What will the big oil companies do with all that money?

As a result of decontrol of oil prices, they will accumulate extra estimated net income of $296 billion -- after paying the newly enacted tax on crude -- by 1990.

This year's first-quarter earnings -- more than double those of the same period in 1979 -- are but a first small installment.

Moreover, as The Washington Post reported Thursday, a controversial new accounting technique ordered by the Securities and Exchange Commission led the 11 largest U.S. oil companies to estimate that the present value of the future flow of cash from their oil and gas reserves rose last year by $95.1 billion -- from $132.7 billion to $227.8 billion.

Over the next decade, will the companies invest most or all of their huge profits in exploration and development as some of them would have the country believe?

Or will the companies go on a binge of acquisitions, thus extending the giant merger wave in which an estimated $100 billion of existing corporate assets has been rejuggled since 1975?

Many experts say the companies really have no choice but to buy up other companies, inside or outside of the energy industry. One reason is that the industry has what University of Maryland economist Dennis C. Mueller terms "a finite time horizon."

Just how soon the world will "run out of oil" is disputed, Mueller told the Senate antitrust subcommittee last year. "Nevertheless, the leading companies in the industry have to diversify if they are to maintain their position on Fortune's 500 well into the 21st century."

In a recent interview, another economist, who has worked in the oil industry and in federal agencies dealing with energy, said the companies are generating more cash than "they intelligently can spend."

He calculated the industry can spend no more than $12 billion for new drilling in the continental United States before it becomes uneconomic.

The economist, who asked that his name be withheld, predicted the companies gradually will produce less domestic oil while shifting emphasis to refining, marketing and acquisitions, particularly of other energy producers.

Three oil company acquisitions are possible omens.

Just last month, Sun Oil won conditional acceptance for the second largest acquisition in the nation's history when it offered to buy Texas Pacific, an oil and gas producer owned by Seagram Co. Sun will pay Seagram either $2.3 billion in cash or, at Sun's choice, $500 million in cash and two notes of $900 million each.

In 1979, Shell Oil made the largest acquisition in U.S. history when it paid $3.65 billion in cash for Belridge Oil.

In another huge cash acquisition in 1979, Exxon paid $1.16 billion -- about double the market price of the stock -- for Reliance Electric, a leading maker of electric motors.