Discovery of massive new oil deposits, confirmed by the force of a blowout at a well off Yucatan, should increase Mexico's proven reserves "significantly," according to Mexican and U.S. officials.

"We have been told a very large increase of proven reserves will be announced in the next few months," said a U.S. source. The United States is an eager customer for Mexico's oil, taking about 80 percent of the 500,000 barrels currently exported daily.

The blowout of the exploratory well in the new deposit area - throwing vast amounts of crude into the Gulf of Mexico - is dramatic evidence that the formation is even deeper and richer than had been expected.

According to Jose Diaz Serrano, chief of the national oil company Pemex, the deposits had already been proved to be three times the size of the famous Reforma formation, which until now was Mexico's most impressive discovery.

Mexico, although not a member of the Organization of Petroleum Exporting Countries, sells its crude at prices in line with those of OPEC. Its proximity reduces shipping costs to the American market, however.

The United States also would like to buy Mexican natural gas. American companies were on the verge of a deal that would bring it by pipeline into Texas when the U.S. government vetoed the pact as too expensive.

Talks on the gas issue have continued since President Carter's visit here in February but no progress has been reported.

Mexico presently sets proven hydrocarbon reserves at 40 billion barrels, about two-thirds of which is crude and one-third gas. This alone makes Mexico about as rich in oil as the United States with its declining reserves.

But potential reserves, where the new-found formation in the gulf has been counted until now, show Mexico as a major petroleum source worldwide, with 200 billion barrels.

For all the cost in equipment and environmental damage of the blowout eight days ago, it is nonetheless drawing excited comments from experts.

The flaming well has spewed half a million barrels of oil already and is now gushing at a rate of 30,000 barrels per day, a flow much stronger than Mexico's richest wells. They have reached 20,000 barrels per day, a rate matched only in the Middle East.

"The pressure below is enormous. This is bigger than anything I've ever seen. It's very promising," said one American expert familiar with the energy picture here.Mexico's average well produces around 6,000 barrels per day, which is very high by U.S. standards.

But promising as the exploded well may be, it is contaminating a large area of the gulf at a disastrous rate. Foreign damage control teams have rushed to Mexico to help the Pemex men stem the huge oil slick. Red Adair, from Houston, who is regarded as the world's foremost oil disaster expert, is on scene.

Just days before the accident, Mexican oil chief Diaz Serrano for the first time publicly revealed details of the massive discovery, called Cantaral, that is now Mexico's richest by far. The Cantaral area, he told reporters, is "700 square kilometers, twice as large as the rich Reforma-Chiapas zone" but its greater depth, he said, made it "three times" the size of Reforma-Chiapas.

Reforma is the heart of Mexico's oil boom and presently produces more than 60 percent of the country's daily output of 1.5 million barrels. Cantaral, Diaz Serrano told reporters, is expected to produce between 400,000 and 500,000 barrels per day in a year's time. Four of 15 offshore platforms already are in place.

Pemex has known of Cantaral's strong potential since fisherman Rumersindo Cantaral reported "oil bubbling up on the water" there in 1971. Exploration began in 1976.

But according to an expert familiar with the recent exploration, "Pemex is only just now proving how big Cantaral really is. It will hike probable an proven reserve figures considerably."

The significance of the blow-out expoloratory well - which has a depth of 8,820 feet - is that it is revealing formations underneath Cantaral, separate from it and much richer than expected. "Its oil is of a similar light grade as the Reforma fields on the coast, so we suspect we're dealing with a continuation of Reforma or a parallel structure. Whatever it is, it looks huge," one expert said.

All indications, however, are that Pemex is faced with an immense problem to prevent even greater damage to the environment.

The direction of the current is expected to circulate the oil slick and, contrary to earlier reports, industry experts calculate the oil slick is likely to damage Mexico's own Yucatan coast and not that of Texas or the Caribbean islands. With present techniques, an oil industry source said, "the slick can be contained without permanent damage to the environment. We see this sort of thing in Louisiana all the time."

Nonetheless, the cost of the rescue operation and the losses to the fishing industry are expected to be enormous. The blowout not only partially destroyed the $22 million offshore platform but the thick layer of oil threatens marine life.

Although the oil is staying afloat and not threatening deep-sea life, the spill is in Mexico's major shrimp fishing area.

Thousands of people around Carmen City, 50 miles away, depend for their livelihoods on the shrimp fleet and the packing and freezing plants along the coast which export largely to the U.S. market and represent a major source of foreign exchange. CAPTION: Map, no caption, By Dave Cook - The Washington Post