In the ground back at the well, in oil tankers floating around the world (they are cheap in comparison to the value of the oil carried), and in oil storage farms around the world that are not well reported or regulated at all.
Sorry. No. People who keep track of these things are very, very sophisticated. They keep track of oil tankers and how low they sit in the water. They know the locations of the storage farms and watch them too (You can't hide them the way you hid pot farms, unless you put them underground). It would be very difficult to massively slow production (hide oil in the ground back at the well) without attracting notice.
Further, the production evidence is consistant with the discovery evidence. New discoveries have been falling for 40 years. Do you really believe that someone has conspired to hide evidence of new discoveries for all that time? If you do, then I'd say you're a textbook case of clinical paranoia.