[link|http://news.bbc.co.uk/onthisday/hi/dates/stories/july/31/newsid_4582000/4582773.stm|BBC].
Change of government shouldn't negate treaties or contracts, in most circumstances.
Nobody external can stop Chavez if he wants to nationalize the oil companies. If the past is any guide though he'll be cutting his nose off to spite his face. (Venezuela needs outside expertise and investment to keep things going as the fields mature.)
Cheers,
Scott.