AI-generated practice question — model-generated for extra practice, not a previous-year CBSE board question.
Gas is lighter than oil, so it naturally rises and accumulates above the oil layer in petroleum-bearing rock formations.
Source: Conventional Sources of Energy, Petroleum section, Chapter 5
The textbook directly states: "Gas, being lighter, usually occurs above the oil." This is the key fact examiners expect. For 1 mark, one crisp sentence stating the reason (gas is lighter → rises above oil) is sufficient. No extra detail needed.