Independent oil explorer UKOG, which grabbed global headlines after initially claiming then backtracking on a potential 100bn barrel oilfield near Gatwick Airport, has admitted that one of its licences for the site is actually set to expire.In an exchange filing on Tuesday, UKOG confirmed that its two exploration licences at Horse Hill - PEDL137 and PEDL246 expire on 30 September 2015 and 30 June 2019 respectively. With the expiry of the first of the licences barely six months away, UKOG said it had applied to the Oil & Gas Authority for a one year extension.The company, which owns around 30% of both licences, said planning permission was already in place for it to commence flow tests on the Horse Hill site.A few weeks ago UKOG had claimed in a BBC interview that the site held "up to 100bn barrels of oil". However, few days later following criticism from geologists and market experts, the company issued a "clarification" saying that its original figure was above expectations.It added that assuming "between 50bn-100bn barrels" was more reasonable, of which between 3-15% could be viably extracted.At 12:05 BST the company's share price was down 4.42% at 2.96p