Experts say that one of the main devices President Reagan employed to keep next year's projected budget deficit under $100 billion is unworkable, and will not produce the revenue claimed.

The president estimated that the Interior Department would collect $18 billion in fiscal 1983 from offshore oil and gas royalties and sales of drilling rights in the Outer Continental Shelf.

But that $18 billion would be 180 percent of the highest year on record, and 230 percent of what is estimated from this source this fiscal year. It would mean gleaning in a single year as much as the entire leasing program produced in the 1950s, '60s and early '70s combined.

"Good grief!" said Douglas Henderson of the American Petroleum Institute when told how much the administration expected for 1983. "I don't see how it would be possible."

Other sources, from the industry and from such centers of expertise as the Congressional Budget Office, congressional committees and environmental groups, also say the projected $18 billion is too high, by $6 billion or more.

The $18 billion is carried in the budget as an offsetting receipt. It is one of the largest non-tax revenue sources the government has. It has been used traditionally by presidents to make forthcoming deficits look smaller than experts expect them to be. But never has it been used on this year's scale.

A spokesman for the Interior Department, James Robinson, concedes that the number is "our most optimistic estimate," and that already some delays being written into the leasing schedule might change matters.

At the Office of Management and Budget a spokesman stuck by the number as a real one, given the administration's determination to accelerate offshore leasing and oil and gas production generally.

But officials at three oil companies--Shell, Exxon, and Union Texas Petroleum--all said they found the projection optimistic; Robert Nanz of Shell said it was "highly speculative."

The Petroleum Institute's Henderson suggested that softness in the world's oil market would work against the OMB expectations. "Costs are going up and oil company revenues are going down. I think that would raise questions."

He said the oil industry has the capability to bid on the huge amounts of offshore acreage scheduled to be offered. The question is how willing they will be to go for the particular tracts involved.

D. Michael Harvey, a Democrat on the Senate Energy Committee staff, worked for years on leasing for the Interior Department. "My initial reaction when I saw this number was that it was really off the wall," he said. "They were doubling it!"

Harvey said, "There is a long history at the Office of Management and Budget of bumping up the OCS Outer Continental Shelf leasing program revenues to make budget figures look good. This goes back a lot of years. I know in the 1960s and '70s when I was there, every time we sent our estimates over to the OMB, they came back a little higher."

When Harvey began to look into the details of the 1983 figure, he found some unusual facts that made the number seem less improbable, but still too high, he said. "It could still be off by $6 billion, or more," he said. The question now is not "whether it's inflated, there's just no question about that. But inflated by how much?" he said.

The Interior Department does have nine offshore-lease sales scheduled for 1983, pending final revision in July. This is more than any other year. Seven sales in 1981 brought revenues of $10 billion, the highest on record.

Because of past court cases, there is also some $3 billion in escrow that Interior has guessed will be released in 1983. Half of it will go to the government and will show up as 1983 revenues.

Moreover, the last two lease sales of 1982 come near the end of the year, and the checks for those sales may not be received until fiscal 1983 has begun, so technically they may count as 1983 revenues.

The OMB also says that some of the 1983 tracts are especially rich; there are believed to be 28 billion barrels of oil and reserves in the land to be leased. This year's set-aside land holds an estimated 4.3 billion barrels, and OMB has projected lease sales yielding about $4.5 billion.

Because next year's tracts are estimated to have seven times more oil and gas than this year's, OMB has lifted its estimate of likely revenue three-fold, to $13.2 billion. Royalties and other lesser amounts will lift total proceeds to $18 billion. OMB said its prediction reflects "conservative probability factors on such items as possible delays in sales, unsuccessful bids, incorrect reserve assumptions, etc."

But such experts as Harvey at Senate Energy and Ann Hoffman at the Congressional Budget Office remain skeptical.

The estimate that 28 billion barrels of oil and gas will be offered in 1983 is based on a number of extraordinary assumptions, they said.

First, the figure of 28 billion depends on speculation about what specific tracts may be offered or leased. "But the way that is estimated is just to take estimates for the whole Gulf of Mexico and divide them in half. There is no way of telling what will actually be offered, or might be bid on," Hoffman said.

It also assumes that all sales scheduled will be held. Over the past decade, there have been some 15 lawsuits that interfered with sales, and about half of them resulted in delays of a few months to two years.

Susan Chasis of the Natural Resources Defense Council said that four or five of the areas up for lease are likely to be hit with suits and other action by environmentalists and state governments. Alaska and California have vigorously resisted plans for leases off their coasts.

Unexpected events such as lawsuits caused the OMB's last set of predictions to be off by 30 percent. Its original estimate of revenues for 1982 was $11 billion. It was recently revised downward, to $7.8 billion, and that figure is not yet certain.

Two recent sales also have proved disappointing when measured against first predictions. The last sale of leases in the Atlantic drew such disappointing bids that Interior rejected all of them.

And this month in a sale in the Gulf of Mexico, the richest and easiest area to drill for oil, the $1.25 billion bid was far less than the original estimates. Only 59 percent of the tracts available were bid on.