This article is very good and explains well various reasons (there are more) why the supposed oil backing is pure fiction. If you know Spanish: Highly recommended.
Remember, the government isn't promising to give you oil in exchange for petros. It's promising to give you the amount of bolívares that a barrel of oil is worth, based on the rate that they themselves set (and have set for 15 years, since exchange controls were enacted).
The government is not even promising this. It only promises to
accept petros
at a rate they themselves set and
at a discount as
payment of "national taxes, fees, contributions and public services". If you don't owe anything to the government and don't need it's services (as will be mostly the case for foreign investors) this promise isn't of any direct value to you.