LONDON (Bloomberg) — Gulf Keystone Petroleum Ltd., a Bermuda-based explorer of Middle East oil, rocketed in London trading after saying its Shaikan-1 well in Kurdistan may contain twice as much crude as originally forecast.

On Tuesday Gulf Keystone rose as high as 109 pence, more than double Monday's closing price, and traded up 71 percent at 90 pence in London. Yesterday the stock retreated eight pence.

The stock is up more than fourfold this year, valuing the company at £430 million ($732 million).

Shaikan-1 well, located near the Northern Iraqi city of Dihok, may hold as much as three billion barrels of oil, Gulf Keystone said in a statement on Tuesday. Initial estimates suggested the well would contain about 1.5 billion barrels, the company said.

The well is a "value-transforming discovery", chief executive officer Todd Kozel said in the statement. The company said in July that it would sell stakes in Nigeria to focus on Kurdistan.

Oil producers have flocked to Iraq since the autonomous Kurdish region in the north started to export crude in June. Exports prompted a surge of takeovers, including China Petrochemical Corp.'s $7.2 billion bid for Addax Petroleum Corp. and a reverse takeover of Turkey's Genel Energy International Ltd. by Heritage Oil Ltd. for $2.5 billion.