The value of debt issued by junk-rated US energy companies has plummeted to the lowest level for more than two decades, sending a warning signal about the outlook for the North American oil industry. The average high-yield energy bond has slid to just 56 cents on the dollar, below levels touched during the financial crisis in 2008-09, as investors brace for a wave of bankruptcies. The slump in bond prices took a further step down last week, as crude dropped to 12-year lows below $28 per barrel. Although oil rebounded sharply, at about $32 per barrel on Friday, it was still 14 per cent lower than at the start of the year. The US shale revolution which sent the country’s oil production soaring from 2009 to 2015 was led by small and midsized companies that typically borrowed to finance their growth. They sold $241bn worth of bonds during 2007-15 and many are now struggling under the debts they took on. Very few US shale oil developments can be profitable with crude at about $30 a barrel, industry executives and advisers say. Production costs in shale have fallen as much as 40 per cent, but that has not been enough to keep pace with the decline in oil prices. “This year in shale will be very hard,” said Bielenis Villanueva Triana, an analyst at Rystad Energy.