لخّصلي

Petroleum (literally rock oil, from the Greek petra- for rock and Latin -oleum for oil) is a general
term used to refer to all forms of oil and natural gas that is mined from the earth. What most people
concern themselves with is crude oil, the liquid mixture of naturally occurring hydrocarbons and
natural gas, which is a gaseous mixture of naturally occurring hydrocarbons.
Hydrocarbons are complex molecules that are formed from long strings of hydrogen and carbon,
such as propane (C3H8) or butane (C4H10).
Petroleum is the final product that we get out of the ground. But how does it get there?
Petroleum begins as living animals, microscopic organisms (like diatoms or plankton) that live in
the oceans. When these organisms die, their bodies sink and collect on the ocean floor. These
organisms live all over the oceans and their bodies fall and collect on the ocean bottoms all over
the world. When the organic matter becomes buried and begin to decompose, they are referred to
as kerogen. Despite the apparent abundance of dead organisms raining down on the ocean
bottoms, there are specific conditions that must be met for these organisms to be transformed into
petroleum.
First, the area that the kerogen collects must be a restricted basin, a depression where
sediment can accumulate and where there is poor water circulation. When the oxygen is gone, the
decomposition stops and the remaining matter are preserved. The kerogen must be buried under
sediment where it will be altered through high temperatures and high pressures. As the heat and
pressure breaks down the kerogen, the hydrocarbon chains are freed. Long chains of hydrocarbon are
oil; shorter chains are gas, generally methane (CH4) and condensates such as ethane, propane and
butane. As the heat and pressure continues, the longer chains will continue to break into shorter
chains. If the process continues long enough, all that will remain will be methane.
Compaction of the sediment and the expansion of the kerogen as it is transformed into
petroleum cause it to be forced out of the rock it was created in (the source rock) and into nearby
sediments. If these sediments are porous enough (have microscopic holes) and permeable enough
(allowing for the flow of liquids), then the petroleum will migrate through the rock. Since gas and
oil are lighter than water, they can travel through water-saturated rock. Eventually the oil will stop
migrating as it meets rock that is not porous or permeable,and will collect in a trap. It is these
petroleum traps that geologists search for and that the oil companies drill into to recover the oil.
Despite the simplicity, there are several conditions that must occur, otherwise, no oil will
be made.
First, there needs to be a source rock that contains the organic matter to be converted into
petroleum. This source rock is generally shale or other mudstones. There must be a reservoir
rock, usually sandstone or limestone that is porous and permeable where the oil can be stored and
transported. There needs to be a trap, something that is non-porous and non-permeable that will
hold the petroleum in the reservoir and prevent it from migrating further. Finally, there needs to be enough heat and pressure to sufficiently cook the oil and gas out of the kerogen. If anyone of
these conditions is not met, then petroleum cannot be formed.
The important step in the process is the trap. Something needs to block or trap the petroleum
so it will accumulate into a large enough deposit for geologists to be able to locate it. Petroleum
traps come in several varieties, in various sizes and can be made through structural processes (like
folds and faults), or by sedimentary processes.