Current world oil supplies are such that any country cut off by Iran could replace much of its lost crude by buying elsewhere, U.S. government and many private experts believe.

Iran has threatened just such a cutoff for any country joining the United States in sanctions against it.

As a result of a drop in oil demand, Iran is exportng less than 2 million, barrels of oil a day, possibly as little as 1.5 million, Carter administration officials said.

Loss of a substantial part of that oil probably would still leave production elsewhere in the world only slightly below demand at a time when seasonal use of oil is about to begin dropping, they said.

Cargoes of curde are available at spot market prices in the Persian Gulf, but are finding virtually no buyers, officials at several major oil companies said. From an average of $40 to $45 barrel, prices have slipped to the $35 to $37 range and still there are few sales. One cargo of heavy crude recently changed hands at about $32 a barrel, one oil executive said.

This easing of the demand for crude oil is a major reason the administration has felt free to pursue the sanctions approach, even with the threatened cutoff by the Iranians, in its effort to free the 50 hostages being held in the U.S. Embassy in Tehran.

The Japanese, who have been importing about 530,000 barrels of Iranian crude daily, have been especially reluctant to go along with the sanctions, however, no matter what the short-term crude outlook. Japan, almost totally dependent on imported energy, is concerned about the longer term effect on its relations with Iran, relations the Japanese have been hoping to cement to guarantee access to oil.

A year ago a halt in Iranian output and then fears about future Iranian production levels drove up oil prices and helped cause last spring's gasoline lines in the United States.

But over the coruse, of the last year, usable oil inventories have nearly tripled and some countries, including the Japanese, are running out of storage space.

In addition, demand for oil is dropping around the world because of sharply higher prices and much slower economic growth in the industrial nations, oil experts noted.

While administration officials would not confirm it, some sources said other Persian Gulf exporters, such as Kuwait, Saudi Arabia and the United Arab Emirates, have assured the United States they will keep their production high in the face of any cut in Iran's exports.

Rulers in those Arab countries have supported the United States in its confrontation with Iran fearing the Ayotallah Ruhollah Kromeini's brand of revolution might spread to their countries.

Government officials are leery of promoting talk of an oil glut, but one said, "In the very near term, I could see a way of riding out the shortage [from a halt in Iranian oil exports] if consuming nations behave sensibly."

The 530,000 barrels a day Japan is getting "could be replaced elsewhere without the spot market roaring back to life," he continued, "if the market had the expectation the Saudis and the Kuwaitis would maintain production at current levels."

Even though there has been no concerted effort by most nations to reduce their dependence on Iranian oil, it has happened anyway in some cases.

West German officials said only about 1 percent of their oil imports now come from Iran, mostly through a direct contract with Veba, the state owned oil company. As recently as last fall about 12 percent of Germany's imports were from Iran.

Other Iranian supplies were reduced when Iran refused to allow U.S. oil companies to load cargoes destined for countries other than the United States. That action came immediately after President Carter embargoed Iranian oil shipments to the United States.

Beyond that, Iran has reduced the amount of crude oil available to Royal Dutch Shell and British Petroleum during 1980, part of which would have gone to West Germany.

Italy and France have been dependent in Iran in recent months for less than 5 percent of their oil.

Government officials are sanguine about an Iranian cutoff, too, because they think Iran simply could not afford to end oil exports no matter what is is now threatening.

The question is whether the Iranians or the rest of the world would feed the pinch first. Ending oil exports would quickly mean "austerity with a vengeance," one administration official said. "With 50 percent unemployment in Iran, what fuels that economy is export earnings from oil."

"The threat of a shutdown doesn't seem likely," he added, Because we could come through it" while the Iranian economy was thrown into chaos.

Even as the market has been forcing a cut in production on the Iranians right now, the official continued, the Japanese, Shell and BP have been signing up for oil for 1980 "because they want to secure supply" since other producers might cut output as some have threatened.

But as long as those threats -- principally from Kuwait and Saudi Arabia -- do not materialize, and they are not likely to in this situation, then there should be enough crude to go around.