The world’s biggest oil explorers are fighting a U.S. plan to toughen offshore drilling rules that Exxon Mobil Corp. said will cost $25 billion over 10 years and render many offshore discoveries worthless. (Bloomberg) — The world’s biggest oil explorers are fighting a U.S. plan to toughen offshore drilling rules that Exxon Mobil Corp. said will cost $25 billion over 10 years and render many offshore discoveries worthless. The Obama administration is expected to issue the sweeping new regulations Thursday, a person familiar with the decision said, as part of an effort to reduce the number of well blowouts after the explosion aboard the Deepwater Horizon rig in 2010. The government has pegged the rules’ costs at less than $1 billion. The changes would arrive amid the worst oil slump in a generation. ConocoPhillips and Chevron Corp. have already abandoned some Gulf prospects because they wouldn’t be profitable at […]