Welcome to Season 5 Episode 4 we are going to discuss the formation of hydrocarbons. Because of their impact on our climate the conversations around hydrocarbons are (rightly) focused on their extraction and impacts from use. However, an important part of the equation is their formation and understanding what they are and where they come from.

An important part of connecting with the world around us comes from understanding where our everyday items come from. General knowledge typically includes wood, metals, and ceramics, but materials like plastics and the fuel that runs our lives are not as well known.

Because the processes that form hydrocarbons are linked to sedimentary processes (the first rock type we are covering) I thought it was important to talk about them on their own.

So do our cars run on dinosaurs?

LIsten here:


What are hydrocarbons? Simply put hydrocarbons are molecules made of carbon and hydrogen atoms. There are a variety of different hydrocarbons that are classified by the number of carbon atoms per molecule. Some of these might even be familiar names
1 methane
2 ethane
3 propane
4 butane
5 pentane
6 hexane
7 heptane
8 octane
9 nonane
10 decane
11 undercane
12 dodcane

When hydrocarbon compounds are burned the carbon is released in the form of carbon dioxide when the released carbon bonds with oxygen.

Formation :
The vast majority of hydrocarbons are either in the form of crude oil, petroleum, coal, or natural gas. To explore the differences in depositional environments we are going to talk about coal and petroleum.

Coal starts its life in a swamp.

The plant life as it dies builds up within the swamp under the water in an environment low on oxygen. As long as the accumulation of plant material is greater than the decomposition rate, peat can form. Peat is the first step in the formation of coal.

Peat has historically also been an important material in the use of making heat and for cooking. Peat is often cut and dried before use as it can still contain some water content

As the material is subjected to increased pressure and temperature the peat losses its water and begins the transformation to coal. Coal is graded by the amount of metamorphism it has undergone.

The lowest quality is called lignite. Of the types of coal it has the least concentration of carbon at 60-70% and the highest moisture content. It is also called “brown coal” and currently is generally burned for electricity.

Subbituminous coal is coal that has undergone more metamorphism and the pressure and heat has driven off more of the oxygen and hydrogen in the coal giving it a higher carbon content of 71-77 %. It is black but not shiny and is also generally used in electricity generation.

Bituminous coal is the most abundant and is about 50% of the coal production in the US it has a carbon content between 77-87% and is a black and shiny coal but if you look closely you can still see some layering in the rock.

The highest quality coal is anthracite coal. It is hard, brittle, black and shiney. It is considered to be a metamorphic rock and has a carbon content over 87%.

For the coal we were in a swamp. For petroleum the water gets deeper and we are now in a shallow sea.

In these seas lived a variety of organisms. As they die they sink down in the water leaving a layer on the ocean bottom. So just like in the case of coal these layers build up over time except in this situation it is these tiny organisms living in the water column building up over time. Much like with coal there is a lack of oxygen in this environment. Once the layer is covered with sediment and begins to compress it transforms into a substance called kerogen. 

Kerogen is the precursor to oil and gas. With increased pressure and temperature the kerogen can undergo a process called catagenesis which is just the way of saying like the metamorphism of coal the kerogen undergoes changes to become a hydrocarbon. This process usually occurs at up to 1 km depth. 

As the material undergoes further changes continue and take a significant amount of time. There are reactions that occur and change the kerogen into one of two classes, one with a low hydrogen to carbon ratio, and one to a high hydrogen to carbon ratio. 

The formation of oil from kerogen occurs within a specific temperature window. Too cool and it stays as kerogen and too hot and it converts to natural gas.

Where are the major deposits
Because of the specific conditions needed for the formation they do not occur everywhere. In terms of oil there are large deposits that are currently being extracted in the middle east, Mexico, Venezuela, Brazil, the United States and Canada, Russia, the North Sea, China, Australia, and some areas of Africa.

With smaller fields in places like SE Asia and New Zealand. Like we covered there are very specific conditions needed to form these deposits and specific conditions for them to accumulate in an economically and minable deposit.

Coal exists on every continent but the largest coal reserves are in the US, Russia, China, Australia and India.

Historically hydrocarbons have not only been used for burning and energy. There are two varieties that you might recognize from the crystal world. These are Shugnite and Jet.


Shungite sample (left photo by: Steven Thomas Pearson-Walsh) and commercial product (right)

Shungite gets its name from the village of Shuga in Krelia, Russia. The carbon content can have a wide range from 5-80% and is generally divided into low, medium, and high carbon shugnite.

The name was first used in 1879. It is a type of pyrobitumen and is formed through the polymerization of petroleum over geologic time analogous to the way tree resin turned to amber.

Although originally found and described in Russia there are also deposits described in Austria, India, DRC, and Kazakhstan.


Jet sample (left photo by: Steven Thomas Pearson-Walsh) and Victorian jewelry (right)

Jet is a type of lignite coal. Remember that this is the lowest grade of coal. The name of get is thought to come from an ancient town of Gagae in modern day Turkey. Several ancient authors mention the stone of gagates that was able to drive away serpents and several other uses. Jet can be soft or hard and that is the main two forms of jet.

Jet has a long history of being used by humans including by the Romans for jewelry. In the medieval period in the west Jet was dominantly used for religious items like crosses and rosary beads. It became very fashionable to have jet jewelry again in the Victorian era.

Call for questions for Extraction
Now that you know how hydrocarbons form we need to talk about their extraction. With a degree in geology I do know a number of people working in the extraction industry.

One of my soap boxes is NIMBYism. Not In My Back Yard. It is important to now how things CAN be extracted because the most responsible way is usually more expensive and when we push extraction out of our backyards it moves to someone else’s where there may be little to no safety or environmental protection.

So please send me comments and questions about the technology and logistics on extraction of hydrocarbons. Send emails and voice notes

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Further reading
If you want to learn more I recommend starting here and then branching out on your own further research! If you need some guidance feel free to reach out.

For reading on Deep Time I recommend Timefulness: How Thinking Like a Geologist Can Help Save the World by Marcia Bjornerud