(Adds detail, BG statement, analyst)
OSLO, Oct 15 (Reuters) - Statoil Chief Executive Helge Lund unexpectedly resigned on Wednesday to take the top job at smaller rival BG Group, the firms said in separate statements.
Lund leaves state-controlled Statoil after more than ten years in the top job and the firm appointed Eldar Saetre, its current head of marketing, processing and renewables division, as its acting chief while it searches for a new CEO.
"(Lund) has built a world-class exploration and production portfolio at Statoil, and the company is now widely admired for its technical expertise, financial performance and strong, values-based culture," BG chairman Andrew Gould said.
Lund, 51, expanded Statoil from a mostly domestic firm to an international major with a $77.5 billion market capitalisation and an international exploration and production portfolio spanning from Brazil to Indonesia.
"In the near-term it means uncertainty (for Statoil), and of course in a Norwegian company like this with a significant holding by the Norwegian state it's not easy to have an international CEO with salaries that are criticised (because they are low)," Handelsbanken analyst Anne Gjoen said.
"He probably believes that BG has been a significant growth story," Gjoen said. "They have been successful in many ways, and he's been the CEO of Statoil for many years, so it's not that surprising."
Under Lund, who had been CEO for just over 10 years, the firm has also become one of the most successful explorers, with big finds in places like Canada, Brazil and Tanzania.
But Lund also leaves at a difficult time as the firm cuts capital spending plans, lays off workers and cuts costs after oil prices slumped to a four-year low and cost inflation squeezed margins. (Reporting by Balazs Koranyi, Gwladys Fouche and Stine Jacobsen, editing by Louise Heavens)