Crude oil was formed over many millions of years, with the oldest deposits estimated to be around 500 million years old and the youngest approximately 50 years. While it may not be immediately apparent from looking at crude oil, it was all, in fact, once living organisms.
Crude oil significantly impacts society, underpinning virtually everything we do. Without it, we would not have transport, plastics, and many domestic products. Its use is not without its issues, however, as it is responsible for many conflicts around the world and vast amounts of emissions contributing to global warming.
We'll learn the definition of crude oil, its formation and composition, the types that exist, an overview of crude oil distillation, and the products made from it.
The Definition of Crude Oil
It is referred to as crude oil, as when it is extracted from the underground geological formations in which it is found, it contains a mixture of hydrocarbons, all with different lengths, properties and uses. Before they can be used, these hydrocarbons need to be separated from one another in a process known as refining.
Crude oil is a liquid mixture of relatively volatile hydrocarbons of varying lengths.
Fig. 1. A beaker and flask containing samples of petrol, which is obtained from crude oil.
The crude oil industry, and its many products, are referred to as the petrochemical industry. Crude oil acts as the primary feedstock of hydrocarbons used in industry, with refined petroleum products used in everything from plastic to skincare to fuels.
The Formation of Crude Oil
When ancient organisms died, they gradually fell to the bottom of the sea or ocean. Here, they mixed with the sediment found at the bottom, slowly building into a deeper and deeper layer. This layer underwent anaerobic decomposition, which meant the bodies were only partially decomposed. Over time, further layers build up on top of this seam of partially broken-down organic matter, compressing it.
A popular misconception about crude oil is that it is formed from dinosaurs. While the image of us pottering about in vehicles powered by the remains of what was once such magnificent animals is quite evocative, it is unfortunately not true. Crude oil is predominantly formed from tiny marine plankton and algae remains.
The sediment eventually compacts into rock kilometres thick, which then exerts massive pressure on the seam of organic matter. This pressure, along with the high temperatures under kilometres of rock, causes the organic matter to convert into oil, along with shattering the vein of rock above the oil into shale. This shale allows the oil to travel up before its passage upwards is blocked by the next layer of impermeable rock, a reservoir rock. We then drill into this layer when extracting oil for use.
Fig. 2. An example of the oil rigs used to extract oil via offshore drilling.
It is important to remember that while an area may not be currently underwater, the world has undergone drastic changes over millions of years, so what is now land may once have been underwater.
This is how we can find oil and gas underground rather than just under the sea.
An example of this transition from underwater to land is that of the Himalayas. Despite now being well above sea level, the rock which makes up much of this mountain range was once underwater in an ancient sea known as the Tethys sea. This sea eventually dried up and receded, leaving the Himalayas hundreds of miles from the nearest ocean. However, fossilised remnants of the ancient sea's inhabitants were left behind and now appear within the mountain range.
The length of time required for the layer of organic material to build up and be converted into oil makes crude oil a non-renewable resource. While more may eventually form, this would be over such a long timescale that for practical human use, it is effectively finite.
Difference Between Crude and Refined Oil
Refined oil has had all the impurities that could cause harm removed. It is more suitable for human use. It is also easier to store than crude oil as crude oil is easily oxidised and deteriorated. Crude oil is refined to meet set standards.
The Composition of Crude Oil
As discussed in the above definition section, crude oil is a complex mixture of relatively volatile hydrocarbons with varying lengths.
As the name suggests, hydrocarbons are molecules comprised solely of hydrogen and carbon.
The carbon generally forms a backbone of varying shapes and configurations, with the hydrogens bonded to them. The ability to create this backbone stems from carbon atoms' ability to form 4 covalent bonds.
These bonds underpin organic chemistry, with the ability of carbon to start families of similar compounds, known as homologous series, which may contain chains or rings of carbon atoms.
Hydrocarbons generally exist in one of 4 states: gases, liquids, low melting or polymerised solids. Hydrogens can be classified using many methods; however, the two main ones are by whether they are fully saturated with hydrogen or not and by the types of bonds present.
Saturated VS Unsaturated Hydrocarbons
Saturated hydrocarbons have the maximum possible number of hydrogen atoms bonded to each carbon. They include only single bonds in the carbon backbone. This backbone may be a straight or branched line of carbons or contain one or more rings.
When classifying hydrocarbons by the bonds present, these are known as alkanes.
On the other hand, unsaturated hydrocarbons contain one or more double or triple bonds between carbons. This means that at least two carbons are bound to less than the maximum number of carbons. Like saturated hydrocarbons, these can have straight, branched or ring-containing structures.
When classifying by bond types, those with double bonds are called alkenes, whereas those with triple bonds are known as alkynes.
If a hydrocarbon contains an aromatic ring, then the hydrocarbon is known as an aromatic hydrocarbon.
Alkenes are hydrocarbons containing at least one carbon-to-carbon double bond. The double bond also enhances their reactivity, which is why these are generally not common in crude oil. The general formula of alkenes is CnH2n.
If you are interested, please read up on the IUPAC naming of alkenes
Alkynes are hydrocarbons containing at least one carbon-to-carbon triple bond. As a homologous series, Alkynes do not have a general formula, but simple examples with only one triple bond can be represented by the formula CnH2n-2.
This means that alkenes and alkynes are unsaturated, as the extra bond takes up a space that could be used for hydrogen.
Alkanes
Alkanes, as discussed above, are hydrocarbons containing only single bonds with no other functional groups. They have a general formula of CnH2n+2, where n is the number of carbon atoms within the molecule and is saturated with hydrogen, meaning each carbon is bound to the maximum possible number.
General Formula - A chemical formula which reflects the formula of an entire class of compounds, such as the homologous series alkanes.
The formula of each alkane differs by CH2 from those on either side of it in the homologous series. The physical properties of the alkanes gradually change as you go along the series, an example of which is the boiling point increasing progressively as the chains get longer.
The fractions produced when crude oil is refined using fractional distillation largely contain mixtures of members of this homologous series.
The naming of alkanes is relatively simplistic, with family members always ending in the -ane suffix.
Alkanes are generally named for the number of carbons contained within their structure. If the fact that it is a straight vs branched chain is vital to their properties, then the prefix n- is added to the name of the straight chain alkane to distinguish it from the branched equivalent.
Some examples of alkanes are:
| Formula | Name |
| CH4 | Methane |
| C2H6 | Ethane |
| C3H8 | Propane |
| C4H10 | Butane |
| C5H12 | Pentane |
| C6H14 | Hexane |
When an alkane has five or more carbon atoms, the naming uses the appropriate numerical prefix, such as penta-, drops the a from the end and adds the suffix ane.
An eight-carbon alkane, for example, would use the prefix octa-. We remove the a from the end to give us oct, then add the -ane suffix to leave us with octane, which you may recognise from its addition in varying amounts to fuels for cars.
Types of Crude Oil
There are four main types of crude oil and this affects fractional distillation which is explained further below.
- Very light oils: these fuels are very volatile and include:
- Light Oils: these are moderately volatile and of medium toxicity. They include:
- Most domestic fuel oils
- Diesel fuel oils
- Medium Oils: these oils are the most common and have a higher viscosity and a lower volatility in comparison to light oils.
- Heavy fuel oils: these are the most toxic of crude oils as well as the most viscous and least volatile.
- They include heavy marine fuels.
Viscosity refers to how easily the oil flows. For example: honey is more viscous than water.
Volatility refers to how easy the oil evaporates. A substance that is a gas at room temperature is highly volatile.
Crude Oil Distillation
As crude oil is a mixture of many different compounds, all with varying chemical and physical properties, these must generally be separated for any of the individual components to be used, generally through a process known as fractional distillation. These refined products can then be manipulated further by modifying their structures. One way this is being done is through cracking.
Fig. 3. An oil refinery in Kuwait, processing crude oil into useful end products such as petrol and diesel.
As discussed in the alkane section, members of a homologous series exhibit a gradual change in physical properties as you progress along the series. This includes boiling point, and this gradual variation allows us to use fractional distillation to separate the mixture that is crude oil into its component compounds.
The crude oil is heated at the base of a large tower, with several condensers at varying heights. The heating causes many components of the crude oil to vaporise and begin to move up the tower. As they rise, they gradually cool. By knowing the temperature at which each desired fraction condenses and the rate at which the vapours cool, we can position condensers to condense and collect each fraction.
Longer hydrocarbons have a higher boiling point and therefore condense first, meaning they are pulled off lower down the tower. The shorter a hydrocarbon is, the lower its boiling point, and the further it will move up the tower before condensing.
For a more in-depth look at fractional distillation, look at our full-length article on the subject!
Cracking
The varying fractions we can extract from crude oil have varying uses, from lubrication to gases to fuel stoves, etc. Because of these varying uses, there are differing levels of demand for each of the fractions. The ratio of the fractions extracted from crude oil does not always match this demand, meaning scientists needed to develop a way to manufacture more of specific fractions. They can do this by shortening, extending or otherwise modifying hydrocarbons.
Cracking is the technique scientists use to shorten hydrocarbons, creating more short-chain fractions.
For example, if the demand for petrol exceeds the need for the lubricating oils produced lower down the distillation tower, the long chains found within this fraction can be broken down, forming shorter chains, which can then be added to the petrol fraction, increasing the amount available for sale.
Cracking is a complex process with several methods, but each generally uses a catalyst and high temperatures.
The exact techniques used within the industry for cracking are explored in more depth in our full-length article on the subject.
Products Made From Crude Oil
Most derivatives of crude oil are used as fuel for diverse activities. Here is a list of the main products of fractional distillation and how we use them:
- Liquid petroleum gases: fuel for domestic use (heating, cooking)
- Petrol or gasoline: fuel for most cars
- Naphta: chemicals and plastic manufacturing
- Kerosene of paraffin: fuel for aircraft
- Diesel: fuel for trains and some cars
- Fuel oil: fuel for ships and power stations
- Bitumen: for roads and roofs
Crude Oil - Key takeaways
Crude oil is a non-renewable resource produced over millions of years from the partially decomposed remains of ancient plankton and algae.
It is generally found within deep rock formations and extracted through drilling.
It is a complex mixture of different hydrocarbons, generally belonging to the homologous alkane series, which are saturated hydrocarbons containing only single bonds.
The different hydrocarbons have varying properties depending on their length, meaning they all have different uses. These are referred to as fractions and are extracted from the mixture in a process known as fractional distillation.
Hydrocarbon chains may also be shortened in a process known as cracking to produce more of the shorter chain fractions.
References
- Fig. 1. Petroleum sample (https://commons.wikimedia.org/wiki/File:Petroleum_sample.jpg) by Nefronus, CC0 Public domain
- Fig. 2. An oil rig offshore Vungtau (https://commons.wikimedia.org/wiki/File:An_oil_rig_offshore_Vungtau.jpg) by Genghiskhanviet, Public domain
- Fig. 3. Mina-Al-Ahmadi oil refinery night (https://commons.wikimedia.org/wiki/File:Mina-Al-Ahmadi_oil_refinery_night.jpg) by Grubb, Public domain
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