been trapped underground in the same layers of rock and
some of the hydrogen and carbon atoms from the oil have combined to form
a mixture of gases. Because the gases are lighter than the oil, they gradu-
ally separate and gather at the top of the oil reservoir. Similarly, oil is lighter
than water and will oat into the pore spaces above it. However, in most oil-
elds, there is not enough space for all three substances as they expand in the
high temperatures deep within the Earth’s crust. This explains why pressure
builds up and drives oil to the surface when a well is drilled.