Marathon Oil has been at the vanguard of the US shale drilling revolution. Year after year it reported double-digit gains in crude lifted from North Dakota’s Bakken, Texas’s Eagle Ford and other hotbed regions. Now, those heady days are over. “Our focus, as we navigate the volatile environment and progress through the planning cycle, will be capital discipline and balance sheet strength, not production growth,” Lee Tillman, Marathon’s chief executive, told an investor conference this month.