Exxon Mobil authorized a $6.8 billion expansion offshore of sparsely populated Guyana,it announced Sept. 22. The project is its seventh in just a decade within the massive South American oil development. The new project, named Hammerhead, is Exxon's first with archrival Chevron as its largest partner in Guyana after Chevron in July closed on its $53 billion acquisition of Hess.