In the context of mitigating the impending global warming due to anthropogenic emissions of carbon dioxide, which of the following can be 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:

Correct Answer: The correct option is 1, 2 and 3.

Explanation:

Carbon sequestration is the process of capturing and storing atmospheric carbon dioxide (${\text{CO}}_2$) to mitigate global warming and climate change. Geologic carbon sequestration involves injecting ${\text{CO}}_2$ into deep underground rock formations for long-term storage.

Let us evaluate each of the given potential sites:

1. Abandoned and uneconomic coal seams: Coal seams contain tiny pores and fractures that adsorb gas molecules. When ${\text{CO}}_2$ is injected into unmineable or abandoned coal beds, it preferentially binds (adsorbs) to the coal surface, permanently trapping the carbon. Additionally, this process can displace coal bed methane (CBM), enabling enhanced gas recovery. Hence, statement 1 is correct.

2. Depleted oil and gas reservoirs: Depleted oil and natural gas fields are prime geological storage locations because they have demonstrated structural integrity by holding hydrocarbons trapped under high pressure for millions of years. Injecting ${\text{CO}}_2$ into these reservoirs securely traps the gas in porous rock formations beneath impermeable caprock (an approach often integrated with Enhanced Oil Recovery or EOR). Hence, statement 2 is correct.

3. Subterranean deep saline formations: Deep saline aquifers are underground layers of permeable rock saturated with saltwater (brine). They possess the largest theoretical capacity among all geological sequestration options. Injected ${\text{CO}}_2$ gets trapped structurally under caprock, dissolves over time into the saline fluid, or reacts with surrounding rocks to form stable carbonate minerals. Hence, statement 3 is correct.

Since all three options (1, 2, and 3) serve as viable and effective geological sites for carbon sequestration, the correct code selection is 1, 2 and 3.