Edinburgh-based Bowleven said a well drilled to appraise the existing IM find had encountered what appeared to be a good quality reservoir nearby.
"Initial indications ... suggests the presence of liquids rich hydrocarbons," said Bowleven.
The company has not done the testing needed to assess the size of the find made with the IM-5 well.
However, chief executive Kevin Hart said the com-pany is highly encouraged with results obtained so far with its latest drilling on the Etinde permit.
Seismic survey date had indicated the area could contain a sizeable reservoir in the Intra Isongo level.
Mr Hart said: "The potentially significant hydrocarbon discovery in the Intra Isongo demonstrates the substantial upside that we believe exists on Etinde."
The encouraging results come ahead of a crucial few months for Bowleven.
The company hopes to decide in the second half of this year whether to sanction the hefty investment required to bring the finds made on the Etinde permit into production. In November, Petrofac agreed to provide up to $500m of the initial development costs on Etinde in return for a share of cashflows generated from production.
Analysts at Jefferies International told clients seismic survey data indicated the find made with the IM-5 well could be "very significant". The original IM find was made by the former Mobil in 1967.
Bowleven acquired acreage on which some majors made finds in what was thought to be a gas prone area. The finds had not been developed amid doubts about the commercial value of gas discoveries in an area in which there may not have been enough demand.
Shares in Bowleven closed up 4.25p at 67.75p.
The firm raised £55 million from investors at 650p per share in 2005 before suffering drilling setbacks under previous management.
Bowleven will continue drilling the IM-5 to appraise the Middle Isongo sands.