The oil terminal of Marsa al-Hariga in eastern Libya, a part of the country where rebel actions have cut off shipments of crude for much of this year. Agence France-Presse/Getty Images LONDON—Six months ago, one of OPEC’s concerns was whether a U.S. shale boom might upend crude markets by providing too much oil. Today, the group of some of the world’s largest producers has a more short-term worry: How to compensate for lost Libyan crude at a time of rising demand and tensions between Russia and the West. “That’s the big question: Who will fill the gap left by Libya?” said one delegate within the group, which meets in Vienna later this week to discuss its output. Members of the Organization of the Petroleum Exporting Countries—which produces one out of every three barrels of oil consumed daily in the world—are facing calls by some to boost output to help […]