This tiny nation on the north coast of South America is about to become the world’s newest petrostate—and potentially the richest. In 2015, Exxon Mobil Corp. made what one of its executives described as a “fairy tale” discovery in the vast Stabroek exploration block off the Guyanese coast. Since then, it’s found so much oil that by the mid-2020s Guyana, with a population of about 778,000, will probably produce more crude per citizen than any other country.