IGCSE Environmental Management (0680) Study Notes

Section 2.1: Fossil Fuel Formation

Hello future environmental managers! This chapter is all about understanding where our main energy sources—coal, oil, and gas—actually come from. It’s like being an energy detective, figuring out the ancient recipe for these fuels. Understanding their origin explains why they are **non-renewable** and why they have such a big impact on our environment today. Let’s dive deep into geological time!

The Big Picture: Fossil Fuels Explained

Fossil fuels are often called "fossil" because they are made from the remains of ancient living organisms (plants and animals) that died millions of years ago.

The Ingredients for Fossil Fuels:
  • Ancient Organic Matter: The dead bodies of plants and small marine creatures.
  • Time: Tens to hundreds of millions of years (this is why they are non-renewable!).
  • Heat and Pressure: Caused by deep burial under layers of rock and sediment.
  • Low Oxygen Environment: Essential to prevent the organic matter from fully decaying or rotting away.

Did you know? All fossil fuels are essentially stored solar energy! The ancient plants and plankton captured sunlight through photosynthesis, and that energy is now trapped within the carbon bonds of the fuels we burn today.

Key Takeaway

Fossil fuels are hydrocarbons (molecules containing hydrogen and carbon) created from decomposed organic matter, transformed by massive heat and pressure over geological timescales.


1. The Formation of Coal (Solid Fuel)

Coal is a solid fuel primarily formed from the remains of ancient land-based vegetation, usually in vast swampy forests.

Step-by-Step Formation of Coal:
  1. The Swamp Stage: About 300 to 400 million years ago, huge forests of ferns, trees, and other plants grew in swampy areas.
  2. Preventing Decay: When the plants died, they fell into the swamp water. Because the water was deep and oxygen-poor (anaerobic), the dead matter could not fully decompose (rot).
  3. Peat Formation: This partially decomposed plant material built up, forming a spongy, brown, water-logged material called peat. (Analogy: Peat is like the very first, damp layer of cake ingredients.)
  4. Burial and Compaction (Pressure): Over millions of years, the peat swamps were covered by layers of sediment (sand, mud, silt) brought by rivers. The weight of these overlying layers caused massive pressure.
  5. Heating (Metamorphism): The deeper the peat was buried, the hotter it got. This heat, combined with pressure, squeezed out the water and other impurities.
  6. Carbon Concentration: As water and impurities are removed, the percentage of carbon increases. This process transforms the peat into coal. The longer and deeper the burial, the higher the quality (carbon content) of the coal: Peat → Lignite → Bituminous Coal → Anthracite (the highest quality).
Quick Review: Coal
  • Source Material: Land plants (swamp vegetation).
  • Environment: Swamps / Low oxygen conditions.
  • Key Process: Burial, compression, and heat remove water and concentrate carbon.

2. The Formation of Oil and Natural Gas (Liquid and Gas Fuels)

Oil (petroleum/crude oil) and Natural Gas form from very different source material and environments than coal.

Source Material and Initial Environment:

Oil and gas are mostly formed from the tiny remains of **marine organisms** (like plankton and algae) that lived in the oceans and shallow seas.

Step-by-Step Formation of Oil and Gas:
  1. Marine Life Dies: Millions of years ago, vast numbers of tiny marine organisms died and sank to the ocean floor.
  2. Sediment Mix: They mixed with mud and silt, forming an organic-rich layer on the seabed. This layer was low in oxygen, preventing rapid decay.
  3. Source Rock Formation: Burial under further sediment layers creates great pressure. This material hardens into a rock rich in organic matter (the source rock).
  4. The "Oil Window" (Heat and Pressure): The organic material must be heated to the correct temperature—usually between 50°C and 150°C—known as the **Oil Window**. This heat chemically converts the organic matter into liquid hydrocarbons (crude oil).
  5. Gas Formation: If the burial is deeper or the temperature exceeds about 150°C (past the Oil Window), the oil starts to crack (break down) into simpler molecules, forming natural gas (mostly methane).
  6. Migration and Trapping: Since oil and gas are less dense than rock and water, they migrate upwards out of the source rock. They continue moving until they hit a layer of non-porous rock (the cap rock or trap rock), which holds them captive in a porous rock layer (the reservoir rock).
Analogy for Oil and Gas Formation:

Think of cooking a complex meal. You need the right ingredients (plankton) and the right temperature. If you heat it a little (50-150°C), you get a nice liquid sauce (oil). If you turn the heat up too high, the liquid evaporates and you are left with just gas!

Common Confusion Point:

Coal vs. Oil/Gas: Remember this simple split for your exams:
Coal = Land plants + Swamps
Oil/Gas = Marine life (plankton) + Ocean floor

Key Takeaway

Oil and gas form from microscopic marine organisms that are heated and pressurised within the Earth's crust. Gas forms when temperatures are higher than those required to form crude oil. They require specific rock structures (source, reservoir, and cap rock) to be trapped and stored.

Don't worry if the sheer scale of time seems tricky at first—just remember that the key processes are always the same: ancient life, burial, huge pressure, and steady heat over millions of years!