How are Fossil Fuels Formed? (Cambridge (CIE) IGCSE Environmental Management): Revision Note

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

• 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

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

Insert image: Petroleum and natural gas formation
The formation of petroleum and natural gas