Gaseous, liquid, or solid organic compounds
consisting of hydrogen and carbon. Petroleum is a mixture of
different types of hydrocarbons (fossil fuels) derived from the
decomposed remains of plants and animals that are trapped
in sediment and can be used as fuel. When plants and animals
are alive, they absorb energy from the Sun (directly through
photosynthesis in plants, and indirectly through consumption
in animals) to make complex organic molecules which, after
they die, decay to produce hydrocarbons and other fossil
fuels. If organic matter is buried before it is completely
decomposed, some “solar energy” may become stored in the
rocks as fossil fuels (less than 1 percent of total organic matter
gets buried). In most industrial nations the chief source of
energy is fossil fuels.
The type of organic matter that gets buried in sediment
plays an important role in the type of fossil fuel that forms.
Oceanic organisms (such as bacteria and phytoplankton) are
buried by shales and muds and form proteins, lipids, and carbohydrates
that form oil and natural gas when heated. Terrigenous
plants (such as trees and bushes) form resins, waxes,
lignins, and cellulose, which form coals. Incompletely broken
down organics in shale form kerogens, or oil shales, which
require additional heat to convert to oil.
The first people on the planet to use oil were the ancient
Iraqis, 6,000 years ago. Oil is fluid and is lighter than water,
which strongly influences where it is found. An oil “pool” is
an underground accumulation of oil and gas occurring in the
pore spaces of rock. An oil field is a group of oil pools of
similar type.
Once oil forms by the decay of organic material, it
migrates upward until it seeps out at the surface or it encounters
a trap. The migration of oil is like the movement of
groundwater. Migration is slow, and since petroleum is lighter
than water, water forces it upward to the tops of the traps.
Because most oil eventually finds its way to the surface, it is
not surprising that most oil is found in relatively young rocks.
The formation of oil requires that the source has been
through a critical range of pressure and temperature conditions,
known as the oil window. If the geothermal gradient is
too low or too high, oil will not form. Oil and gas can only
accumulate if five basic requirements are met. First, an
appropriate source rock is needed to provide the oil. Second,
a permeable reservoir is needed, and third it must have an
impermeable roof rock. Fourth, a trap (stratigraphic or structural)
is needed to hold the oil, and finally, the formation of
the trap must have occurred before the oil has escaped from
the system. Thus, it is extremely lucky if all five criteria are
met and a petroleum deposit is formed.
Geologists know the location of approximately 600 billion
barrels of oil; estimates for total global oil reserves are
1,500–3,000 billion barrels. We have used up about 500 billion
barrels. Many of the unknown reserves include small
deposits, but not tars, tar sands, and oil shales, which must be
heated and extensively processed to make them useful, and
thus are very expensive. We now use 30 million barrels of oil
a year, therefore, known reserves will last 20–50–100 years at
the most. Oil is running out and is becoming an increasingly
powerful political weapon. The oil-rich nations can effectively
hold the rest of the world hostage, being that we have become
so dependent on oil. Future energy sources may include nuclear
fuels, solar energy, hydroelectric power, geothermal energy,
biomass, wind, gas hydrates, and tidal energy.
See also COAL; GAS HYDRATES.
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