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Which Rocks Commonly Contain Oil Deposits?

Oil forms as organic matter is put under extreme pressure beneath layers of crushing earth. As it compresses, the matter breaks down and is transformed into kerogen. Kerogen is a dark waxy substance that again breaks down to either petroleum or natural gas. It takes vast amounts of organic matter to create oil deposits, so the type of rock it forms in is characteristic of this type of environment. Places with high concentrations of life, such as oceans and swamps, are likely to form oil deposits after hundreds of thousands of years.

Sedimentary
- In order for sedimentary rock to form, it takes specific circumstances and a lot of time. Organic matter falls to the bottom of a sea bed and settles, then is quickly covered by another layer of matter. This matter could be plankton, plants and other organisms that die and settle on the sea floor. After years of accumulation and pressure on top of these layers they form into stone. All stone that forms this way is called sedimentary. Its grain size and composition are what determine which type of sedimentary rock it is. Oil is found in fine-grained sedimentary stone.
Shale
- Shale is the most common stone containing soil. It is a very fine-grained sedimentary rock. "Oil shale" is a term applied to any sedimentary rock containing bituminous (mixed hydrocarbon) materials. This stone is comprised of silt sized mineral particles. They build up in layers that can be seen as they break away from the rock. Shale can be yellow to red or black depending on its composition. Shales that are rich in organic matter and likely to contain oil are black because of the heavy organic deposits within.
Siltstone
- The Borden Siltstone Delta in southwestern Illinois is a deposit of organic rich sedimentary rocks containing oil pools. Siltstone, limestone and sandstone are the major types of sedimentary stone in this area, according to the Illinois Department of Registration and Education. Siltstone grains are slightly larger than shale and gritty to the touch. Unlike shale, siltstone will not break off it layers. The Borden features wells of oil in grey dolomite siltstone on top of a bed of limestone with some interspersed sandstone deposits.
Tar Sands
- Tar sands or oil sands form when extra heavy oil accumulates in a porous stone. According to the JRank Science Encyclopedia, extra heavy oils contain high levels of resins and asphaltenes (hydrocarbon components). Because of this they must be extracted in different ways than regular crude oil. Extra heavy oils accumulate in porous sedimentary rock such as sandstone and mudstone. This type of oil is very viscous and must be mined in rock form and then extracted through heating and chemical treatments. This type of oil extraction creates more carbon emissions than traditional oil refining.

Nature