Q1. [4] medium exam-ready
Read the following passage and answer the questions that follow:
India's petroleum industry is largely concentrated in rock formations of the tertiary age. The physical conditions under which crude oil accumulates are determined by the geological structures of the area. Oil may be trapped in folded structures, fault zones, or other geological formations that create pockets where hydrocarbons can collect over millions of years. The oil lies sandwiched between layers of rock with different permeability characteristics. Natural gas is almost always found in association with petroleum, and its occurrence follows the same trapping principles.
(i) In an anticline petroleum trap, where does natural gas typically collect and why? [1]
(ii) Why must at least one rock layer in a petroleum trap be non-porous? [1]
(iii) Name any two important petroleum-producing states of India and one major oil field from each. [2]
Generated by claude-sonnet-4-6 · 2026-06-26 13:33 · grounding rag
Model Answer
(i) In an anticline, natural gas collects at the crest (top) of the upfold because gas is lighter than oil. Being the lightest component, it rises above the oil layer and gets trapped at the highest point of the folded rock structure.
(ii) At least one rock layer must be non-porous to act as a cap rock, preventing oil from rising or sinking further. Without it, oil would escape through the rock and no trap would form.
(iii)
- Gujarat — Ankeleshwar is the most important oil field.
- Assam — Digboi (oldest oil-producing state); other fields include Naharkatiya and Moran-Hugrijan.
(Mumbai High/Maharashtra may also be cited as a major offshore area.)
Source: Conventional Sources of Energy — Petroleum, Chapter 5
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Explanation
- (i) The key phrase from the text is: "Gas, being lighter, usually occurs above the oil." Always mention why (lighter than oil) for full marks.
- (ii) The textbook states oil "is prevented from rising or sinking by intervening non-porous layers" — this is the cap rock concept. One line is enough for 1 mark.
- (iii) For 2 marks, name two states + one field each. Gujarat and Assam are directly named in the passage. Avoid vague answers — name specific oil fields.