How are Fossil Fuels Formed? (Cambridge International IGCSE Environmental Management)

Coal formation

• Fossil fuels include:

  • Coal
  • Natural gas (mostly methane), which is used in domestic boilers and cookers
  • Crude oil, which is refined into petrol, diesel, and other fuels

Fossil fuels are formed from the remains of plants and animals

• Chemical energy stored in fossil fuels originally came from sunlight
• Energy from the sun was transferred to chemical energy stores within plants through photosynthesis (plants use energy from sunlight to make food)
• Animals ate the plants and the energy was then transferred to their chemical stores

Fossil fuels include coal, oil and natural gas

• Coal is a solid fossil fuel formed from the remains of ancient plants

Stages of coal formation

• Stage 1: Plant accumulation—millions of years ago, dense forests grew in swampy areas

  • When plants died, they fell into waterlogged ground
  • The absence of oxygen slowed their decay

• Stage 2: Peat formation—dead plants built up and slowly changed into peat

  • Peat—a soft, brown, spongy material made of partly decayed plant matter

• Stage 3: Burial and compression—layers of sediment built up over the peat

  • The weight of these layers compressed the peat and forced out water and gases

• Stage 4: Heat and pressure—as heat and pressure increased, peat gradually turned into different types of coal

  • First into lignite—a soft, brown coal with low carbon content and lots of moisture
  • Then into bituminous coal—harder, darker, and containing more carbon, producing more energy when burned
  • Finally, into anthracite—the hardest and purest form, with the highest carbon content and energy value

• The whole process took millions of years underground

• Coal is mainly made of carbon and is burned to produce energy in power stations

The formation of coal

Petroleum (oil) and natural gas (methane) formation

• Petroleum (crude oil)—a liquid fossil fuel formed from the remains of marine organisms
• Natural gas—a gaseous fossil fuel made mainly of methane

Stages of petroleum and natural gas formation

• Stage 1: Organic matter—tiny plants and animals (plankton) lived in ancient seas

  • When they died, their remains settled on the sea floor

• Stage 2: Burial—layers of mud and sand covered the remains

  • Lack of oxygen slowed decay, so the organic matter was preserved

• Stage 3: Heat and pressure—as sediments built up, temperature and pressure increased

  • Over millions of years, this changed the remains into oil and natural gas

• Stage 4: Migration and trapping—oil and gas moved through porous rocks until trapped under impermeable rock layers

  • These traps form oil and gas reservoirs

• Natural gas is often formed alongside oil from the same marine deposits

The formation of petroleum and natural gas

Exam Hint Be careful—many students think all fossil fuel forms come from the same source material and are formed in the same way. Remember: coal forms on land from plants, while oil and gas form under the sea from plankton.