The new find, as well as some of the drilling concessions, are in a disputed area whose possible annexation Venezuela is putting to a national referendum.
Guyana, much smaller than its neighbour, has the world’s highest reserves of crude per capita.
US oil giant ExxonMobil made the discovery of an estimated “20m of hydrocarbon-bearing reservoir” and about another “81m of additional hydrocarbon-bearing sandstone,” Guyana’s natural resources ministry said in a statement.
The new reservoir would undergo a “comprehensive appraisal process,” it added.
Later, vice-president Bharrat Jagdeo told reporters that drilling bids had been awarded to companies including ExxonMobil, French multinational TotalEnergies, and local company SISPRO Inc.
“We now have to meet with the parties to which the blocks have been awarded to have a negotiation on the contracting, that is to move towards the completion of the award,” said Jagdeo.
Tiny Guyana boasts oil reserves of at least 10 billion barrels, more per capita than Brunei, Kuwait, or the United Arab Emirates.
ExxonMobil, with its partners Hess and CNOOK, has made 46 discoveries off the Guyana coast since 2015, and four so far this year.
The oil is in the 160,000sqkm Essequibo region, administered by Guyana, that Venezuela has for decades argued should fall within its borders.
The Essequibo region makes up more than two-thirds of Guyana and is home to 125,000 of its 800,000 residents, according to a decade-old census.
A former Dutch and British colony, Guyana says its border with Venezuela was fixed by an arbitration tribunal in 1899.
However, Venezuela says the Essequibo river to the east of the region forms a natural frontier recognised at the time of independence from Spain.
The dispute, which is before the International Court of Justice in The Hague, intensified after ExxonMobil’s first oil discovery there eight years ago.
In the latest volley, Venezuela announced it will hold a referendum on Dec 3 on whether or not to annex the area.
Guyana has called the plebiscite illegal, with backing from the Caribbean Community (Caricom) and the Organization of American States (OAS).
Caricom in a statement on Wednesday said the planned referendum “has no validity, bearing, or standing in international law” adding that it “earnestly hopes that Venezuela is not raising the prospect of using force or military means.”
Guyana, meanwhile, has denounced the deployment of 200 Venezuelan soldiers in the border region, which Caracas said was to combat illegal mining.
The referendum will also ask Venezuelans to express themselves on whether Caracas should continue to recognise the ICJ or not.