News of the find pushed US stocks up, while oil prices bounced back from two month lows at $68.71 per barrel for light, sweet crude oil.
A test well indicated the find could be the biggest domestic oil discovery since Alaska's Prudhoe Bay a generation ago, the companies said.
However, the oil deposit, roughly 6.4km beneath the ocean floor, will not reduce significantly the country's dependence on foreign oil, nor will it help to lower prices at the pumps.
Art Smith, chief executive of energy consultant John S Herold, said: "It's a nice positive, but the US still has a big difference between its consumption and indigenous production.
"We'll still be importing more than 50% of our oil needs."
Chevron on Tuesday estimated the 768 square kilometre region where its test well sits could hold between three billion and 15 billion barrels of oil and natural gas liquids.
The US consumes roughly 5.7 billion barrels of crude oil in a year.
The test well is thought to be the
deepest in the Gulf of Mexico
It will take many years and tens of billions of dollars to bring the oil to market, but the discovery comes as Western oil and gas companies are finding fewer opportunities in politically unstable parts of the world, including the Middle East, Africa and Russia.
The proximity of the Gulf of Mexico to the world's largest oil-consuming nation makes it especially attractive and could bring pressure on Florida and other states to relax limits they have placed on drilling in their offshore waters for environmental and tourism reasons.
The country's reserves currently are more than 29 billion barrels of oil equivalent, according to the US energy department.
But the US imports most of its oil from abroad and its overall supply is tiny when compared with Saudi Arabia, whose reserves exceed 250 billion barrels.
Chevron's well, called Jack 2, was drilled about 8.5km below sea level in the Walker Ridge area of the Gulf, about 282km off the coast of Louisiana. It followed up a discovery made by Chevron in 2004.
Chevron said the well set a variety of records, including the deepest well successfully tested in the Gulf of Mexico.
The company has a 50% stake in the field, while partners Statoil ASA of Norway and Devon Energy of Oklahoma City own 25%.
During the test, the well sustained a flow rate of more than 6,000 barrels of oil a day, but analysts and executives believe the payoff could be much larger than that.
While the industry was generally pleased by the discovery, Cambridge Energy Research Associates said challenges in maximising output would include a shortage of rigs capable of drilling in very deep water and the length of time needed to drill very deep wells.