Ecuador's government said that the oil block known as ITT holds 750 million barrels more than originally estimated, boosting the field's reserves to 1.67 billion barrels.
The increase followed the drilling of an exploratory well in the Ishpingo, Tambococha and Tiputini (ITT) field located in a national park in the Amazon.
The government of President Rafael Correa in 2013 opened ITT to oilfield development despite criticism by environmentalists.
"This means that national oil reserves are now 4 billion barrels," Vice President Jorge Glas told reporters.
Authorities expect in August to start production at Tiputini, the only area of ITT that lies outside the Yasuni nature reserve. Output is expected to reach 20,000 barrels per day (bpd) by the end of the year.
Glas said the new reserves figures boost ITT's estimated production to 300,000 bpd in 2022.
Ecuador had initially asked wealthy nations to donate billions of dollars in exchange for leaving ITT's oil reserves underground as a way of protecting the Yasuni National Park, which is home to endangered species.
Correa's government scrapped that initiative after failing to raise enough money.
Ecuador, the smallest member of the Organization of the Petroleum Exporting Countries, produces about 540,000 bpd.