Saudi is liquid oil and they are by far the largest single source.
Venz is bitumen that is solid or near solid underground and requires huge amounts of steam to make it liquid to pump it out. They are ultra heavy crude or tar sands and that is also super dirty with sulfur that must be removed down to 15ppm or less by the time it is used down from 6% in the ground.
Canada has huge tar sands which require the same treatments it’s dirty energy intensive and expensive. Look up tar sands pits and think of that on a scale the size of Texas to fuel even a portion of the world’s use.
Venezuelan heavy-oil sands require only modest temperatures relative to Canadian bitumen, to release the hydrocarbons.