Passage Summary: Some scientists think oil comes from ancient earth minerals, but a geologist says that's impossible because oil contains traces of life.

Conclusion: The theory that petroleum formed from deep carbon deposits rather than living organisms is incorrect.

Reasoning: Petroleum contains biomarkers, which are molecules that only come from living organisms.

Analysis: The geologist assumes that if biomarkers are in the petroleum, they must have been there from the start. To weaken this, we need to show that biomarkers could have entered the petroleum later. If petroleum formed from deep carbon and then picked up biomarkers as it migrated through layers of the crust containing fossils, the geologist's objection vanishes. Look for an answer that provides an alternative explanation for how those biological markers got into the oil.