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
Close up of fractured reservoir
A. High permeability;
D. Low permeability