Mr Fox's government wants Mexico to maintain its current output

The field is in the Gulf of Mexico, and Mexico says it could be bigger than its largest oil field, Cantarell.

Production there is said to have declined sharply in recent years.

Mr Fox made the announcement as figures showed the country's total oil reserves had fallen 2% between 2003 and 2005.

Perforation of the well known as Noxal 1, which is located about 100km (60 miles) from the port of Coatzacoalcos on the coast of Veracruz state, started in December.

MEXICO'S OIL Mexico is Latin America's largest oil producer ahead of Venezuela and Brazil Output: 3.4m barrels per day Oil provides one third of the state income Half of the production is exported, mainly to the US

The oil is under 930 metres (0.6 miles) of water and a further 4,000 metres (2.5 miles) underground.

Visiting the drilling platform, Mr Fox said: "With Noxal we will begin a new era of oil exploration in our country."

The government says its investment in exploration will enable Mexico to maintain its current output in the future.

With at least 3.4m barrels per day, Mexico is Latin America's largest crude producer ahead of Venezuela and Brazil, according to the International Energy Agency (IEA).

The oil industry provides one third of the Mexican state income. More than half the crude extracted is exported, mainly to the United States.

The state-owned company Petroleos Mexicanos (Pemex) is among the biggest players in the international oil market.

Mexico is not a member of oil producers' cartel Opec.