President Reagan's proposal for removing federal controls from the price of natural gas would produce one certain winner: the major oil companies.

The administration's bill would free "old gas," which accounted for about 56 percent of gas bought by the major interstate pipelines last year, from all controls as of Jan. 1, 1986.

The top 20 producers of such gas--led by Mobil, Exxon, Texaco, Gulf and Shell--control more than 70 percent of it and stand to gain a $40 billion windfall between now and 1990 if controls are lifted, according to industry analysts.

These producers now are selling old gas to the 20 largest interstate pipelines at an average of about $1.38 per 1,000 cubic feet, according to the latest government figures.

The rationale for the government controls that hold down the price of "old gas" from wells that were producing before 1977 is that where drilling and exploration costs have been recovered, the cost of continuing production is low.

"Many of these wells were brought in years ago, they were easy to find because they were in or near known gas-producing geological formations, they are shallow, so they didn't cost much," a government analyst said. "The marginal cost of producing gas from some of these wells is only pennies."

But the bill lifting controls would enable the companies that own this old gas to double the average price for it. In some cases in which they are selling old gas for as little as 29 cents per 1,000 cubic feet, they may be able to boost their prices by 700 or 800 percent.

New gas is selling to interstate pipelines at an average of $3.30 per 1,000 cubic feet, according to the latest government figures. Analysts say they believe that the present market-clearing price where most gas sales would take place if no controls existed might now be as low as $2.50.

But the Energy Department projects that the average price of gas under its proposal will rise from the present $2.70 to $3.60 in 1986, when the decontrol process would begin. Thus, the price of old gas after Jan. 1, 1986, could be expected to approach the $3 level.

Moreover, producers of old gas would be able to demand this higher price immediately--even though some are locked into long-term contracts that run into the late 1980s and 1990s--because the administration bill would permit them to break every contract on Jan. 1, 1985.

"On Jan. 1, 1985, there will be a massive price shock as gas producers secure high prices for their old gas or exercise their newly granted market power to bring the natural gas market to its knees," said Rush Moody Jr., a member of the Federal Power Commission from 1971 to 1975.

Energy Secretary Donald P. Hodel, asked at a news conference to identify the winners and losers under the administration's proposal, conceded that producers of low-cost gas "are optimistic that they are going to get something better."

Hodel said producers of high-cost supplies, on the other hand, "are very fearful that their prices may be driven down."

But the highest cost gas--which comprises only about 5 percent of all domestically produced gas and has been selling at more than $7 per 1,000 cubic feet--is "the domain of relatively small companies," according to a Federal Energy Regulatory Commission official.

Thousands of independent companies also provide a majority of the remaining new gas, which may or may not drop somewhat in price under decontrol. While the major companies also own a considerable amount of this new gas, industry analysts say their stakes are not nearly as high as in old gas.

"Old gas is overwhelmingly skewed to the majors," the FERC official said.

These top 20 producers of old gas, according to a study performed by the Consumer Federation of America, had contracts calling for them to supply 72.3 percent of the old gas purchased last year by the nation's 15 largest pipelines, according to the federation.

The federation calculated that if decontrol were to occur in 1983, a prospect that under the administration's plan would not occur until 1986, these top 20 natural gas producers "stand to gain a $68.3 billion windfall from their sales of old gas to only 15 interstate pipelines for the period 1983-1990."

While the federation has not updated its projections to take into account the different schedule of the new administration proposal, other industry and government analysts say the White House plan could conservatively be expected to produce a $40 billion windfall.

The administration's plan would enable producers of this old gas to bring pressure on pipelines to renegotiate their contracts immediately following passage of the legislation, since failure to do so carries the certainty that producers would be free within months to break their contracts.

Most of the bargaining leverage, moreover, will rest with the major producers, industry analysts say.

Moody contended in a written analysis of the administration plan that all producer incentives to accept a bargain price for old gas through renegotiation prior to Jan. 1, 1985, "are removed when the producer is told that he has only to wait until that date and then will be free to walk away from all existing service obligations" and sell his gas to the highest bidder.

The top 20 old gas producers, in addition to the five mentioned earlier, are Tenneco, Standard of Indiana, Standard of California, Phillips, Atlantic Richfield, Getty, Cities Service, Union, Superior, Sun, El Paso, Conoco, Pennzoil, Marathon and Columbia, according to the consumer federation.