Shares in Tony Hayward oil firm slump 40%
Shares in Genel, the oil explorer chaired by former BP boss Tony Hayward, slumped to a record low after it reduced the estimated value of a key field by $1bn.
Genel cut the amount of oil it expects to recover from its Taq Taq oil field in Iraqi Kurdistan.
It now forecasts "recoverable proven and probable reserves" will reach 356 million barrels of oil for 2015.
Genel had expected the area to yield 683 million barrels of crude.
It also blamed falling oil prices for the writedown on the value of Taq Taq. The price of Brent crude has fallen by 70% since June 2014 and is now $35.58 a barrel.
Analysts said the cut was much larger than expected.
"I was expecting perhaps a 10, maybe 20% cut in reserves and obviously this is more akin to a 50% cut," said Arden Partners analyst Daniel Slater.
Genel's shares tumbled over 40% to a record low of 74p and have fallen 87% over the past 12 months.
Mr Hayward stepped down as chief executive of BP in 2010 following the Gulf of Mexico oil disaster that claimed 11 lives and caused damage to the coastline.