How was oil formed?

Millions of years ago, prehistoric plant and animal remains washed into the seas along with sand and silt. Layer upon layer of the organic materials piled up on the sea bottom. These thick layers were buried with layers of mud, sand, and silt that trapped the organic material. Without air, the organic layers could not rot away. The mud thickened and pushed down on the organic material with increasing pressure. The temperature of the organic material also increased as other processes in the earth heated it. The mud sediment was buried by more sediment. The sediment changed into rock as the temperature, pressure and anaerobic bacteria—microorganisms that can live without oxygen—increased. The plant and animal remains were “cooked” by this process and slowly changed into crude oil. Crude oil is held inside the rock formation, similar to how a sponge holds water.

Not all organic material turned into oil. There must be a trap of nonporous rock to keep the oil from seeping out along with a salt or clay seal to prevent the oil from rising to the surface. With these conditions, only 2% of the organic material became oil. (Source: http://lsa.colorado.edu/summarystreet/texts/petroleum.htm ).

Oil is a nonrenewable resource because it has taken millions of years to form, and new reserves cannot be created.

Development through the geologic ages

350 million years ago: Vegetation became widespread on the land. Land animals were still scarce. The tiny sea creatures and plant vegetation died and were buried on the ocean floor by layers of mud, sand, and silt.

50 to 100 million years ago: The ever-deepening layers on top of the remains generated tremendous heat and pressure that created oil and gas.Present day: We drill through the layers to the rock formations that contain oil and gas deposits.

Want to learn more?

Energy Kids’ Page US Energy Information Administration
Provides text and graphic illustrations with information on various energy resources, including how each resource is formed and used

Energy Quest California Energy Commission
Includes the energy story (all types of energy), science projects, games, a library, profiles of “super scientists,” and many other resources

How Crude Oil is Formed BBC’s Bitesize Revision
Uses text and animated graphics to illustrate how crude oil developed millions of years ago

Kindergarten through 12th grade Learning Resources Canadian Centre for Energy
Provides teaching guides targeted toward specific school grade levels

How Oil and Gas were Formed Cartage.org
Gives a text description of the formation of oil and gas

What is Crude Oil? Chevron Learning Center
Discusses crude oil from an historical perspective using text and a few graphic illustrations

How Is Oil Formed? Columbia University
Uses text and graphic representations to show how oil formed and what geographical conditions were needed to produce oil and natural gas