Describe how crude oil forms and explain why it is described as a finite resource.

OCR GCSE Chemistry A: Gateway Science (J248) — C6.3 Interpreting and interacting with Earth systems · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Crude oil is one of the most widely used resources on Earth, providing fuels and raw materials for the chemical industry. However, concerns have been raised about its long-term availability.

Model answer (4 marks)

Crude oil is formed from the remains of ancient biomass, mainly plankton, which were buried in mud and subjected to heat and pressure over millions of years.
It is described as a finite resource because it cannot be replaced on a human timescale; it is being used up faster than it can be reformed.

Examiner tips

  • Use the word 'plankton' and 'buried in mud' to show source and process. Mention 'heat and pressure over millions of years' to explain formation. State 'finite' as 'cannot be replaced' and 'used up faster than it can be reformed'.
  • Keep the answer concise – 4 marks only, so avoid extra detail.

Common mistakes

  • Confusing crude oil with natural gas. Forgetting to mention the time scale (millions of years). Using vague terms like 'organic material' instead of 'plankton'.

Mark scheme (4 marks)

  1. Crude oil is formed from the remains of ancient biomass / organisms, consisting mainly of plankton
  2. The organisms were buried in mud (over millions of years) / subjected to heat and pressure over time
  3. A finite resource is one that cannot be replaced / is being used up faster than it can be reformed
  4. Crude oil takes millions of years to form, so once used up it cannot be replenished in a human timescale

Key terms in this question

crude oil · finite resource

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