Concern is growing among investors that Shell's double downgrade of its oil reserves could herald similar moves from some of its competitors.
The Anglo-Dutch giant has had to cut proven reserves by almost 4.1 billion barrels since the start of 2004.
But its partners in the Norwegian oil field on which its most recent cut focused are still quoting much higher reserves.
That, analysts say, means downgrades may also be necessary elsewhere.
Shell's new dose of bad news comes at a time when oil prices are soaring.
The cost of a barrel of oil on US markets is at 13-year highs and closing in on $40.
Shell's troubles over reserves predate the recent gains in prices, however.
Early in the year, the firm shocked investors by warning that - according to tightened US market rules - it had over-stated its reserves by 3.9 billion barrels or 20%.
That news cost its two top executives their jobs.
But according to Deutsche Bank analyst JJ Traynor, the latest downgrade - the result of Shell's decision to go over all its reserve calculations anew - could rebound onto other firms.
"We remain convinced that reserves bookings are a sector-wide issue, albeit amplified at Shell," he wrote.
The new Shell downgrade means it is now booking just 20% of potential reserves at the Ormen Lange Norwegian field, Dr Traynor pointed out, adding that despite recent troubles he still saw Shell as a good-value investment.
But while state-owned Statoil is booking about the same, another partner in the field, Norsk Hydro, is claiming 71%.
And the equivalent figure for Shell's UK rival BP may be close to 80%.