Sedimentary or tectonic structure where oil and/or natural gas has accumulated. These are structural highs where a porous rock unit is capped by an impermeable rock unit. Oil and gas trapped within the porous rock unit migrate to a high point in the structure because of their low density. A trap is any feature or characteristic of a formation which will allow the accumulation, but not the escape, of oil or gas. Traps can be structural – namely the trap has been produced by deformation of the beds after they were deposited either by faulting or folding or they can be stratigraphic traps – formed by changes in the nature of the rocks or their layering. Structural traps are usually easy to see on seismic; multiple targets are possible in a single prospect and they are usually lower risk. Stratigraphic traps are more difficult to see on seismic and entail higher risks.