Oil, natural gas and coal are known as fossil fuels. They are extremely useful substances and humans spend lots of time and energy to extract them from the earth. But why are they called fossil fuels, when and how did they form?
In an town or village in Ireland on a cool evening there is often a delicious pungent smell filling the air. It comes from the smoke from peat burning on open fires in houses and pubs. Peat is thousands of years old and is compressed plant matter formed in wet areas called bogs. As plants grow and die in a swampy area, their remains build up over the years. Underwater there is little oxygen, so the plant material doesn’t get eaten by bacteria of fungi like they would in the soil. Eventually thick layers build up and can be dug and dried to use as fuel.
In Ireland the layers of peat are on the surface and so easy to get at. What would happen if they got buried under layers of sand and mud over millions of years? As it got buried deeper, the pressure of the rock above would squeeze the peat flat, pushing out much of the water. The rock will warm up as it gets buried deeper (the earth is hot inside) and this will cause chemical reactions to change the peat into coal. Coal is fossilised plant material. It may also contain fossils that record shape of the plants that used to live there.
Plants extract carbon from the atmosphere and combine it with water and other elements to make leaves, stems roots and so on. Plant material is therefore a mixture of the elements Carbon, Hydrogen and Oxygen, with much smaller amounts of Nitrogen, Sulphur and traces of others. As the plant matter is buried and heated, the hydrogen and oxygen is driven off, leaving behind the carbon. Coal comes in different grades, depending on how much is was heated. The lowest grade, known as brown coal or lignite contains only 60-75% Carbon. The highest grade, anthracite has over 90% carbon and so contains much more energy per kilogram. It is a valuable fuel.
What about oil? This forms in a similar way – organic matter accumulates in a place with little Oxygen, is buried, ‘cooked’ and turned into a useful substance and store of energy. Coal forms from peat which is plant matter in wet places on land. Oil forms from organic matter that settled on the sea bed to form a substance called oil shale.
There’s a wide source of organic matter that sinks to the bottom of the sea. Plankton is the name for an enormous range of life that floats in the sea, ultimately powered by photosynthesis. It’s an amazing source of energy and powers many of the food webs within the oceans. In the summer, huge areas of ocean turn green as plankton grown in the sun. This green phytoplankton uses photosynthesis to grow. Zooplankton are tiny floating animals that eat the phytoplankton. Small pink shrimp called krill eat the plankton and are in turn eaten by blue whales, the largest living creatures.
Plankton that doesn’t end up in whales or other animals eventually sinks to the sea bed. Plant material from the land also flows into the sea, including spores and pollen, also resins and lipids, parts of plants that decompose slowly.
Imagine yourself in a warm ocean about 200 million years ago, in the Jurassic period. You are near the equator and the sun is bright overhead. The ocean is warm and shallow. The ocean is growing wider as well, and the mid-ocean ridge where new crust is forming mixes iron and other nutrients into the water. Plankton is everywhere, supporting a rich ecosystem including ammonites – like squid but with hard coiled shells – hunted by fast swimming ichthyosaurs, creatures that look like modern dolphins but which are actually reptiles. The ammonites can’t eat all the plankton and it settles onto the sea bed where it forms a dark smelly mud made up of over 5 percent organic carbon.
As the millions of years pass, the mud gets buried and slowly heated. Plankton is different from peat. Chemically it is much more complicated and contains organic compounds like proteins and lipids that are long chains of hydrogen and carbon. As the mud turns into rock a substance called kerogen is formed as the organic compounds decompose. It may contain more Hydrogen than Carbon, but it’s composition is extremely varied.
Rocks containing kerogen can be found at the surface today – they are called oil shales. With a little effort they can be made to burn, but as they are they are not a useful fuel. To produce oil, kerogen needs to be cooked.
Cooking is using heat and sometimes pressure to turn organic matter into something useful to humans. Heating oil shale to around 60 to 120 degrees C produces petroleum (otherwise known as crude oil). This range of temperatures is known to oil geologists as the oil window. At higher temperatures it becomes over-cooked and natural gas is formed but not oil. These temperatures are not super high. You could cook kerogen in your kitchen, but I wouldn’t recommend it.
In the oil industry’s early history people extracted petroleum by directly heating oil shale. But they stopped when they realised that nature had already done it for them. Let’s go back to the oil shale that formed 200 million years ago. It’s now sitting deep under the Middle East, in Saudi Arabia or Iraq and it has been heated through the oil window. As a source of the oil, these layers are known as the source rocks. Oil is a light liquid, so it flowed up into the rocks above. Some of these contained spaces within them for the oil to sit in. The spaces are sometimes the gaps between grains in a sandstone, or maybe cracks in a limestone. These are known as the reservoir rocks. In many places the oil keeps rising up until it reaches the surface, where it might form a tar pit on the surface. Often the oil is stopped from rising up. This can be done by a seal or cap rock, a layer that it can’t flow through perhaps made from salt that evaporated from the ocean, or another layer of shale. The place where the oil accumulates, where it is caught beneath the seal, is known as a trap.
The way I’ve described oil being formed from the remains of past life is known as the biogenic origin theory. Against that, some scientists believe that oil or gas can form underground from carbon trapped deep within the earth. This abiogenic (“without life”) origin theory is not widely believed. We don’t well understand how carbon behaves deep in the earth and it’s not impossible that small amounts of petroleum form in this way, however there is a lot of evidence for the biogenic origin theory. The most convincing piece of evidence is that oil fields are never found in rocks that aren’t sedimentary ones. Igneous rocks, formed from molten rock, contain very little carbon and never any oil. Metamorphic rocks have been heated too much. Geologists have identified source rocks in metamorphic areas, but now they contain only graphite, a form of carbon that is no good as a fuel.
Another line of evidence for a biogenic origin is that we can link oil in a trap to a particular source rock by studying chemicals called biomarkers. These are complex organic chemicals that have survived unchanged from the remains of life that formed the source rock. We can even use these to infer the age of the organisms that formed the oil. Some biomarkers are only found in oil from really old sediments, others in oil from recent sediments. As life evolved into new forms, so new biomarkers were created, to survive and be found as chemical fossils in oil.
Thick layers of sediments are found in places called basins – this is where most oil is found. Oil geologists investigating a new sedimentary basin will study it in a variety of ways. They will drill holes and look at the fragments of rock that come back to the surface as they go deeper. They’ll also perform seismic surveys by making small explosions and listening to the sounds that come back. Like bats using sonar, they can use this to understand how the layers look underground. They’ll also try and understand the geological history of the area. They are looking for suitable source rocks, with reservoir rocks above. The source rocks must also have been heated so they pass through the oil window. If any of these things are missing, oil will not be found. Finally oil traps must be found, which can be more difficult – there can be a lot of luck and drilling the holes to find them is very expensive.
Oil geologists are not looking so hard for new oil fields at the moment, as the price of oil is relatively low. The reason for this is the way in which new technologies, such as fracking have been used in the USA. Sometimes source rocks still contain large volumes of oil and gas that hasn’t migrated upwards. Fracking is using water and sand to fracture the source rocks and extract this oil and gas from a small area. This technique can be used in areas without ‘conventional’ oil and gas reserves, where it is pumped out of the reservoir rocks via a single oil well.
Fracking has transformed our understanding of the amount of oil that can usefully be extracted. Before fracking changed things, some experts believed that we had reached ‘peak oil’, a point where global supplies of oil had started to run out. This idea was based on studying individual oil fields, where production reached a peak and then afterwards rapidly declined. They believed that known oil fields were close to their peak and that new ones were not going to be found. Fracking has changed this. It turns rocks previously thought to unsuitable for extracting oil into rich sources of it. Estimating the global amount of oil remaining is extremely difficult – new technologies might continue to improve our ability to extract oil more efficiently.
Just because we can find and extract more oil doesn’t mean we should. There are very important reasons to leave as much oil as possible within the ground. To explain why we need to understand the carbon cycle.
The Carbon atoms inside your body got there via food you ate that helped you grow. The atoms themselves were made in the heart of a star billions of years ago. Since then they haven’t changed at all, only moved around. They made their way to earth and since then they have moved from place to place countless times. The way in which carbon moves around the earth is known as the ‘carbon cycle’.
Part of the carbon cycle is purely biological. A plant pulls CO2 from the air and uses it to build organic compounds and grow. You eat the plant and digest it, turning it into energy storing chemicals in your body. Inside you body, you use Oxygen (taken from the air via your lungs and blood) to react with those chemicals, producing energy and CO2 that returns to the air. Some Carbon atoms inside you right now may once have been part of a famous person, or a dinosaur.
The geological part of the carbon cycle is also complicated. Carbon captured by plants or plankton can be buried underground and so does not return to the atmosphere for a long time. Limestone is a rock made of the remains of animals that build shells or structures out of calcium carbonate, CaCO3, which is another way of ‘fixing’ carbon and removing it from the atmosphere.
If the carbon is swallowed in a subduction zone, like under the Andes in South America, it may be buried hundreds of kilometres under the ground. There some of this Carbon is converted into precious diamonds. Later some of these diamonds return to the surface via odd volcanic eruptions. Places where oil seeps to the surface or coal or limestone is eroded and turned back into CO2 complete the geological cycle, putting the carbon back into the atmosphere. The Carbon may have been buried for hundreds of millions of years, but it returns eventually. No doubt some carbon atoms have followed that journey many times.
We know from studying ancient climates that levels of CO2 in the atmosphere have varied over time. We can also see that the climate was hotter when CO2 levels were higher. We can also see that the changes are very very slow. Natural erosion of limestone, or of coal or oil seeps increase CO2 levels but slowly, over millions of years.
Over the last few hundred years, humans have changed the carbon cycle in a really dramatic way. Through the burning of fossil fuels, huge amounts of Carbon that was buried for hundreds of millions of years have been returned to the atmosphere.
It’s hard to describe how quickly this change has happened, from the Earth’s point of view. Carbon that was produced by 100s of millions of years of sunlight has been released to the atmosphere over 100s of years. The earth has never seen anything like this before.
First publication by Xiaoduo Media in Front Vision. Front Vision is a Chinese online science magazine for children. Reproduced with permission.