How do geological processes form coal, oil and natural gas over millions of years?
Explain how coal, oil and natural gas form and why they are non-renewable
A focused answer to the WACE Year 12 Earth and Environmental Science dot point on fossil fuel formation. Covers coal from peat, oil and gas from marine organic matter, source rocks, maturation, migration, reservoir and trap, and why fossil fuels are non-renewable, with WA examples such as the Collie coal basin and North West Shelf gas.
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What this dot point is asking
SCSA wants you to explain how each fossil fuel forms and to justify why they are classed as non-renewable. The unifying idea is that fossil fuels store ancient solar energy: photosynthesis captured sunlight in organic matter, which burial then preserved and concentrated into a fuel.
Coal formation
Coal forms from terrestrial plant matter.
- Dead plants accumulate in waterlogged swamps where low oxygen slows decay, forming peat.
- Burial under younger sediment compacts the peat and drives off water and volatiles.
- Increasing heat and pressure transform peat through lignite and bituminous coal to anthracite, raising the carbon content and energy density at each stage.
Western Australia's main coal resource is the Collie Basin, south of Perth, which has supplied coal for electricity generation.
Oil and natural gas formation
Oil and gas form mainly from marine organic matter.
- Microscopic plankton and algae die and settle into oxygen-poor sea-floor mud, where they are buried before decaying.
- As burial deepens, heat converts the organic matter first into a waxy substance and then, within a particular temperature range often called the oil window, into liquid oil. At still higher temperatures, natural gas forms.
- Being less dense than water, oil and gas migrate upward through porous rock until trapped.
A petroleum system needs four elements working together:
- a source rock rich in organic matter,
- a porous, permeable reservoir rock to hold the fluids,
- an impermeable cap rock or seal, and
- a trap, a geological structure that stops further migration.
The North West Shelf off WA is a major gas province where these elements combined to form large offshore gas accumulations.
Why fossil fuels are non-renewable
A resource is non-renewable when it is consumed far faster than natural processes can replace it. Fossil fuels take millions of years to form through burial, heating and maturation, but are extracted and burned over decades. The rate of use vastly exceeds the rate of formation, so on any human timescale the stock is effectively fixed. This is the core reason the syllabus classifies coal, oil and gas as non-renewable, and it underpins later arguments about transitioning to renewable energy.