Peter Voser said the failure of Royal Dutch Shell's huge bet on US shale was a big regret of his time as chief executive of the company.
Speaking to the Financial Times three months before he is due to step down, Mr Voser also described the technical setbacks Shell has suffered in its exploration campaign off the coast of Alaska as one of his greatest disappointments in the job.
Shell has invested at least $24 billion in so-called unconventional oil and gas in North America. But it is a bet that has yet to pay off. Its North American upstream business has struggled to turn a profit and in August Shell announced a strategic review of its US shale portfolio after taking a $2.1 billion impairment. "Unconventionals did not exactly play out as planned," Mr Voser said.
He will be replaced next year as head of Europe's largest oil company by market value by Ben van Beurden, the company's current head of refining and marketing.
A Swiss national, Mr Voser was part of the executive team that steadied Shell in the aftermath of a reserves misreporting scandal in 2004 that rocked investor confidence in the company.
As CEO from 2009, he is credited with overhauling the company's notoriously complex structure and delivering some of the largest projects in Shell's history, including a $19bn gas-to-liquids plant in Qatar.
He also reaffirmed Shell's status as one of the leading innovators in the oil industry by moving ahead with the world's first floating liquefied natural gas project.