BHP leaves door open to U.S. shale gas write-down ADELAIDE (Reuters) - BHP Billiton's (BHP.AX) petroleum chief executive left the door open to the possibility of a write-down on the company's U.S. shale gas assets on Monday, but defended their long term value. The company will review its assets at the end of the fiscal year on June 30, BHP chief petroleum executive J Michael Yeager told reporters in a briefing on Monday. "If we have to take an accounting snapshot here, we hope everybody knows that we'll take another accounting snapshot in the future and whenever those circumstances are changed, that whatever action we take now, may get reversed later on," Yeager said. BHP bought the Fayetteville and Petrohawk shale gas assets last year for $17 billion all together. Shale gas prices have halved since the acquisitions, which has led to speculation that the world's biggest miner will write down the value of the assets. Reporting by Rebekah Kebede; Editing by Jonathan Hopfner