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What is Crude Oil?

By Brendan McGuigan
Updated May 16, 2024
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Crude oil is a liquid found naturally in rock, containing mostly complex hydrocarbons, with some additional organic material. It is the major fuel used on the planet, and is used in the production of many synthetic materials like plastics as well.

This oil can come in many different weights and colors, and can differ greatly in its composition. As little as half of the composition of heavy oils can be made up of hydrocarbons, while the lightest oils can be up to 97% hydrocarbons. There are four main hydrocarbons found in crude oil, in varying amounts depending on the oil. Around half of the hydrocarbons in most unrefined oil are naphthenes, one-third are paraffins, one-sixth are aromatics, and the rest are asphaltics. The color can range from pure black or dark brown to greenish or yellowish, depending on the composition.

Crude oil is considered light if the level of hydrocarbons relative to organics and metals is high, making its density low, and it is considered heavy if the level of hydrocarbons relative to organics and metals is low, making its density as high. Additionally, unrefined oil is classified as sweet if it has very little sulfur in it, and is classified as sour if it has a great deal of sulfur in it. So a crude oil will usually be called something like a sweet, light oil, or a sour, heavy oil. Sweeter oils are more valuable than sour oils, because most countries have sulfur regulations for environmental reasons, and sweet oils require less treatment to remove the sulfur. Light oils are more valuable than heavy oils, because more gasoline can be created from a smaller amount.

Different regions on earth tend to have different types of oil, so unrefined oil is often classified based on where it comes from. Certain regions will act as a sample of a broader region, since they are seen as relatively representative of that broad region. For example, Dubai-Oman oil is a sour crude oil, and is used to benchmark most sour crude from the Middle East; West Texas Intermediate is a sweet, light oil; and the OPEC Reference Basket is a composite oil sample that averages oils from all over the Organization of the Petroleum Exporting Countries.

While conventional unrefined oil is currently the major source of petroleum on the planet, it actually makes up a minority of crude oil currently in reserve. A bit less than one-third of the unrefined oil known on the planet is in conventional form. Another one-sixth is a heavy oil, and a quarter is extra-heavy oil. Another one-third, roughly equal to the amount of conventional oil, is in the form of oil sands, or crude bitumen. This is not a liquid form of crude oil, but is mixed with sand into a somewhat solid form. Huge reserves of bitumen can be found in Venezuela and Canada, which also contain large amount of extra-heavy oil, making the two countries’ reserves equal to about twice the known reserves of conventional oil.

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Discussion Comments

By anon347219 — On Sep 04, 2013

What is the difference between crude oil and petroleum? Are there any alternate sources of crude oil on earth? How can we reduce our dependence on petroleum (mainly petrol and diesel)since the sources are limited? --Ana

By anon215993 — On Sep 20, 2011

The hagonoy plant is one of the organic plants that ends up as oil when it decays.

By anon160653 — On Mar 16, 2011

what is in crude oil?

By anon145443 — On Jan 23, 2011

Why is crude oil such a harmful substance?

By anon134894 — On Dec 16, 2010

what are the uses of crude oil?

By Amphibious54 — On Jun 17, 2010

@ parmnparsley - I'll try to pick up where Fiorite left off. Petroleum is renewable, just not relative to the needs of humans. The processes of maturation and sedimentation will create more oil; however it will take millions of years.

I would also like to point out that although the earth is mostly covered by oceans, not all dead organisms will become deposits of oil. It is thought that almost 99% of all the petroleum the earth has ever created has escaped into the atmosphere through seepage and decay (Don't assume that this debunks global warming because the seepage and decay process occurred over the 4.5 billion year history of the planet, while the remaining one percent has been released by humans in a matter of two centuries).

For petroleum to create a reserve, a layer of impermeable rock called cap rock covers a layer of permeable rock called the reservoir rock. There are six common rock configurations that can create petroleum reserves. The cap rock prevents the oil from reaching the surface, and is usually made up of shale.

These rock configurations are called traps, and in almost all cases the water table forces the oil out of the reservoir rock into a cavity. Think of how a coffee percolator works. This cavity is what is then drilled into to collect the precious fossil fuels.

By Fiorite — On Jun 17, 2010

@ Parmnparsley - I can answer some of your questions. Crude oil is the liquid form of petroleum, natural gas is the gaseous form of petroleum, and oil shale and sands are the semi-solid forms of petroleum. Both petroleum and coal come from decayed organic matter that was compressed underneath layers of sedimentary rock deep below the Earth’s surface.

What distinguishes crude oil and all other forms of petroleum from the different types of coal is that they are almost always formed from the decaying matter of marine animals. Coal on the other hand is formed from decaying plant and animal matter that was found above the ocean’s surface. Even oil and gas reserves found on land are the result of long dead marine ecosystems.

The reason that the marine life that forms oil did not just completely decay is because the process of decaying requires oxygen. The underwater decay process quickly uses up the available oxygen making it possible for all of these dead organisms to be preserved and buried. This is the same reason shipwrecks can be preserved for thousands of years under the sea.

Eventually the increasing pressure and heat of the maturation process breaks down the preserved organic matter into gaseous and liquid complex hydrocarbons. The non organic sediment that was buried with the dead organisms is subsequently turned into sedimentary rocks like limestone and shale. Sometimes the oil is homogeneously mixed with the permeable source rock creating oil shale, other times it is mixed with fine rock particles creating the oil sands.

The end result is huge buried deposits of oil sands, oil shale, natural gas, and different grades of crude oil.

By parmnparsley — On Jun 17, 2010

This article did a great job describing the different types of Crude oil, and their uses, but how do the different types of crude form? Why are some crude oils light and sweet while others are heavy and sour? What makes crude become oil sands or oil shale? What time period is crude oil from, or is it being formed during every era of the earth's development? I would be curious to know any or all of these things.

People always talk about how oil is a limited resource, but I would love to know how limited this resource is. Will any of our organic plant and animal matter end up as oil one day, or do conditions have to be just right? Can anyone clue me in on the natural processes required to form crude oil?

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