Fossil Fuels

You’ll be able to

Describe how coal, oil, and natural gas form and are extracted
Compare the fossil fuels by their carbon emissions and energy content
Use net energy (EROEI) to compare energy sources

What fossil fuels are and how they form

Fossil fuels — coal, oil (petroleum), and natural gas — are the buried, compressed remains of ancient organisms transformed over millions of years by heat and pressure. Coal formed from plant matter in ancient swamps; oil and natural gas formed mostly from marine plankton buried in seafloor sediments. Because they take millions of years to form but are burned in decades, fossil fuels are nonrenewable on any human timescale. Together they still supply the large majority of the world’s energy.

Extraction and its impacts

Each fuel is extracted differently. Coal is mined by surface or subsurface methods (Unit 5), then burned in power plants. Oil is pumped from underground reservoirs and refined; spills during drilling or transport (the Deepwater Horizon blowout, the Exxon Valdez tanker) devastate ecosystems. Natural gas is increasingly obtained by hydraulic fracturing (fracking) — injecting high-pressure fluid to crack shale and release trapped gas. Fracking can contaminate groundwater, trigger small earthquakes, and leak methane, a potent greenhouse gas.

Carbon and energy differences

The three fuels are not equal in their pollution. Burning any of them releases CO₂, but per unit of energy, coal emits the most CO₂, oil is intermediate, and natural gas emits the least (roughly half of coal’s) while producing far less sulfur, particulate, and mercury pollution — which is why natural gas is often called a "bridge fuel." Coal combustion is the leading source of sulfur dioxide (acid rain), mercury, and particulates. All fossil fuels drive climate change; natural gas simply does so less per unit of energy delivered.

Net energy (EROEI)

EROEI = energy returned / energy invested

The energy return on energy invested measures how much usable energy a source yields per unit of energy spent obtaining it. A higher EROEI means a better net-energy source; a ratio near 1 means the source is barely worth extracting.

Worked example

Conventional oil returns about 30 units of energy for every 1 unit spent extracting it, while a certain biofuel returns only 2 units per 1 unit invested. Compare their net energy using EROEI.

1.Write each as EROEI = energy returned ÷ energy invested.
2.Conventional oil: EROEI = 30 / 1 = 30 (a 30:1 return).
3.The biofuel: EROEI = 2 / 1 = 2 (a 2:1 return).
4.Compare: 30 is far greater than 2, so conventional oil delivers much more usable energy per unit invested.

Answer: Oil’s EROEI of 30 vastly exceeds the biofuel’s 2. The higher the EROEI, the more net energy a source provides — a key reason high-EROEI fossil fuels have been so hard to replace.

Checkpoint

Per unit of energy produced, which fossil fuel releases the LEAST carbon dioxide and the fewest air pollutants such as sulfur and mercury?

Tip

Rank the fossil fuels by "dirtiness": coal (most CO₂ and pollutants) > oil > natural gas (least). If a question asks which fuel to switch to for lower emissions, natural gas beats coal and oil; if it asks which is worst, the answer is coal.

Checkpoint

Hydraulic fracturing (fracking) is used to extract which resource, and what is one of its major environmental concerns?

On the exam

Net energy (EROEI) is a favorite comparison tool: higher EROEI = more usable energy per unit invested. Sources with EROEI near 1 (some biofuels, tar sands) barely pay back the energy used to produce them — a strong argument against them even before considering emissions.

Answer the 2 checkpoints as you read.