Shares in Gulf Keystone Petroleum have fallen by nearly 15% after Excalibur Ventures claimed that it is entitled to a 30% stake in the AIM-quoted oil and gas explorer's interests in northern Iraq. Gulf Keystone chairman Todd Kozel says that he is confident that he will be able to fight off this claim. Before Christmas, Excalibur commenced an arbitration case in New York arguing that it is entitled to an interest of up to 30% in the Gulf Keystones' blocks in Kurdistan. Excalibur has started similar proceedings in London.Gulf Keystone says that it received notice of the claims on 23 December. Washington-based Excalibur, whose Iraq office is in the Ishtar Sheraton Hotel & Towers in Baghdad, appears to offer services to the Iraq authorities, the military and businesses. According to its own website: "Excalibur provides clients with locally experienced managers and technicians, and can contract a local labor force to execute projects both large and small". There is no indication of what dealings Gulf Keystone may have had with Excalibur. It appears that Excalibur may have given the oil company some sort of assistance but there has been no mention of Excalibur in previous announcements from Gulf Keystone. Gulf Keystone's subsidiary holds Production Sharing Contracts for four exploration blocks in Kurdistan. There are estimated to be 1.9bn barrels of oil in place in Gulf Keystone's Kurdistan assets - based on P90 estimates (90% probability).On 21 December, Excalibur applied to the London courts for a "worldwide freezing injunction" over Gulf Keystone's assets but the Judge refused the application. In October, Gulf Keystone raised £109m at 140p a share, in order to finance drilling on the Shaikan field. The cash should last until 2012. That followed a £114m fundraising at 75p a share in May. The current share price is 159.25p.