"This upgrade of the Shaikan mean oil resources from 7.5 (billion) to 10.5 billion of barrels of gross oil-in-place is a result of the increase in the lowest known oil areas for each reservoir," the company said in a statement.
The estimated was calculated by Houston exploration consultant Dynamic Global Advisers and marks the third upgrade since the Shaikan discovery was announced in 2009.
Gulf Keystone said it might need another pipeline to get oil from its Shaikan field as most of the oil is transported by truck.
The company during the first half of 2011 has seen significant progress in its exploration activity in the Kurdish regions of Iraq.
John Gerstenlauer, chief operating officer at the company, said he was "delighted" with the results from the Shaikan discovery. He said that while the latest assessment gave his company a better understanding of the field, the overall picture was "far from being concluded."
More drilling is expected during the next few years.