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.
But his last months in the job were tarnished by Shell's setbacks in the US. Like other majors, it entered the American shale sector late in the game and was accused by some investors of overpaying for assets. Its earnings were then hit when a supply boom pushed U.S. gas prices to 10-year lows.
As well as its $2.1 billion writedown, mostly related to its U.S. tight oil assets, Shell also said its US exploration and production business was lossmaking and would likely remain so to the end of the year and possibly beyond.
Just last month, Shell said it had put its acreage in the Eagle Ford shale in Texas up for sale, as part of a strategic review of its US shale portfolio.
"We expected higher flow rates and therefore more scalability for a company like Shell"
Mr Voser said Shell's Upstream Americas business was in the red because of a "strategic decision to slow down" on shale in the face of low gas prices. "Therefore you are hit with more than $3 billion of depreciation whilst you don't have the revenues against it," he said.
He also acknowledged that exploration results in the US shales had been disappointing. "We expected higher flow rates and therefore more scalability for a company like Shell," he said.
Shell's US unconventional oil and gas operation was an "emerging strategic business which needs attention, needs fixing over the next two, three, four years". He said an expected increase in the company's tight oil production in the U.S. "will help us get into a more reasonable profit and cash position in the future".
Mr Voser also said rhetoric about the U.S. shale revolution being exported to other countries was "hyped", and that the rest of the world was in an early "exploration phase" which could yield "negative surprises".
He singled out China, where Shell has drilled 22 wells, as one of the most prospective countries for shale gas, but warned that costs there were higher than in the US.
Mr Voser acknowledged problems in Alaska, where Shell has spent nearly $5bn on an offshore exploration campaign but has yet to complete a single well, amid regulatory and technical problems.
He pointed to the failure of a containment dome, a piece of equipment designed to catch any oil leaking on to the seabed, which was damaged during testing last year.
"That was a big disappointment to me personally," he said. The incident forced Shell to delay its drilling plans, and Mr Voser said the company still didn't know "if we'll go back [into Alaskan waters] in 2014 or 2015".