It's the same thing as when there were sanctions and embargoes on Iraq. Companies were not allowed to sell to Saddam Hussein or buy his oil.
Remember the United Nations scandal over the "Oil for Food" program to allow Iraq to sell oil only for humanitarian aid, only to find corruption and diversion of funds by major European companies?
Companies like Chevron maintain sanctioned-country lists in their compliance departments to regularly audit themselves to ensure they don't do business with embargoed countries, or with other companies that do business with embargoed countries.
President Trump isn't singling out Chevron, he's embargoing Venezuela, and it's just that Chevron is the only American oil company still doing business there.
-PJ
What PJ said above. We do this all the time. We sanctioned Iran and then told European countries that we would sanction them if their companies did not stop doing business.
Venezuela is an unstable narco-state in our hemisphere. Chevron gambled and lost just like others. All they are doing down there now is pumping money to a bunch of people indicted by the US for narcotics trafficking.
Close up shop and tell the opposition party that you will come back when stability is returned to the nation and hope China does not swoop in to buy it all up at pennies on the dollar.