Carbon Capture and Storage 101: The Basics

The cap rock (aka 'seal' in petroleum industry) is not necessarily 'dense' (1:43) - density is irrelevant to the purpose served by a cap rock. A cap rock is impermeable (lacks pore space for fluids to migrate), that is what seals the carbon dioxide in the porous reservoir rock below. Good video.

Passing the Cap and Trade legislation in the US is an important first step; it will give an economic incentive to the coal-energy industry to pursue carbon sequestration.