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burkburnett texas oil field 1919.jpg

Source: Library of Congress

Burkburnett Oil Field, Texas, 1919


What is petroleum?

Photosynthesis: The influx of energy

Organic material: Basis for hydrocarbon fuels


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Energy Use at a Glance:

Petroleum in its various forms currently supplies more than 40% of total U.S. energy, and more than 90% of the fuel used in cars and trucks.  In 2005, the United States used about 21 million barrels of oil per day (25% of world total), compared with worldwide demand of about 84 million, according to the United States Energy Information Administration ( This makes the U.S. the largest world consumer of oil, followed by the European Union at 18% of global demand.

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Source: Energy Information Administration




What is Petroleum?

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Petroleum is a naturally occurring, flammable liquid found within rock formations in the earth, and is comprised of a mixture of hydrocarbons and other organic compounds. The hydrocarbons, although varying widely depending on location, are mainly alkanes (or “paraffins”), cycloalkanes, and various aromatic hydrocarbons. The organic compounds may contain nitrogen, oxygen, and sulfur, in addition to trace amounts of metals, such as iron.

Petroleum (like coal) is formed, in general, through the compression and heating of organic materials over geologic time. Contrary to coal, which owes its origins to the decomposition of terrestrial plant life, petroleum can be traced to the burial of marine (and also lake dwelling) organisms, primarily prehistoric zooplankton (protozoans, some types of copepods, worms, krill, crabs, jellyfish, and the larvae of fish and other invertebrates) and algae.

In other words, we can think of oil (or gas or coal) as organic material that is prevented from complete decay. Or, to think of it another way, the rotting tree in your backyard will not turn into a lump of coal; but bury it beneath a swamp, wait a few million years, and you might have a shot.

But, we’re getting ahead of ourselves. To make the carbon compounds that will one day fuel our cars and heat our homes, we need a more basic constituent and, as we’ll see, that constituent gets its energy from the sun.



Photosynthesis: The influx of energy

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Phytoplankton bloom
Nearly all energy that we utilize on earth is directly or indirectly a result of sunlight. Wind energy, for example, is merely the result of convective atmospheric circulation caused by uneven heating of the earth by the sun’s rays. There are exceptions, such as geothermal energy (energy from the interior heat of the earth) and nuclear energy (energy that is stored within the structure of atoms); although one could certainly argue reliance between these energies (the sun (nuclear fusion), and even the earth (radioactive decay), utilize nuclear energy, for example). Fossil fuels are no exception; they gain their energy (albeit, over extremely long periods of time) from the sun’s participation in photosynthesis. The general idea of photosynthesis is as follows: Hydrogen (from water) combines with carbon (from carbon dioxide) using energy from the sun to form organic matter (glucose) and free oxygen:

You may recall that chlorophyll plays a role in this reaction, and you are correct. Chlorophyll is the party responsible for absorbing the correct wavelength of sunlight, which then allows the photosynthesis reaction to take place.

The glucose from this reaction is then synthesized by autotrophic organisms (those that get their energy from the sun) to construct all of the organic matter that they need. This is the basic process that produces all of the organic matter on earth. Primitive autotrophic organisms such as bacteria and algae were among the first to accomplish this on our planet. In our oceans, phytoplankton are the sunlight gathering autotrophic organisms that do this, thus starting marine the food chain, and they inspire the course of events that will eventually lead us to petroleum. But first, let’s build an organism…




Organic material: The basis for hydrocarbon fuels

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Source: U.S. Joint Global Ocean Flux Study (U.S. JGOFS).
Process of organic deposition in oceans.


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Source: USGS
Petroleum…before it was petroleum (zooplankton to be more specific)
The [organic] chemical composition of bacteria, phytoplankton, higher order plants (trees etc.), and the things that eat them, can generally be grouped into proteins, carbohydrates, lipids, and lignin. Proteins are amino acids; they account for nitrogen compounds in organisms and form materials such as muscle fibers, silk, and sponge and can even behave as enzymes to catalyze biochemical reactions. Carbohydrates are sugar compounds such as cellulose and chitin. They are sources of energy and form supporting tissues in plants and certain animals. Lipids are substances that, due to their structure, are insoluble in water, such as animal fats, vegetable oils, and waxes. Fats form a portion of the energy budget in organisms, while waxes are primarily of value for protection. Lignin is an integral part of cell walls in plants and is one of the most slowly decomposing components of dead plant matter. It is highly prevalent in wood (25-30%) and is crucial in helping plants to transmit water in their stems. It is used mainly in supportive skeletal structures (stems or trunks), which, of course, are not needed in aquatic plankton.

Marine plankton are comprised mainly of protein (50% or more), with 5-25% lipids, and not more than 40% carbohydrates. Higher order plants, on the other hand, are largely cellulose (carbohydrate) (30-50%) and lignin (15-25%), and average less than 3% protein. Therefore, we have a significant division between terrestrial and marine biomass. The former is high in lignin and carbohydrate and is more aromatic and oxygen rich, exhibiting a hydrogen- carbon ratio of 1.3 to 1.5. The latter is rich in protein and lipids, reaching a hydrogen-carbon ratio of 1.7 to 1.9. These differences incur different properties in the fuels that they will one day become, and alter the way that they decompose.

Percentages and ratios are from: Tissot and Welte, 1984. Petroleum formation and occurrence, Springer-Verlag, 699pp.




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