Venezuela's state petroleum company says the Latin nation's Orinoco "heavy oil" tar fields can produce about 500 billion barrels, about the equivalent of the world's known conventional oil reserves, at costs well below current international prices.

In a scientific paper to be delivered Wednesday at a United Nations energy conference, Venezuela claims its vast Orinoco fields can yield "a high-quality synthetic crude costing in the range of $5 to $13 a barrel."

Citing a "range in recoverable reserves" of from 70 billion to 900 billion barrels, a Venezuelan oil official said that 500 billion barrels is "a realistic planning figure for the time being." The estimate is far more optimistic than any ever used by Venezuela.

If the Venezuelan claims are borne out, the price and supply dimensions of the world's oil markets could change significantly during the next decade, sharply alleviating the present pessimistic outlook.

[OPEC's outgoing chairman has said he belives the organization will increase oil prices only slightly at next month's meeting in Venezuela. Story on Page A20.]

State Department officials said yesterday that much depends on the willingness of the recently elected Caracas government to proceed with an ambitious heavy oil development program.

Economic and profitable heavy oil production on a commerical scale in Venezuela, Energy Department officials said, could lead to more aggressive exploitation of heavy oil deposits in the United States, Canada and elsewhere in Latin America. California alone has an estimated 30 billion barrels of heavy oil deposits.

Citing an analysis of current small-scale commerical production facilities in the Orinoco River Basin, Venezuela's oil company says, "All cost figures indicate levels well below those of other nonconventional sources of energy such as tar sands, shale oil, etc . . ."

Venezuela's Orinoco Tar Belt, as it is known in petroleum circles, was first discovered in the mid-1930s and extends east to west across a 13,000-square-mile strip along the Orinoco River. It contains billions of barrels of heavy oil, an extremely thick, molasses-like petroleum deposit, that is also laden with heavy metals and sulfur.

International oil companies such as Exxon and Occidental Petroleum, operating in Venezuela until there were nationalized in 1976, developed plans to produce oil along the Orinoco belt, but never started up commerical operations because of the Caracas goverment's policies and the lower world oil prices then prevailing.

The paper, entitled "Venezuela's Heavy Oil Development Prospects and Plans," was written by Arnold Volkenborn, a senior executive from Maraven, S.A., one of Venezuela's four state oil companies. Maraven now produces about 600,000 barrels of Venezuela's 2.35 million barrels a day in exports. Venezuela is an important U.S. supplier, sending more than half its oil to this country.

A summary of the paper was distributed yesterday in Montreal by the U.N. Institute on Training and Research, which has convened a two-week conference attended by scientists from 100 countries.

Yesterday in Montreal, officials, said that the heavy oil discussion is "a signal Venezuela is going to be undertaking development."

In recent years Venezuela, which was one of the Organization of Petroleum Exporting Countries' founding members, has been faced with declining oil reserves. Currently, Caracas lists its official proven oil reserves at 18 billion barrels, compared with about 30 billion for the United States and more than 172 billion barrels for Saudi Arabia.

Volkenborn says "the Orinoco oil belt is probably the largest essentially untapped oil accumulation in the world," with oil deposite at depths as shallow as 600 feet, and extending as deep as 7,000 feet.

For some years Venezuela has been exploiting heavy oil fields on the northern fringe of the belt and today 80,000 barrels a day are being pumped to the surface, utilizing technology that can be applied to other heavy oil formations. Most of this heavy oil, Volkenborn says, can be produced by pumping steam into the formation, heating oil, and therby making it easier to flow to wells that carry it to the surface.

The Maraven engineer says that "alternative development schemes" could lead to production rates of up to 3 million barrels a day by the year 2000. Existing plans, however, only call for an additional 1 million barrels a day of Orinoco oil output by the turn of the century.

Former Venezuelan President Carlos Andres Perez put off a major program to sput development of the Orinoco fields, largely because of opposition political pressure against early exploitation of the resource, and arguments that it would result in a new ear of "exploitation" at the hands of the multinational oil companies. In the years ahead, however, Venezuela theoretically could get the largescale technology without turning directly to companies such as Exxon.

The new president Luis Herrera Campins, elected last March, has yet to unveil a major heavy oil development plan, but his oil minister, Humberto Calderon-Berti last summer said that the government was "very interested" in moving ahead.

Earlier this year Calderon-Berti and other Cabinet members met with former Energy Department Deputy Secretary John F. O'Leary, and the President's science advisor, Dr. Frank Press, to discuss cooperating in heavy oil production. West Germany's Economics Minister Otto Graff Lambsdoroff also recently asked Venezuela to enter into a joint heavy oil development effort.