Petroleum (also known as crude oil or simply oil) is a fossil fuel that was formed from the remains of ancient marine organisms.
Millions of years ago, algae and plants lived in shallow seas. After dying and sinking to the seafloor, the organic material mixed with other sediments and was buried. Over millions of years under high pressure and high temperature, the remains of these organisms transformed into what we know today as fossil fuels. Coal, natural gas, and petroleum are all fossil fuels that formed under similar conditions.
Today, petroleum is found in vast underground reservoirs where ancient seas were located. Petroleum reservoirs can be found beneath land or the ocean floor. Their crude oil is extracted with giant drilling machines.
Crude oil is usually black or dark brown, but can also be yellowish, reddish, tan, or even greenish. Variations in color indicate the distinct chemical compositions of different supplies of crude oil. Petroleum that has few metals or sulfur, for instance, tends to be lighter (sometimes nearly clear).
Petroleum is used to make gasoline, an important product in our everyday lives. It is also processed and part of thousands of different items, including tires, refrigerators, life jackets, and anesthetics.
When petroleum products such as gasoline are burned for energy, they release toxic gases and high amounts of carbon dioxide, a greenhouse gas. Carbon helps regulate the Earth’s atmospheric temperature, and adding to the natural balance by burning fossil fuels adversely affects our climate.
There are huge quantities of petroleum found under Earth’s surface and in tar pits that bubble to the surface. Petroleum even exists far below the deepest wells that are developed to extract it.
However, petroleum, like coal and natural gas, is a non-renewable source of energy. It took millions of years for it to form, and when it is extracted and consumed, there is no way for us to replace it.
Oil supplies will run out. Eventually, the world will reach “peak oil,” or its highest production level. Some experts predict peak oil could come as soon as 2050. Finding alternatives to petroleum is crucial to global energy use, and is the focus of many industries.
Formation of Petroleum
The geological conditions that would eventually create petroleum formed millions of years ago, when plants, algae, and plankton drifted in oceans and shallow seas. These organisms sank to the seafloor at the end of their life cycle. Over time, they were buried and crushed under millions of tons of sediment and even more layers of plant debris.
Eventually, ancient seas dried up and dry basins remained, called sedimentary basins. Deep under the basin floor, the organic material was compressed between Earth’s mantle, with very high temperatures, and millions of tons of rock and sediment above. Oxygen was almost completely absent in these conditions, and the organic matter began to transform into a waxy substance called kerogen.
With more heat, time, and pressure, the kerogen underwent a process called catagenesis, and transformed into hydrocarbons. Hydrocarbons are simply chemicals made up of hydrogen and carbon. Different combinations of heat and pressure can create different forms of hydrocarbons. Some other examples are coal, peat, and natural gas.
Sedimentary basins, where ancient seabeds used to lie, are key sources of petroleum. In Africa, the Niger Delta sedimentary basin covers land in Nigeria, Cameroon, and Equatorial Guinea. More than 500 oil deposits have been discovered in the massive Niger Delta basin, and they comprise one of the most productive oil fields in Africa.
Chemistry and Classification of Crude Oil
The gasoline we use to fuel our cars, the synthetic fabrics of our backpacks and shoes, and the thousands of different useful products made from petroleum come in forms that are consistent and reliable. However, the crude oil from which these items are produced is neither consistent nor uniform.
Chemistry
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 way molecules are organized in the hydrocarbon is a result of the original composition of the algae, plants, or plankton from millions of years ago. The amount of heat and pressure the plants were exposed to also contributes to variations that are found in hydrocarbons and crude oil.
Due to this variation, crude oil that is pumped from the ground can consist of hundreds of different petroleum compounds. Light oils can contain up to 97% hydrocarbons, while heavier oils and bitumens might contain only 50% hydrocarbons and larger quantities of other elements. It is almost always necessary to refine crude oil in order to make useful products.
Classification
Oil is classified according to three main categories: the geographic location where it was drilled, its sulfur content, and its API gravity (a measure of density).
Classification: Geography
Oil is drilled all over the world. However, there are three primary sources of crude oil that set reference points for ranking and pricing other oil supplies: Brent Crude, West Texas Intermediate, and Dubai and Oman.
Brent Crude is a mixture that comes from 15 different oil fields between Scotland and Norway in the North Sea. These fields supply oil to most of Europe.
West Texas Intermediate (WTI) is a lighter oil that is produced mostly in the U.S. state of Texas. It is “sweet” and “light”—considered very high quality. WTI supplies much of North America with oil.
Dubai crude, also known as Fateh or Dubai-Oman crude, is a light, sour oil that is produced in Dubai, part of the United Arab Emirates. The nearby country of Oman has recently begun producing oil. Dubai and Oman crudes are used as a reference point for pricing Persian Gulf oils that are mostly exported to Asia.
The OPEC Reference Basket is another important oil source. OPEC is the Organization of Petroleum Exporting Countries. The OPEC Reference Basket is the average price of petroleum from OPEC’s 12 member countries: Algeria, Angola, Ecuador, Iran, Iraq, Kuwait, Libya, Nigeria, Qatar, Saudi Arabia, the United Arab Emirates, and Venezuela.
Classification: Sulfur Content
Sulfur is considered an “impurity” in petroleum. Sulfur in crude oil can corrode metal in the refining process and contribute to air pollution. Petroleum with more than 0.5% sulfur is called “sour,” while petroleum with less than 0.5% sulfur is “sweet.”
Sweet oil is usually much more valuable than sour because it does not require as much refining and is less harmful to the environment.
Classification: API Gravity
The American Petroleum Institute (API) is a trade association for businesses in the oil and natural gas industries. The API has established accepted systems of standards for a variety of oil- and gas-related products, such as gauges, pumps, and drilling machinery. The API has also established several units of measurement. The “API unit,” for instance, measures gamma radiation in a borehole (a shaft drilled into the ground).
API gravity is a measure of the density of petroleum liquid compared to water. If a petroleum liquid’s API gravity is greater than 10, it is “light,” and floats on top of water. If the API gravity is less than 10, it is “heavy,” and sinks in water.
Light oils are preferred because they have a higher yield of hydrocarbons. Heavier oils have greater concentrations of metals and sulfur, and require more refining.
