A General Accounting Office investigation has concluded that offshore oil and gas sales--which the Reagan administration had counted on to help reduce budget deficits--will not produce the amount of revenue the administration claims.

The White House first estimated that the Interior Department would collect $18 billion in fiscal 1983 from lease sales for offshore oil and gas and from royalties in the Outer Continental Shelf.

In April, the administration lowered the estimated take to $15.7 billion because of depressed oil prices and "public concern that the $18 billion estimated was too high," according to the GAO. The GAO now reports that both the original estimate and the April revision are both probably too high.

A more reliable figure is the $12.8 billion estimated by the Congressional Budget Office, the GAO suggested.

Rep. Edward Markey (D-Mass.), chairman of the oversight and investigations subcommittee which will hold hearings Monday on the president's new accelerated lease program, said that the GAO report "demonstrates the willingness of Interior to manipulate its financial projections for the appearance of deficit reduction."

An Interior Department spokesman declined to comment on the report, saying the department has not had a chance to review it.

The OCS revenues are carried in the budget as a receipt to offset expenditures. Revenue projections for the lease sales and royalties, one of the government's largest non-tax revenue sources, apparently often have been used by presidents to make forthcoming deficits look smaller.

The White House revenue estimates depend largely on two offshore sales in the Gulf of Mexico. The administration's projections of revenue from those two sales account for $8.7 billion, or two-thirds of the whole year's "bonuses"--lease sale prices not counting royalties. The Congressional Budget Office estimates the same sales will bring in no more than about $4.5 billion.

"Bonuses of this magnitude seem questionable since most of the Gulf areas have already been considered by industry in the past" but have been passed up in sales, the GAO report said.

The GAO went on to say that:

* Proposed Gulf sales "include some deep water tracts of high economic risk and uncertainty," which will drive the price down.

* The last two sales in the Gulf of Mexico brought in far less cash than expected, and oil experts have said that the industry has far less interest in oil from the Gulf than in previous years.

* Although offshore leasing increased in the last decade, "Interior's goals were never achieved . . . . Only about 60 percent of the planned sales were held, a small fraction of the land planned was leased, and nine frontier areas were not opened up for leasing as programmed."

Over the past decade 15 lawsuits have interferred with sales, and about half of them resulted in delays of a few months to two years.

In the most recent OCS sale, last week in California, 24 of 140 tracts were withheld from sale pending a trial. The court ruling permitted lease sales on 140 tracts from Point Conception to San Clemente Island, but no bids were received on 105.

The total amount brought in was $132 million, much less than previous California sales. Despite the relatively low bid total, Reid Stone, minerals manager of the department's Pacific Outer Continental Shelf Region, said he considered the sale a success.

"Considering all the difficulties we've had in reaching this point, it looks like we have a successful sale," he said. Past auctions have been much richer, with $2 billion in high bids accepted at a 1981 sale of 81 lease tracts north of Point Conception. One tract alone went for $333.6 million in that sale.