United Oil & Gas said an independent report indicated its Crown discovery in the North Sea had a contingent resource estimate of around 6.4m barrels.
The competent persons' report, prepared by ERC Equipoise, also led to an increase in the contingent resource estimate at the Waddock Cross field to around 1.55m barrels from 1.23m.
Contingent resource estimates are based on geological analysis but drilling is required to bring them into probable or proved status.
Chief executive Brian Larkin described the estimates as an 'excellent outcome'.
At 9:21am: (LON:UOG) United Oil Gas Plc share price was +0.25p at 4.85p