President-elect Donald Trump’s selection of Rex Tillerson as his secretary of state may accelerate a planned leadership transition at Exxon Mobil Corp., placing a new chief executive atop the oil and gas giant as it faces extraordinary challenges. Succession planning is serious business at Exxon, and Mr. Tillerson, chairman and chief executive since 2006, is supposed to retire in March when he turns 65, according to company rules. Although Exxon hasn’t formally announced a successor to Mr. Tillerson, Darren Woods, 51 years old, emerged as Mr. Tillerson’s heir apparent last December when he was appointed president of the company and took a seat on the board. “The board of directors of Exxon Mobil Corporation congratulates Rex W. Tillerson, chairman and chief executive officer, on his nomination for the position of U.S. Secretary of State,” Suzanne McCarron, Exxon’s vice president of public and government affairs, said in a statement Tuesday […]