U.S. shale oil and gas producers are buying international assets to maintain supply amid revisions of oil demand outlooks for the long term. From South America to the Middle East, frackers are going global. Continental Resources is one example. The company of fracking icon Harold Hamm has been expanding in Argentina’s Vaca Muerta shale play, widely considered the second-largest shale oil and gas deposit after the Permian. In the last three months, Continental made two asset acquisition deals in the Vaca Muerta, with its chief executive, Doug…