DEFINITION OF PETROLEUM

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. The petroleum group of hydrocarbons includes oil, natural

gas, and gas condensate. 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. After these

plants and other organisms die, they may decay to produce

hydrocarbons and other fossil fuels if the plant remains are

buried before they completely decay.

Crude oil and natural gas may become concentrated in

some regions and become mineable for use under some special

conditions. First, for oil and gas to form, more organic

matter must be produced than is destroyed by scavengers and

organic decay, conditions that are met in relatively few

places. One of the best places for oil and gas to form is on

offshore continental shelves, passive margins, or carbonate

platforms, where organic productivity is high, and the oxygen

contents of bottom waters is low so organic decay is low and

inadequate to destroy the amount of organic material produced.

Organic material may also be buried before it decays

in sufficient quantities to make petroleum in some deltaic and

continental rise environments.

Once the organic material is buried it must reach a narrow

window of specific pressure and temperature conditions

to make petroleum. If these temperatures and pressures are

not met, or are exceeded, petroleum will not form or will be

destroyed. When organic rich rocks are in this petroleum

window of specific temperature and pressure, organic rich

beds known as source rocks become compacted and the

organic material undergoes chemical reactions to form hydrocarbons

including oil and gas. These fluids and gases have a

lower density than surrounding rocks and a lower density

than water, so they tend to migrate upward until they escape

at the surface, or are trapped between impermeable layers

where they may form a petroleum reservoir.

Oil traps are of many varieties, divided into mainly

structural and stratigraphic types. Structural traps include

anticlines, where the beds of rocks are folded into an upward

arching dome. In these types of traps, petroleum in a permeable

layer that is confined between impermeable layers (such

as a sandstone bed between shale layers) may migrate up to

the top of the anticlinal dome, where it becomes trapped. If a

fault cuts across beds, it may form a barrier or it may act as a

conduit for oil to escape along, depending on the physical

properties of the rock in the fault zone. In many cases faults

juxtapose an oil-bearing permeable unit against an impermeable

horizon, forming a structural trap. Salt domes in many

places form diapirs that pierce through oil-bearing stratigraphic

horizons. They typically cause an upwarping of the

rock beds around the dome, forming a sort of anticlinal trap

that in many regions has yielded large volumes of petroleum.

Stratigraphic traps are found mainly where two impermeable

layers such as shales are found above and below a lensshaped

sandstone unit that pinches out laterally, forming a

wedge-shaped trap. These conditions are commonly met

along passive margins, where transgressions and regressions

of the sea cause sand and mud facies to migrate laterally.

When combined with continuous subsidence, passive margin

sequences typically develop many sandstone wedges caught

between shale layers. River systems and sandstone channels

in muddy overbank delta deposits also form good trap and

reservoir systems, since the porous sandstone channels are

trapped between impermeable shales.

Most of the world’s industrialized nations get the majority

of their energy needs from petroleum and other fossil fuels,

so exploration for and exploitation of petroleum is a major

national and industrial endeavor. Huge resources are spent in

petroleum exploration, and thousands of geologists are

employed in the oil industry. In the early days of exploration

the oil industry gained a reputation of being environmentally

degrading, but increased regulations and awareness by these

companies has greatly alleviated these problems, and most

petroleum is now explored for and extracted with minimal

environmental consequences. The burning of fossil fuels,

however, continues to release huge amounts of carbon dioxide

and other chemicals into the atmosphere, contributing to

global warming.

See also HYDROCARBON; PASSIVE MARGIN.

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