The size of BHP Billiton Ltd.’s $2.8 billion writedown for its U.S. shale assets took some banks by surprise: at least two thought it was too small. After the latest charge, Melbourne, Australia-based BHP values its U.S. onshore business at $24 billion. That’s more than 40 percent higher than what analysts at JPMorgan Chase & Co. think it is worth and over twice Citigroup Inc.’s estimate. The producer said its U.S. onshore assets would generate positive cash flow in the year to June 2016 with oil at $60 a barrel, above current prices. “Given the deteriorating conditions of the U.S. oil and gas market, we thought an impending impairment could have been larger,” analysts at JPMorgan including Lyndon Fagan wrote in a research note Wednesday. After four years of record supply, America’s natural gas output is shrinking as producers retreat from shale amid tumbling oil prices. That’s hurting investments […]