Traps are formed mainly by folding and faulting of sedimentary strata. The rocks that are above and below must be impermeable so it does not allow gas or oil to move through them.

Oil and gas can move through rocks if they have high permeability. The pore space of these rocks would be great and they would not be cemented very well. Pure sands are the best reservoirs and the oil acts as the glue to keep the sands together. When we extract oil from strata we sometimes have to "pump" another liquid, like salt water to prevent the overlying ground from sinking. Strata that is fractured can also allow oil to be trapped within the "cracks".

Close up of fractured reservoir

A. High permeability;
D. Low permeability