Exxon Mobil Corp. and Chevron Corp., the biggest U.S. energy producers, hunkered down for a prolonged stretch of weak prices after posting their worst quarterly performances in several years. Exxon reported its lowest profit since 2009 as crude prices fell twice as fast as the world’s largest crude producer by market value could slash expenses. Chevron recorded its lowest profit in more than 12 years after the market rout forced $2.6 billion in asset writedowns and related charges.