Giant oil group Royal Dutch Shell has said it is trimming its figures for proven oil and gas reserves by 20%.
Despite rising profits, investors have turned their back on Shell
Stunned investors promptly began a sell-off that knocked more than 7% off the Anglo-Dutch firm's share price in both London and Amsterdam.
Shell said it does not expect the reassement to have any impact on its financial results, as 90% of the reserves involved remain undeveloped.
But analysts were unconvinced. Shares in fellow oil firm BP also fell 2%.
Shell said it was making a one-off revision of its proven reserves by moving some of them into a category known as "scope for recovery".
It expressed confidence that it may be able to add the reserves back to its proven pumpable holdings later, once it is more certain of their commercial viability.
"It is anticipated that most of these reserves will be re-booked in the proved category over time as field developments mature," said Shell.
Meanwhile its 2003 proven reserves statement, which is due out on 6 February, will be lighter by 3.9 billion barrels of oil equivalent than the end-2002 list.
Investors and oil analysts were startled, and puzzled, by the move.
"It was shocking, to say the least," the Agence France Presse news agency quoted one oil analyst who did not wish to be named as saying. "They gave no detailed explanation why this has happened."
"This reduces the value of the company by 10% using discounted cash flows," said Richard Brackenhoff, an oil analyst for Kempen & Co.
Most of the downgraded reserves are in Nigeria and Australia.
Meanwhile, BP announced a drop in margins at its North American refining and retail operations.
BP said its refining margins dropped $3.4 a barrel in the final three months of 2003 from $4.59 in the third quarter.