From early on in the hostage crisis, the Carter administration was looking at a Pentagon plan to occupy Kharg Island at the head of the Persian Gulf to gain control of Iran's oil spigot.

The idea of a seaborne attack that would seize Iran's central outlet for the international oil trade was rejected, for reasons that help explain why President Carter has been so reluctant to use military force to free the hostages in Tehran.

Officials yesterday, for the first time, confirmed details of the military option and why it was turned down.

The main reason, beyond the obvious one of jeopardizing the lives of the hostages, is that President Carter does not want to risk losing Iran, and its oil, to the Soviet Union or anyone else.

This overall diplomatic objective, not the military difficulties, is what has kept the Kharg Island option, and other war plans like it, in the bottom drawer, at least for now.

According to informed sources, the main elements of the Kharg Island plan put together in a thick-document by the staff of the Joint Chiefs of Staff are:

Marines on amphibious assault ships would steam up the Persian Gulf and disembark onto Kharg Island, presumably unopposed by Iranian forces.

Once on the island, they or technicians with them would take over the oil lines on the island that are used to fill up tankers form all over the world with Iranian oil.

In advocating this plan in "informal" government discussions, former CIA director Richard Helms was said to be among the alumni of the intelligence community who argued that the United States could decide, once it got control of the spigot on Kharg, which countries got the oil.

Besides obtaining leverage on other nations, backers of taking over Kharg contended that denying the Ayatollah Ruhollah Khomeini oil revenue would put tremendous pressure on the Iranian leader without reaping the whirlwind of criticism which bombing Iran would get from the Moslem world.

It was aassumed by the military planners that Iran might shell U.S. troops from the mainland, forcing the Marines to dig in. But this problem was considered manageable.

The big worry of administration officials as they reviewed the Kharg plan was what the Soveits would do in response. Would they offer Iran military aid, thus forging the link the president wants to keep from being formed?

Even if the Soviets did nothing more than show their flag in Iran, perhaps by sending warplanes onto an Iranian airfield near Kharg, officials feared that a relationship born of Iranian desperation would be established.

Of course, taking any kind of military action anywhere entails risks. And some Pentagon planners are among those American chafing over President Carter's refusal to take more risks.

Information the United States has gathered through electronic intelligence heightens the frustrations of government insiders. The intelligence confirms that Iranian officals feel confident that Carter will take no military action to free the hostages, despite the occasional saber-rattling in Washington.

Carter's plan, Iranian officials have concluded, is to suffer through the humiliations infilicted by Tehran in hope of reestablishing a relationship with the country when a stable government finally emerges.

Although the president's patience with Iran is not infinite, and presumably would come to a quick end if the hostages were harmed, Pentagon planners have been told to keep their eyes on the long range objective of reestablishing relations with Iran.

"Bombing Iran would make us feel good for about a week," said one Carter administration official in backing restraint. Because of this philosophy at the top, the military's plans for occuppying Kharg Island, bombing Iran and mining and blockading the Persian Gulf straits have been written but not implemented.