Libyan oil-facility guards prevented two of the country’s biggest fields from resuming production, days after the National Oil Corp. reached an agreement to restart operations there to boost output in the politically divided OPEC state. The El Feel, or Elephant, and Sharara fields still aren’t operational after they were shut more than a year and a half ago, an NOC official said Sunday by phone, asking not to be identified for lack of authorization to speak to news media.