A report from Sweden yesterday that the Soviet Union had discovered an oil field larger than all those in Middle East combined temporarily battered the stocks of oil companies, drilling concerns and oil-field equipment suppliers on U.S. financial markets.

Soviet officials promptly denied the report, and American oil experts also debunked it. Nevertheless, many U.S. oil company stocks were delayed in opening because of the report and suffered an initial drop in their value.

The report came from PetroStudies, an oil-consulting firm based in Malmo, Sweden, that has consistently issued optimistic projections on the Soviet Union's hydrocarbon reserves. It claimed a newly discovered area in western siberia, larger in size than France, contained estimated recoverable oil reserves of 4.5 trillion barrels.

"This is the biggest oil find in history by far," a PetroStudies spokesman said. "This is foolishness," snapped a geologist with the U.S. Geological Survey very familar with Soviet oil resources.

In Moscow, a Soviet Ministry of Geology official confirmed that there was oil at the site mentioned by PetroStudies but said the fields was small and had been operating for years. He said PetroStudies' claim that the field covers about 600,000 square miles -- 5 percent of the Soviet Union's total land mass -- strained credulity.

The "field" is actually a large area in western Siberia in which oil-bearing shale is found at a depth of about 10,000 feet. Except in one limited area, called the Salym field, well drilled into the shale have either been dry or produced so little oil they were not considered commercial.

The Samotlar field, the largest in the Soviet Union and the 11th largest in the world, lies near the edge of the shale area, but its production is from other, younger rocks closer to the surface.

In contrast with the PetroStudies claim of a 4.5 trillion barrel discovery, the Samotlar field contained approximately 14 billion barrels of recoverable oil when it was found in 1966. The world's largest known oil field, Ghawar in Saudi Arabia, contained between 70 billion and 80 billion barrels of recoverable oil, geologists estimate.

In an article published last summer in the Oil and Gas Journal, James W. Clarke and Jack Rachlin of the U.S. Geological Survey estimated the shale area probably contains as much as 15 trillion barrels of oil, but that only a tiny portion of it can be produced. The oil is contained in the somewhat porous shale. But since the rock is not extensively fractured, it remains in the rock rather than accumulating in pools from which it can be pumped.

This is a familiar problem to the oil industry, and many techniques have been developed, with varying success, to fracture such oil-bearing rocks. The Soviets, according to Clarke and Rachlin, apparently attempted to fracture the shale formation in October 1979 by detonating an underground nuclear explosion close to the presently producing Salym field.No information is available about the success or failure of that venture.