How Does Nature Make Oil? Diagenesis, Catagenesis and Hydrocarbons

The single most important natural resource in human history — with respect to the development and use of technology — is oil. Fossil fuels generate our electricity and propel our vehicles, ships, and airplanes. Fossil fuels are the reason we have plastics and synthetics, the materials we use to make computers, telephones, televisions, and home appliances. With almost no exceptions, fossil fuels are a part of every person’s life on the planet. From the rich to the poor, regardless of skin color, religion or nationality, and around the world, everyone’s life is fossil fuel dependent. 

Why we need fossil fuels is obvious. But, how fossil fuels come to be is a different matter. While most people have a general idea as to how oil comes to be, most of us do not know the specifics. That is understandable given the complexity of the geological processes that produce crude oil and natural gases.

But, to understand how geological processes produce crude oil and natural gas, it is first necessary to understand exactly what fossil fuels are.

What is Oil Made of?

Though an elementary explanation, it is accurate nevertheless: crude oil is the consequence of compressed, heated, non-degraded biomatter. 

The process of sediment piling upon, compressing, and heating biomatter is called diagenesis. As hundreds of feet of sediment stacked upon biomatter heats and compresses it, diagenesis produces kerogen — the precursor of hydrocarbons. Hydrocarbons are the valuable component of oil. Hydrocarbons are the molecules and molecular chains in oil that burn, combust, and ignite. 

As the name implies, hydrocarbons are made of two elements: hydrogen and carbon. “Carbon and hydrogen make up around 98 percent of the content of a typical crude oil type. The rest is made up of sulfur (1-3 percent), nitrogen, oxygen, metal and salts (less than 1 percent each).” 

The hydrocarbons in crude oil are called paraffin hydrocarbons. The word “paraffin” is synonymous with universal. A paraffin hydrocarbon is one of many numbers of saturated, alkane hydrocarbons found in crude oil.

The list of paraffin hydrocarbons is so large that the total number of them is unknown. As such, the chemical formula for paraffin hydrocarbons includes an integer that expresses an unknown quantity. The letter “n,” in the formula CnH2n+2 indicates that the number of both hydrogen atoms and carbon atoms in paraffin hydrocarbons can vary. 

Varying hydrogen and carbon atom counts in a fuel molecule — or fuel molecular chain — determines a fuel’s type. For example, the molecule chains in diesel have a large number of carbon and hydrogen atoms — C12H24  — while natural gas (methane) is a short chain with a small number of carbon and hydrogen atoms: CH4.

In other words, because there is a wide range of different types of paraffin hydrocarbons, there are different types of crude oil. And, there are different types of crude oil because there are different variables in the geological processes that produce fossil fuels. 

Step-By-Step Process of Fossil Fuel Production

Originally, the energy found in fossil fuels came from the sun. All biomatter is a product of the sun’s solar radiation and all fossil fuels come from biomatter. The biomatter that produces gas-state fossil fuels and oil comes from, “Dead plankton – both phytoplankton and zooplankton – as well as algae and bacteria that sink to the bottom of an ancient ocean and mix with inorganic, clay-like materials that enter these oceans from streams and rivers.”

An important note, the formation of oil occurs in an anoxic environment. In order for biomatter to eventually become oil, it must not have too much exposure to oxygen or it will decompose and the energy in it is lost. 

In other words, oil is the product of buried, un-decomposed biomatter. 

As sediment settles over the un-decomposed biomatter — hundreds upon hundreds of feet of layers of sediment, — both the sediment and the biomatter mixed in with it begin to heat and undergo a chemical change, diagenesis. “Diagenesis results from any form of physical, chemical, or biological alteration to these relatively young sediments as they are lithified, or converted into a rocky material. The end result of these processes will be to alter the mineralogy and texture of the source material.”

It is during the diagenesis process that the proteins and carbohydrates in un-decayed biomass break down into smaller parts. Broken down carbohydrates and proteins form a substance called kerogen. 

But, diagenesis is only the first of two processes that transform biomass into oil and gas-state fossil fuels. The second process transforms kerogen into hydrocarbons — crude oil and/or natural gas.

Diagenesis and Catagenesis

The process of diagenesis does not produce hydrocarbons. Therefore diagenesis is not the process during which crude oil appears. But diagenesis is the first step in a two-part process that does. “Diagenesis is a process of compaction under mild conditions of temperature and pressure.” Biomatter — like plants, bacteria’s, carrion, algae, pollen, wood, vitrinite, and structureless material — containing proteins, lipids, carbohydrates. More specifically, biomatter contains hydrogen and carbon. 

Biomatter also contains a great deal of water, nitrogen, and sulfur. As biomass undergoes its chemical change into kerogen — as a result of pressure and heat, — the water, nitrogen, and sulfur content of the biomass drops. 

The result is a generation of two biomass byproducts, one organic and the other inorganic: kerogen and bitumen. 

The Organic and Inorganic Consequences of Diagenesis

The first step in the generation of crude oil — diagenesis — converts biomatter into two substances. One is organic — kerogen — and the second is inorganic, bitumen. It is the organic substance kerogen that contains hydrocarbons. And, again, hydrocarbons are the valuable component of crude oil. 

Bitumen has a fraction of the monetary value of kerogen because bitumen has no hydrocarbons. Therefore, bitumen does not combust, ignite, or burn. 

As additional layers of biomatter and inorganic sediment stack on top of kerogen — hundreds of feet of layers which produce extraordinary amounts of pressure and heat — kerogen undergoes further chemical changes. 

This stage of fossil fuel development is called catagenesis. “Catagenesis involves heating in the range of 50° to 150°C [122° to 302°F]. At these temperatures, chemical bonds break down in kerogen and clays within shale, generating liquid hydrocarbons.” 

It is because of differences in the diagenesis and catagenesis processes that different types of oil and gas-state fossil fuels exist. Specifically, the amount of heat and pressure and the non-biomass-related elements in the sediment that determine the different types of crude oil and gas-state fossil fuels. 

Types of Crude Oil

There are hundreds of different types of crude oil. In fact, no two barrels of crude oil are exactly alike. And the differences between crude oils taken from different wells are even greater. And, the differences between crude oils taken from different regions are even greater. So, most crude oils are named after the region from which they are extracted. Examples of crude oil types include West Texas Intermediate (WTI); Brent Blend — which is from the North Sea; — Dubai and Oman Crude; and Canadian Crude.

But, more important than the region from which crude oil is extracted are the qualities of the oil. 

There are three qualities that are important with respect to crude oil. The weight of crude oil is the most important component, the American Petroleum Institute (API) gravity. The second aspect of crude oil that determines quality is sulfur content. Total acid number (TAN) is the third quality of crude oil that determines its value.

Crude Oil Weight

The weight of crude oil is measured in relation to water. While all but the heaviest of crude oils float on water, some float to a greater degree than others. The reason the weight of a crude oil is important is that there is a direct correlation between the weight of oil and its energy density, how many kilojoules of energy an oil will produce per volume unit of measure. 

The reason the weight of oil in relation to water is important is that hydrocarbons are lighter than water. On the other hand, most of the contaminants found in oil — including water, nitrogen, and sulfur — are equal to in weight or heavier than water. 

Simply, the lighter a crude oil, the more fuel it contains per gallon, barrel or liter. 

Causes of Different Crude Oil Weights

The difference in crude oil weights is a symptom of the amount of heat and pressure applied to crude oil during the catagenesis stage of oil formation. During the catagenesis process, hydrocarbons are formed by the heating and application of pressure to kerogen. But, in addition to transforming kerogen into hydrocarbons, that heat and pressure can crack hydrocarbons. 

The cracking of hydrocarbons is the breaking of hydrocarbon molecules and molecule chains into smaller molecules and chains. Long-chain and large molecules constitute heavy fossil fuels like fuel oil and bunker fuel.  Medium size molecules and chains constitute the different weights of diesel. Small molecule chains are the components of light fossil fuels like gasoline and kerosene. 

If cracked under enough pressure and heat, hydrocarbon molecules become gas-state fossil fuels like natural gas (methane) and propane. 

Crude Oil Sulfur Count

Like weight, sulfur count is an indication of how much usable energy is in a gallon or barrel of oil. The higher the sulfur count, the less usable energy a given crude oil type contains. 

Additionally, a high-sulfur crude oil’s emissions contaminate the environment to a greater degree than a low-sulfur crude. The reason being, sulfur combines with elements in the air during combustion to create polluting emissions like sulfuric acid and nitrogen sulfides. These are the pollutants associated with acid rain. 

Furthermore, high sulfur content crude oils are difficult and expensive to refine. The sulfur oxidizes and corrodes the components of a refinery as well as the delivery system used to transport the crude from the well to the refinery.

Cause of High-Sulfur Content

High-sulfur crude oils are the result of a lack of iron in the formation of sediment into sedimentary rock. If there is iron in the sediment/biomatter mixture, sulfur will chemically react with it to create elements like pyrite. However, if the sediment is poor in iron, the sulfur is left as a free agent and contaminates the oil.

Crude Oil Total Acid Number

Similar to the sulfur count of crude oil, the total acid number (TAN) is an indication of the different contaminants in the crude oil. The higher the acid number of a crude oil, the more difficult it is to refine. 

Unless a refinery is equipped to deal with high-tan crude oil, its oxidizing and corroding qualities can destroy the infrastructure of the refinery. As such, refining high-TAN crude oil is expensive because it requires investing in the necessary machinery and auxiliary components of a refinery and refining high-TAN crude require regular and intensive equipment maintenance and upkeep. 

Cause of High TAN Crude Oil

It is imperative that biomatter move into the diagenesis state in an anoxic environment and through the catagenesis process in the same. Exposure to oxygen and bacteria cause the hydrocarbon in forming oil to biodegrade. The biodegradation of hydrocarbons generate carboxylic acids, the two most common types being naphthenic acids (NAs) palmitic acids.

Though the process of crude oil formation is highly complex, there are only two stages: diagenesis and catagenesis. During the diagenesis stage, biomatter under pressure and heat breaks up into the organic and inorganic matter, ketamine and bitumen. During the catagenesis stage, ketamine transforms into hydrocarbons and the pressure and weight of the sediment layering break hydrocarbons into different fossil fuel types. 

Without iron or with oxygen and bacteria, the oil formed is impure. While the formation of oil is never ideal under natural circumstances, the greater the iron-to-sulfur ratio and the lower the oxygen content, the lighter and lower-sulfur content crude oil is.


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