Mexico opened the valves today on the flow of $1 million worth of natural gas per day across the border into the United States.

The gas flow, at the rate of 300 million cubic feet per day, represents only 8 percent of current U.S. imports. But informed sources here said the contract leaves room for doubling or tripling the amount in the near future, depending on U.S. needs and Mexican production.

The deal which took two years to conclude, was the subject of much diplomatic controversy following the U.S. government's veto of a private deal for 2 billion cubic feet per day in 1977. At the time, the price of $2.60 per thousand cubic feet was regarded as too high by then secretary of energy James Schlesinger.

Personal intervention by the U.S. and Mexican presidents last December concluded the deal between the Mexican government and six U.S. distributors. The gas price -- $3.625 per thousand cubic feet -- is the highest in the world for pipeline gas. It is tied to world crude oil prices and hence is expected to go up quarterly.

The gas deal has improved the bilateral energy climate and enhanced U.S. chances for steady access to Mexico's growing reserves of oil and gas.

For Mexico, today's turning of the valves means it can virtually stop flaring off its excess natural gas, which it was doing at the rate of 200 to 300 million cubic feet per day.

Mexico announced yesterday the discovery of yet another vast oil field, estimated at 1.5 billion barrels. Proved oil and gas reserves already stand at 45.8 billion barrels.

The discovery is near the southeastern Reforma-Chiapas fields that yield 60 percent of Mexico's current production. The remainder comes from equally rich deposits in the Gulf of Mexico, where new wells are breaking world records by producing up to 42,000 barrels per day.

Earlier this month, Pemex announced it was now producing 1.9 million barrels a day, and would almost certainly reach its 1980 target of 2.2 million barrels per day ahead of time.