Petroleum is a FOSSIL FUEL

What are fossil fuels?

how oil is formed

Source: Library and Archives Canada,

Fossil fuels are hydrocarbon compounds such as coal, natural gas, and oil. They are formed by the anaerobicwithout oxygen decomposition of buried ancient organisms. Generally, coal forms from land sediments, and natural gas and oil form from marine sediment. Natural gas is found sometimes with petroleum, with coal, or by itself. Being less dense, natural gas is most often found on top of oil pools. Fossil fuels are classified as non-renewable resources because they take millions of years to form, and reserves are being depleted much faster than new ones are being formed.

Oil & Gas Start with Organics

The formation of natural gas and oil begins with the accumulation of organic material (mostly the bodies of dead microscopic organisms) on the sea-floor (figure 1). Heavy sediment accumulation then buries the organic material before scavengers, oxygen, and microorganismsA microorganisms is an organism invisible to the naked eye, so it must be viewed with a microscope. can break it down (figure 2). As the sediment builds up, the trapped organic material experiences high heat and pressure, which eventually change the material into oil and then gas (figure 3).

Most natural gas and oil formation dates back between 10 (Cenozoic) and 180 (Mesozoic) million years ago. Only 10% of oil deposits are Paleozoic (more than 200 million years ago). As time passes, oil is trapped in spaces or pores of rocks such as limestone and sandstone where the oil remains until it is extracted (figure 4).

To understand fossils, you have to understand the history of time. The geologic time scale describes the geological and biological chronologyA chronology is an arrangement of events in the order in which they occurred. of the history of the Earth. The time scale is divided into eons, eras, periods, and epochs.

time scale

Modified from: U.S. Geologic Society, Fossils, Rocks, and Time

quizExamine the time scale. In what eon, era, and period do you live?

quizSince the 1930s Sinclair Oil Company has used a dinosaur as its logo. The dinosaur represents Mesozoic deposits that yielded oil in Pennsylvania. But why is using a dinosaur not exactly right?

coal fern fossil

Source: Wikimedia

Making Coal the Hard Way

Coal is generally formed by the accumulation and anaerobic decomposition of plant material. Much coal was produced during the Carboniferous Period of the Paleozoic Era over 300 million years ago when giant swamp forest dominated many parts of Earth. The accumulation of the bodies of these plants through the years resulted in vast coal deposits.

The process of coal formation is called coalification. The stages of coal formation proceed from plant debris through peat, lignite, sub-bituminous coal, bituminous coal, anthracite coal to graphite.

Fossils & Oil

What Are Fossils, and What Stories Do They Tell?


Fossil trilobite, Cyphaspis, approximately 400 million years old. Credit: Michael Johnson

The term fossil is derived from the Latin word fossus that literally means “having been dug up.” The German scientist and father of mineralogy Georgius Agricola (1494-1555) coined the term. Today, fossil refers to the remains or traces of an organism that lived in the past. To be considered a true fossil, a specimen must be at least 10,000 years old.

small fossil fish

Phareodus encaustus. Source: National Park Service, Credit: Arvid Aase

People who study fossils are called paleontologists. Fossils literally tell a story to paleontologists. They can provide hints about ancient geography and where organisms lived. Surprisingly, dinosaur fossils are found in the Antarctic, mastodon fossils are found in Louisiana, and seashells are found high in the Andes. Fossils can also give us a glimpse of how organisms have changed over time. For example, fossils of an ancient whale Ambulocetus natans tell the story of how whales evolved back to the sea from the land.

miscroscope view of foraminifera

Miscroscopic foraminifera.
Source: U.S. Geologic Society, 2000, Wikipedia

Fossils can also be used to identify rock layers that may contain oil deposits. By collecting and identifying fossils from different rock strata, it is possible to describe the ancient environments of different times (and depths) and to find areas more conducive to oil production. After drilling oil over many years, it became obvious that specific types of fossils are found in layers of rock that are more likely to contain oil.

image of a live ammonia

Live ammonia. Credit: Scott Fay

These fossils are called index fossils because they indicate the likely presence of oil. Petroleum geologists and micropaleontologists look for the presence of index fossils in test well samples to make decisions about drilling locations.

Three foraminifera are commonly used as index fossils in the petroleum industry: Haynesina orbiculare, Cibicides robertsonianus and Elphidium excavatum. Forams are single celled ameboid organisms in the Supergroup Rhizaria. PseudopodiaAlso called "false feet," pseudopodia are cellular extensions of eukaryotic cells used in moving and feeding extend through pores in their calcium carbonate-hardened shells (notice the threadlike structures in figure). Ninety percent of all identified species exist only as fossils.

How are fossils made?

The chances of fossilization are rather slim. After dying most organisms decay, are eaten, or are scattered by other animals. The chances of fossilization increase if an organism has hard parts or is quickly buried. Shells, teeth, and bones are more likely to fossilize than soft tissues, so the fossil record of mollusksMollusks include snails, slugs, squids and octopuses. (hard shells) and vertebratesanimals with spinal columns (hard bones) is better than that of flatworms (soft tissue). Sometimes the remains of an organism is buried in silt, ash, or even peat moss, which preserves the organism. Most soft-bodied organism, including bacteria and algae, fossilize this way.

Fossils can be complete organisms, parts of an organism, or traces of an organism. Occasionally a complete organism is preserved in ice, amber, or tar. These fossils are particularly valuable because tissues can be collected and studied, stomach contents can be analyzed, and important observations and tests can be conducted. More commonly a part of an organism is preserved, like the dragon fly in amber (pictured below). Examples of partial fossils include seashells, pieces of wood, bones, and teeth. Trace fossils may include footprints, burrows, gizzard stones (gastroliths), and fossil feces (coprolites).

amber, shark teeth, coprolite fossils

(Left) Dragon fly. Source: U.S. Geologic Society. (Middle) Shark teeth. Source: morgueFile, Credit: Nina Mollnau.
(Right) Coprolite. Source: North Dakota Dept of Mineral Resources

Types of Fossilization

  1. Fossils in ice – Several Pleistocene mammals have been found preserved in ice including the wholly mammoth.
  2. Fossils in amber – Fossilized tree sap can preserve the bodies of animals as well as plant parts.
  3. Fossils in tar – Animals may become trapped in tar pits and preserved. The La Brea tar pits in Los Angeles, California have yielded many quality fossils.
  4. Fossils in peat – Organisms that fall into a peat bog (Sphagnum moss) can become preserved.
  5. Fossils in sediment – Many fossils are preserved in sediment. After dying and sinking into the sediment in a water body, the organism is preserved. Driveway gravel and limestone are sedimentary fossils.
  6. Fossils in coal – Coal itself is a fossil, but many times plant parts or animals can be found in coal deposits.
  7. Mummification – Natural mummification can preserve organisms. The most familiar use of mummification is the traditional burial methods of wrapping and sealing the bodies of royalty.
  8. Petrification – In this process living material is replaced by non-living material such as in petrified wood.
  9. Casts and Molds – A mold forms when an organism has been quickly covered with sediment that hardens, and the organism’s body decays leaving an imprint. A cast is formed when the mold is filled with minerals that then solidify.

quizCan you match the 9 types of fossils with the pictures below? Rollover each picture to check your answer.

(1) Rare fire beetle. Source: Beetle Fossils, The Virtual Fossil Museum. (2) Sediment bat from the eocene at Greenriver. Source: University of California Museum of Paleontology. (3) Bivalve mold. Credit: Mark A. Wilson. (4) Mummy displayed at U.S. Embassy in Lima, Peru. (5) Aurochs skeleton recovered from Danish peat bog. Credit: James S. Aber. (6) Mammoth discovered by scientists at the University of Michigan. Credit: Bernard Buigues. (7) Sabertooth from Fossils: Mammals. Credit: Greg Goebel. (8) Petrified wood. Credit: John Sullivan. (9) Coal fern. Source: Wikimedia.

quizWant to know more about fossils? Check out State Symbols USA. What is your state fossil? What is the state fossil of five other states?

Geologic time

Geological, climatic, or other events serve as markers dividing the time scale. For example, the Cretaceous Period of the Mesozoic Era was brought to an end by an extinction event that began the Cenozoic Era.

quizIf you had a time machine, when would you have to set the dial to see the following?
Use the time scale to identify the period name and an approximate time.

  1. Dragonflies with a 3-foot wingspan
  2. A trilobite
  3. Archaeopteryx
  4. Eurypterids
  5. A mammoth
  6. A velociraptor
  7. Lepidodendron

T-rex animation

Fossil fuels are millions of years older than dinosaurs. That's old!

how old?

Scientists use radioactive (a.k.a. radiometric) dating to determine the age of bones and fragments. Learn more about how fossils are dated with a process called Carbon-14 Dating.

diagram of carbon formation

Diagram of carbon formation. Source: California State Parks

did you know?

The use of oil dates back to glue for ships and roads in 3000 BC. In 1000 BC, the Chinese began refining oil to use in lamps.

But it was the internal combustible engine that really increased our use of oil because oil powered the engine better than steam and electricity.

diagram of a combustion engine

Diagram of the combustion engine. Source: U.S. Patent & Trademark Office

all about coal

According to the U.S. Dept of Energy, "Coal has been used for heating since the cave man." The first commercial coal mine didn't start until 1740s. Learn more about the history of coal use.

Underground mining began in the United States in Virginia and has been an active industry since the 18th century.

Learn how underground mines are excavated. Click the Underground Mining button to view a video showing the mining process.

oil art

This graphic poster depicts the relationship between fossils and fuel. The artist, Sei Shimura, says that he created this poster to "make people realize that fossil fuel is a thing of the past and we need to start turning to alternate sources of fuel and energy like bio fuels and recyclable energy."

poster of dinosaurs and oil rigs

Source: Designboom, Credit: Sei Shimura

Visit the Designboom website to see a larger version of the poster.

fossil free fuels?

Scientists have been trying to create fossil-free fuels, too. Read this story in National Geographic about scientists using diamonds, lasers, and methane to test whether fossil fuels could be created without fossils, that is, abiotically.