In the context of mitigating the impending global warming due to anthropogenic emissions of carbon dioxide, which of the following can be the potential sites for carbon sequestration? 1. Abandoned and uneconomic coal seams 2. Depleted oil and gas reservoirs 3. Subterranean deep saline formations Select the correct answer using the code given below:

All three are recognised geological storage sites for CO2 under carbon capture, utilisation and storage (CCUS), so the answer is D.

Statement 1 is correct. Uneconomic coal seams can adsorb CO2 onto coal surfaces. The CO2 also displaces methane, which is why the approach can be paired with enhanced coal bed methane recovery.

Statement 2 is correct. Depleted oil and gas reservoirs have proven trapping capacity, since the rock has already held hydrocarbons under pressure for millions of years. Enhanced oil recovery projects inject CO2 into such fields.

Statement 3 is correct. Deep saline aquifers, typically below about 800 m where CO2 becomes dense (supercritical), have the largest estimated storage capacity of the three, though they are the least explored.

The core concept is geological sequestration, which sits alongside ocean and mineral carbonation options. For Mains GS-III, link this to India's climate commitments, the net-zero debate and the cost and safety questions around CCUS.

Trap: Aspirants assume only depleted reservoirs are viable and drop statement 3 or 1 because they sound unfamiliar. The test is whether the formation is geologically sealed, not whether it was previously productive.