(ShareCast News) - Cluff Natural Resources said it was stepping up its hunt for a farm-out partner after a "very significant" increase in natural gas was confirmed at its wholly owned licence in the North Sea.An independent survey of the six most mature prospects on licence P2248 in Block 43/11 of the Southern Gas Basin was estimated at between 760bn cubic feet (bcf) to 7.2trn cubic feet (tcf) of gas initially in place (GIIP), with a central estimate (P50) of 2.6tcf.Prospective resources were estimated in the range of 480bcf to 4.75tcf, with P50 of 1.73tcf.Across Cluff's total portfolio, the aggregate best estimate GIIP and P50 prospective resource are now estimated to have increased to 4.8tcf and 2.5tcf respectively, not including any other prospects still to be fully quantified.Chairman and chief executive Algy Cluff said the report was an endorsement of "the significant exploration potential associated with proven and new play types on our Licence P2248" and confirmed the board's belief that the basin still has many large prospects still to be drilled. "We now have a significant inventory of drilling targets across a range of play types and the key focus going forward is to attract the required partners and funding to ensure these prospects are fully appraised."Shares in Cluff quickly topped 6p for the first time since 2012 and by 1115 BST were off slightly but still up 34% to 5.97p.