The new estimates are separate to the one made last month claiming another part of the Weald Basin could hold up to 100bn barrels. UKOG was later forced to issue two separate clarifications on the size of that find.
A new report on behalf of Horse Hill Developments, the consortium of companies behind the exploration in the area and which UKOG is a part of, found the upper portland sandstone reservoir area has an estimated 21m barrels in place, up on the previous estimate of 8.2m barrels made in December last year.
"The Xodus Report supports the company's view that the Horse Hill and Collendean Farm oil pool constitutes a significant conventional upper portland sandstone oil discovery in the Weald Basin," said UKOG chief Stephen Sanderson.
The company will now apply for approval to conduct flow tests later in the year.
A report on another part of the area where Horse Hill Developments holds a license, pegged oil reserves at 100bn barrels. Aim-listed UKOG was forced to clarify the number was contingent on certain assumptions on how much could be recovered.
The announcement at the beginning of April sent shares rocketing more than 300 per cent. UKOG shares are currently trading at 2.751 pence per share - still more than 150 per cent higher than before the original discovery.