Mexico's state-owned oil company says studies on the 70-year-old Ixachi field in southern Veracruz state indicates it holds a whopping one billion barrels of proven, probable and possible reserves.
That would boost what are known as Mexico's total "3P" reserves, which currently stand at about 25.5 billion barrels of crude equivalent.
The Ixachi field has been drilled since 1948, but the extent of the reserves wasn't realized until 2017.
The Petroleos Mexicanos company said the field was now rated as the biggest onshore oil reserve confirmed in the last 25 years.
The field is expected to produce about 80,000 barrels of oil and 700 million cubic feet of gas per day when it is fully producing in 2022.