“This is the most important achievement” so far in Mexico’s overhaul, said Pablo Medina, a Houston-based analyst at the consulting firm Wood Mackenzie.
The companies won the rights to drill in the zone in 2015, during Mexico’s first auction of exploration rights.
In an interview, Alfredo A. Marti, a former BP executive who is now a Riverstone managing director, said that high-tech data processing indicated that the Mexican government’s estimates for the amount of oil in the field might have been too low.
But after drilling, he said, “we found that even our bullish expectations were conservative, and we have something much more interesting.”
Because of Mexico’s rich resources and proximity to the United States, Mr. Marti continued, “we think that over the next few decades, Mexico is going to be a very important source of growth in the oil and gas sector.”
Also on Tuesday, Eni, an Italian oil company, said a well recently drilled to test a previously announced discovery had shown that the trove also held more than one billion barrels of oil — much more than previously thought.
Eni’s chief executive, Claudio Descalzi, said in an interview that the oil would be cheap and easy to produce because it was in shallow water only a few miles from shore. He added that his company, which won the rights to explore the field in 2015, would keep drilling in the area and that the size of the find might grow.
“I imagine that after these big discoveries, the appetite of other companies will increase,” he said.
Oil companies have long eyed Mexico for exploration and production of energy, banking on the rich geology that has produced so much oil and gas in American waters in the Gulf of Mexico.
“Geologically it is part of the same big basin,” Tony Durrant, Premier’s chief executive, said in an interview.
Mr. Durrant said that while the American side was “completely pockmarked” with oil wells, the Mexican side was not as heavily drilled because the state-owned Petróleos Mexicanos, or Pemex, had long been the “only game in town.”
Last year, Mexico’s oil production averaged 2.5 million barrels per day, making it a midsize producer, comparable with Brazil or Venezuela. But output has been declining, and last year’s figure was more than a million barrels per day fewer than it was a decade earlier.
Those poor levels of production compelled the government to take on nationalist pressures and to bring in competitors to try to turn around one of the country’s most important industries.
But the timing of the overhaul coincided with a sharp fall in oil prices, which had been at more than $100 a barrel but are at about $48 a barrel now.
With companies chopping exploration budgets, interest in Mexico’s early auctions was muted, allowing companies like Eni and the consortium including Premier to pick up what are turning out to be gems.
Now, most major oil companies have acquired positions in Mexico. “After this discovery, it is going to be quite hot,” Mr. Durrant said.