Some of the riskiest energy stocks — indebted producers of costly oil from shale rock — surged Wednesday after oil jumped 9 percent on news of an OPEC production cut.
The move by the global crude cartel gave a new lease on life to the U.S. shale firms by prompting higher prices and opening the door for frackers to reclaim market share.
Shares of Whiting Petroleum, North Dakota's largest oil producer, soared 30 percent. More than a quarter of its float was bet short, or positioned for an incorrect wager that the shares were going to decline, according to FactSet.
Whiting Petroleum 5-day performance
Whiting Petroleum was the largest gainer in the VanEck Vectors Unconventional Oil & Gas ETF (FRAK), which includes companies involved with shale oil exploration and production.
The second-best performer in the ETF was Oasis Petroleum, followed by WPX Energy, both closing more than 27 percent higher. Nearly 19 percent of Oasis shares available for trading were sold short, according to FactSet, while short interest in WPX was 7 percent.
Overall, FRAK closed 10.4 percent higher Wednesday, while the Energy Select Sector SPDR Fund ETF (XLE) closed nearly 5.1 percent higher.
—CNBC's Evelyn Cheng contributed to this report.