An exploratory well-head at Horse Hill, is seen at a site in Surrey, southern England, April 9, 2015. A small British oil producer said it had discovered a possible world class oil resource in the countryside to the south of London that could contain billions of barrels of oil. The London-listed oil explorer UK Oil & Gas Investments (UKOG) said on Thursday up to 100 billion barrels of oil could lie below the green fields around Gatwick Airport, however it cautioned that only a fraction of that could be recovered.REUTERS/Peter Nicholls