Cook, 50, was a candidate to replace Chief Executive Jeroen van der Veer, who is retiring July 1. Instead, the top job went to Peter Voser, the company's chief financial officer.

"It's a mutual decision," said Shell spokesman Shaun Wiggins of Cook's departure. He said the company had nothing to add.

Cook was a Shell employee for 29 years, joining the company in Houston when she was 21. She has a degree in petroleum engineering from the University of Kansas.

In a statement, Van der Veer thanked Cook and noted the company's liquid natural gas capacity has risen by 60 per cent since she took her job five years ago.

As oil production declined in the same period, Shell's total production is now almost equally split between gas and oil.

As part of her duties, Cook also oversaw Shell's renewable energy arm.

In March, she told reporters in an unusually candid discussion that the company would not make any new investments in wind or solar energy, focusing instead on biofuels.

"On wind and solar, they're interesting but they continue to struggle in comparison with the other investment opportunities we have in our portfolio, even with substantial subsidies," Cook said.

She rejected criticism that the company's investment in renewables was too small: US$1.7 billion over the past five years, compared with US$126 billion in net profit.

"We have to start somewhere," she said. "If there aren't investment opportunities that compete with our other projects in the billions and billions then we won't spend the money."

She said the company's focus was on investments that would "get the best return for shareholders. If those were in renewables today, we'd be putting money there, we'd be happy to. But that's just not the case."

Cook was paid US$3.9 million in 2008, including a US$1.9 million bonus. Shareholders rejected the company's executive bonuses at Shell's annual meeting last week in a non-binding vote, noting that the company had missed performance targets.

- AP