The government may have lost as much as $7 billion in 18 months by auctioning offshore oil tracts under the Reagan administration's controversial "areawide" leasing program, according to a new General Accounting Office study.

According to the GAO, the government lost an average of $3.1 million on every tract it leased between April 1983 and September 1984, largely because the areawide system "significantly decreased" competition for oil leases and reduced the size of industry bids.

The study sharply contradicts the Interior Department's calculations, and industry groups immediately attacked it as "misleading."

But Rep. John D. Dingell (D-Mich.), who requested the study, said it raises questions about the areawide leasing system, and environmental groups said it confirms their belief that the administration's aggressive leasing policies have been an "economic disaster."

At the heart of the debate is a leasing system designed by former Interior secretary James G. Watt as the centerpiece of his five-year plan to speed offshore oil exploration. For nearly three decades, the department had offered leases on specific sites nominated by industry; under Watt's system, the department put vast areas of the ocean up for bid all at once.

Conservation groups were quick to criticize the new scheme, arguing that the government could not possibly conduct adequate environmental studies on such massive areas. But in recent months, the debate has centered on the economic impact of the new system.

The GAO report confirms findings by Texas officials last year that the areawide leasing system has dramatically reduced competition for oil tracts and depressed bids, meaning that the government is putting more land under lease but is getting less money for it.

Bonus bids -- the "up-front" money the government gets for leasing oil tracts -- dropped from an average of $2,624 an acre under the old leasing system to $686 an acre under the areawide system, the GAO said.

GAO investigators also questioned whether the government is getting fair market value for offshore oil resources, as required by law. Interior has little or no geological data on some of the tracts it is offering, the report said, and is instead relying on "the marketplace" to determine if bids are adequate.

Oil industry groups agree that bids have been lower under areawide leasing, but attribute that to a sagging oil market and offshore lease sales in less productive areas of the outer continental shelf.

But the GAO, which adjusted its figures to account for variables such as low crude oil prices and less desirable sites, said it found that "areawide leasing, by itself, significantly decreased competition and government bid revenues for individual tracts."

For the 13.03 million acres leased in the first 10 areawide sales, the report said, "The federal government received about $7 billion less than it would have received if the same acreage were leased under the tract-selection program."

Interior officials have staunchly defended the program, arguing that the up-front money tells only part of the story. The department contends that the government will make up the difference in revenue from royalties, rents and taxes when the newly leased lands start producing oil and gas.

GAO investigators acknowledged that royalties and rents would add to government revenues, but they noted that bonus bids have historically made up nearly 70 percent of oil revenues.

Interior released its study on areawide leasing earlier this month, contending that the system has boosted industry interest and earned more revenue for the Treasury. William D. Bettenberg, director of the department's Minerals Management Service, said the program was designed to "increase the rate of investment in exploration" and it had met that goal.