The oil discovery was made in the sea about 175km (110 miles) off the Amazon coast but the size of the field is not yet clear.
It is also still unknown whether extracting the oil, which is located at a depth of 2,886m (9,469ft), would be commercially viable.
Conservationists have warned that the area where the exploratory drilling is taking place is very biodiverse.
They fear that any accidents could threaten not just the nearby reefs but that currents could carry spilled oil to coastal wetlands teeming with wildlife.
Lula has in the past rejected criticism by conservationists as “meddling” and on Monday he again said that he would not accept any foreign interference.
“This means national sovereignty. Any country which has oil of this quality won’t allow anyone to butt in,” he said.
Petrobras started the exploratory deep-water drilling operation in the area in October 2025.
The company’s chief executive Magda Chambriard said that the discovery confirmed that Petrobras’s “optimism” of finding oil in the Brazilian Equatorial Margin had been warranted.
She added that the company was committed to “ensuring the country’s energy security”.
President Lula has argued that oil revenues are needed to help fund Brazil’s transition to non-fossil fuels.
Speaking on Monday, he insisted: “I’m not denying my commitment to the fight for a day when we no longer need fossil fuels, because Brazil will continue to be the king of biofuel innovation and will remain a major player in renewable energy.”
Environmentalists were aghast when Lula announced he had given Petrobras the green light to start the exploratory drilling just weeks before the COP30 climate conference hosted by Brazil last November.
In his remarks on Monday, the president made a reference to the anger at the time, singling out opposition in Europe in particular.
“Many people didn’t want me to announce that Petrobras had won the licence because they said that the Europeans would be annoyed,” he said.
“I took the decision to announce that people wanted to do exploratory drilling there before COP because the interests of our country and our company were at stake.”
The prospect of oil discovery off the coast has already given a boost to the nearest town.
In Oiapoque, in Amapá state, the hope of jobs and opportunities has led to an influx of people, according to the local housing authorities.
With less than two months to go before the presidential election, Lula will be trying to attract voters hoping for economic development in this remote part of Brazil without alienating those concerned about the potential impacts on the environment.