Oil prices would need to fall at least another $20 a barrel to choke off the U.S. energy boom, industry experts say, though some smaller American producers would face serious problems from a more modest decline. Small and midsize companies—not global giants—are behind the surge in U.S. oil output, which hit 8.97 million barrels a day earlier this month, according to federal statistics. Some of these drillers have taken on a lot of debt, which was easier to justify when oil was going for as much as $107 a barrel just four months ago.