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  • Writer's pictureSarah Whiteford

How is crude oil formed?

This Fun Fact Friday we’re going to answer the question of how crude oil (petroleum) is really formed. We’ll cover how oil is classified, where it comes from, how it’s extracted, carbon’s role on the Earth, and how alternatives to petroleum help meet demand.

Image Credit: National Geographic

Hundreds of millions of years ago, algae, plants, and other tiny animals lived in the sea. They absorbed and used sunlight for energy, and some of that energy was trapped inside them in the form of carbon. They died and sank to the seafloor, where over time they were covered by layers of sediment. Bacteria and other plants and animals piled on and they were pushed deeper into the Earth. Pressure and temperature increased, transforming the remains into coal, oil, and natural gas. According to OilPrice, “This process, known as diagenesis, changes the chemical composition first into a waxy compound called kerogen and then, with increased heat, into a liquid through a process called catagenesis.”

Most of this oil is trapped underground, retrieved through drilling, although occasionally it makes its way back to the surface on its own. Depending on the composition of different metals and chemicals, oil can be a wide variety of colors from black or brown to taking on a yellow, red, tan or green hue. With very little metals or other chemicals, oil is usually a light hue, almost clear. It’s processed and used to make thousands of everyday items from gasoline and fuel to plastic and medicine. Petroleum exists below the deepest wells humans can build.

Chemistry and Classification

As mentioned above, the crude oil that comes out of the ground varies in color but also in many other ways. According to National Geographic, “Crude oil is composed of hydrocarbons, which are mainly hydrogen (about 13% by weight) and carbon (about 85%). Other elements, such as nitrogen (about 0.5%), sulfur (0.5%), oxygen (1%), and metals such as iron, nickel, and copper (less than 0.1%) can also be mixed in with the hydrocarbons in small amounts.” The molecules’ organization is dependent on the original composition of the organisms and the environment it was exposed to as it formed into oil. Since oil varies widely from light oil, at about 97% hydrocarbons to heavy with about 50% hydrocarbons, it must be refined to make useful products.

There are three classifications for oil: location, sulfur content, and American Petroleum Institute (API) gravity.

Geography is simply from where the oil is retrieved. There are three main locations, plus OPEC:

  • Brent crude comes from 15 oil fields from Scotland to Norway.

  • West Texas Intermediate is produced mostly in Texas, United States – it is considered a sweet and light high quality oil.

  • Dubai-Oman crude is light, sour crude from Dubai in the United Arab Emirates and nearby Oman.

  • Also, OPEC, or the Organization of Petroleum Exporting Countries, is a group of 12 countries that average their price of oil: Algeria, Angola, Ecuador, Iran, Iraq, Kuwait, Libya, Nigeria, Qatar, Saudi Arabia, the United Arab Emirates, and Venezuela

Sulfur is a corrosive and pollution causing impurity in petroleum production. Less than 0.5% sulfur is “sweet” and more than 0.5% sulfur is “sour”.

API gravity is the standard for density of petroleum liquid versus water. Greater than 10 means the oil is light and floats, while less than 10 is heavy and sinks. Lighter oils have more hydrocarbons and require less refining, and are therefore more valuable.

Image Credit: Research Gate

What is a petroleum reservoir?

The underground pockets that hold petroleum are called reservoirs. Since pressure is high deep under the surface of the Earth, petroleum moves toward the surface where there is less pressure. When there is a rock layer than it can’t get through, it collects there, in a reservoir. Different types of rocks affect the movement of oil differently. For example, oil can move through sandstone while shale will trap oil and is impermeable. To find these reservoirs, “seismic reflection” is used. A small explosion sends sound waves underground and as they return to the surface, sensors read the waves and can tell the possibility of a reservoir.

Image Credit: National Geographic

How is the petroleum extracted?

Sometimes, petroleum can reach the surface on its own, as with some parts of Saudi Arabia and Iraq, but most is trapped underground. Oil-in-place is the amount of petroleum in a reservoir. Depending on the location, it may be too difficult to drill to the oil. We won’t get too much into drilling, but there are three main types:

  • Developmental: this is drilling where oil reserves have been found.

  • Exploratory: “wildcat” drilling is drilling where there are no known reserves.

  • Directional: start by drilling vertically, then turn the drill bit to access other resources.

What about oil rigs?

Oil rigs can be on land or offshore. Different methods of drilling include the pump-jacks you may see in a field to the huge offshore oil rigs in the ocean. Offshore rigs are some of the largest human-made structures in the world. They are either floating and tethered to one area or fixed to the bottom of the ocean. They can also act as artificial reefs and attract marine life. Getting all the oil out of a well can be tricky and is called secondary recovery. This is done mostly through drilling below the well to a natural gas reservoir to help force the oil out of the ground.

What is bitumen?

Bitumen is a very viscous liquid that is usually mixed with oil sands, making it difficult to remove. It’s also high in sulfur and heavy metals, making it difficult to refine while producing more carbon emissions. Since it is similar in consistency to cold molasses, hot steam is pumped into a well to soften and remove it. Since the process is complex, it takes two tons of oil sands to produce a barrel of oil. About 85% of it is used to make asphalt for roads, while some is used for other purposes like roofing. Most of the world’s tar sands are in Alberta Canada, in the Athabasca Oil Sands, with other reserves in Kazakhstan and Siberia. Bitumen reserves are beneath the boreal forest, or taiga.

National Geographic on the taiga:

“The taiga circles the Northern Hemisphere just below the frozen tundra, spanning more than 5 million square kilometers (2 million square miles), mostly in Canada, Russia, and Scandinavia. It accounts for almost one-third of all of the forested land on the planet.

The taiga is sometimes called the “lungs of the planet” because it filters tons of water and oxygen through the leaves and needles of its trees every day. Every spring, the boreal forest releases immense amounts of oxygen into the atmosphere and keeps our air clean. It is home to a mosaic of plant and animal life, all of which depend on the mature trees, mosses, and lichen of the boreal biome.”

Image Credit: ArcGIS

What is carbon’s role?

Carbon is essential to life on Earth, cycling between land, water, and atmosphere. It’s a part of every living organism and released through volcanoes, soil, and evaporation. When it becomes trapped in the atmosphere, it absorbs and retains heat, regulating Earth’s temperature. Much of the carbon on Earth is below ground, or sequestered in fossil fuels and soil. Releasing this carbon can change the atmosphere, affecting its ability to regulate temperature. Since 85% of the hydrocarbons in petroleum are carbon, we have to be careful in how we use it.

What are the alternatives to petroleum?

There are some interesting alternatives to oil that are being explored to help meet demand. Bioasphalts are made from crops like sugar, corn, or potato starch. Although they are non-toxic and do not require oil, they require huge amounts of crops, placing a larger demand on agriculture. Algae can act as a source of biofuel, takes up a small amount of space, and does not need freshwater to grow. Many countries like Sweden and Norway are reducing dependence on oil, while companies like bp and Shell are diversifying into many other forms of energy. While other sources of energy are becoming available, petroleum will still play a large part in our energy for the foreseeable future.

Image Credit: National Geographic

Happy Friday!


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