Trillions of barrels of heavy oils, the foul-smelling ooze strewn in deposits in more than 45 countries, are being offered by the United Nations as a near-term solution to the world energy crunch.

This proposal, coupled with immediate prospects of increasing production of heavy oils and even-lower-grade tar sands, was the focus of the First International Conference on Heavy Oil sponsored by the United Nations Institute for Training and Research in oilrich Alberta here this month.

Long overlooked by the major oil companies, and most oil-exporting and consuming countries, heavy oil's have become the growing focus of attention as the Organization of Petroleum Exporting Countries has ratcheted oil prices up from about $3 a barrel in 1973 to nearly $20 a barrel today,

Dr. Joseph Barnea, a UNITAR special fellow, said that if heavy oils had been developed in the United States and elsewhere in the western consuming countries, the current oil squeeze could have been averted.

Ironically, California, which has been the state hit the hardest by gasoline and diesel fuel shortages, has massive deposits of heavy oil, estimated at from 20 billion to 30 billion barrels - nearly the size of the U.S. total proven oil reserves. One field alone - the giant Wilmington Field - has nearly as much oil in places as is being produced in the Prudhoe Bay field on Alaska's North Slope.

Still larger deposits are scattered outside the oil cartel, including nearly 600 billion barrels of heavy oil tar sands in canada, and billions more in Madagascar, offshore Italy in the Mediterranaen, in Turkey, in Equitorial Africa, and throughout Latin America.

And the most well-known deposits of heavy oil, in Venezuela's Orinoco Tar Belt, have been estimated to amount to more than 1 trillion barrels by Venezuelas' Ministry of Mines and Petroleum, along with many of the American major oil companies that operated there before they were nationalized.

Most of the major international oil companies were unwilling to discuss heavy oil prospects at the UNITAR Conference, and refused to disclose up-to-date information on their heavy-oil programs, the extent of reserves, or the costs of production as requested by the U.N.

Barnea, an Israeli energy expert with more than two decades of experience at the UN, says that many of these deposits can be produced at today's world oil prices, but have not because the major oil companies prefer to develop more profitable conventional oil reserves first.

Even then, heavy oil deposits now account for nearly 10 per cent of world oil production, and 15 per cent of the U.S. output.

Days before the UNITAR conference convened, Deputy U.S. Energy Secretary John F. O'Leary conceeded in a televised interview that the oil companies have neglected heavy oils.

"There are a lot of apportunities that have not been explored," O'Leary said, adding that the oil companies prefer to invest in more profitable oil-producing ventures. As for research and development to enhance the prospects for heavy-oil and tar-sands production, O'Leary said, "the government has not done very much."

O'Leary, who was scheduled to give the keynote address at the UNITAR conference, withdrew at the last minute without an official explaination of his absence. Members of the U.S. delegation privately said that O'Leary's optimism over heavy-oil prospects was viewed by the White House as "inappropriate at this time," and he was asked to withdraw.

(In Washington O'Leary in a phone interview said that he cancelled his speech before the UNITAR convention because of a scheduling problem, and that the suggestion it was under White House pressure was "a pure fabrication.")

During the conference, the U.S. and Canda agreed to establish a mechanism for formal R and D cooperation in tar sands and heavy oils. A similar accord between the Ottowa government and Venezuela was agreed to at the convention, calling for committing millions of dollars to heavy-oil development in the years ahead.

The key to heavy oil's future is the development of advanced production technologies that are more efficient than current methods which, by themselves, are energy-intensive. Also needed is new refining technology to "upgrade" heavy-oil and tar sands production, removing metallic impurities and sulfur.

A variety of production techniques calling on both tried and novel means of recovery are being used at nearly two dozen test sites in western Canada.

Heavy oil is being extracted in California and Alberta with steam and chemical injections that either heat up or loosen the petroleum deposits, which in turn are pumped to the surface using conventional methods.

Lower-quality Canadian tar-sand deposits are strip-mined, then processed on site.

Two commercial tar-sand plants - the only such operations in the world - are operating 250 miles northeast of here, and will account for 7 percent of Canada's domestic crude oil supply this year.

Other tar-sand plants either are being constructed or are clearing regulatory hurdles in Canada,, including a 125,000-barrel-a-day, $2.4 billion facility commissioned last autumn by Syn-crude Canada Ltd., a consortium of Gulf Oil Canada Cities Service, Imperial Oil and the Canadian national oil company; a $4 billion plant financed by a consortium led by Shell Oil has been proposed; and still another facility that would cost Exxon's Canadian facility $5 billion also is scheduled for construction.

"You need not be a prophet to predict that massive deposits of heavy oil will be discovered in the years ahead," said UNITAR's Barnea.