I think this may provide an answer. It seems they make them and...

I think this may provide an answer. It seems they make them and they break them.

"Another kind of structural trap is the fault trap. Here, rock fracture results in a relative displacement of strata that forms a barrier to petroleum migration. A barrier can occur when an impermeable bed is brought into contact with a carrier bed. Sometimes the faults themselves provide a seal against “updip” migration when they contain impervious clay gouge material between their walls. Faults and folds often combine to produce traps, each providing a part of the container for the enclosed petroleum. Faults can, however, allow the escape of petroleum from a former trap if they breach the cap rock seal."

http://petgeo.weebly.com/structural-trap.html