The Soviet Union is believed to have conducted an underground nuclear explosion in western Siberia last October to hasten the flow of oil from Russia's second-largest field.

Two geologists from the U.S. Geological Survey said yesterday that an underground explosion that took place in the Middle Ob region of Siberia on Oct. 4, 1979, almost had to be an experiment to stimulate production in the huge Salym oil field where the flow has been so slow that some wells are producing as little as 35 barrels a day.

"The fractures in the ground that allow oil to flow freely are just not there in the Salym field," said Dr. James W. Clarke, who together with Dr. Jack Rachlin released the report."This explosion could have been a way to create those fractures to increase production."

The site of the explosion, which was pinpointed by 115 seismic stations the USGS operates, lies 200 miles from the giant Samotlor oil fields in Siberia and about 1,000 miles from Semipalitinsk, where the Soviets conduct underground nuclear weapons tests.

The explosion occurred in a region of Siberia where earthquakes are exceedingly rare; Clarke and Rachlin described the region as "aseismic," meaning even small earthquakes are recorded infrequently.

The explosion registered as an "underground event" with a magnitude of 5.4 on the Richter scale, suggesting it was an explosion with a force of 100 kilotons. This is below the 150-kiloton limit imposed by the partial test ban treaty. The explosion is believed to have taken place at depth of almost 10,000 feet.

Clarke and Rachlin said that estimates of reserves in the Salym field run as high as 10 billion barrels, making it the second largest in the Soviet Union and one of the 10 largest in the world. However, production at Salym has been no more than 10 million barrels, almost all of it in 1976.

Many of the wells in the field have dried up, Clarke said, in part because the rock where the oil is trapped is so tight that there are few pathways for oil to flow to the surface.

Clarke and Rachlin said there could be two reasons for the Soviets to resort to a nuclear explosion: to shut in some wells to raise the pressure below ground to force the oil up, and to create fresh fractures in the rock to open up new paths to the surface.

"If the October expolsion stimulated production at Salym," Rachlin said, "then the Soviets will probably conduct quite a few more atomic explosions in the same field."

The two geologist said it has been at least two or three years since the Soviets discussed with their American counterparts any plans to stimulate oil production with nuclear explosives. The last time the Soviets detonated an atomic device in a nonmilitary experiment was at least five years ago.

Clarke and Rachlin said the reason the device was exploded in October when Siberia had begun to freeze is that winter is the only time oil drilling can go on. In spring and summer months the rivers flood and the flat Siberia land grows soft with mud.

"When the ground is frozen, it is much easier to drill." Clarke said. "Besides, the rivers are all frozen and you don't have to worry about floods that make heavy equipment work so difficult."