It only takes a few months to wipe out an economy that relies on a single resource by cutting off access to all credit to fund production. It's pretty simple. Plus the sanctions blocked CITGO from sending profits/dividends back to Venezuela. The result is essentially the same as seizing assets. We've basically wiped 7% from their GDP overnight.
You probably know a lot about oil but you don't know shit about finance. You're out of your league on this one junior.