If Congress simply makes no change in existing law, President Carter will automatically achieve within three years the same large increase in oil pirces he unsuccessfully sought as part of this year's energy plan.

That is because the present system of oil price controls is scheduled by law to expire by the fall of 1981.

Carter also has legal authority to hasten that expiration; in fact, he has power to abolish controls next spring.

If oil were his only concern, aides say, he might well do this. The administration wants higher oil prices to discourage consumption while stimulating domestic production, thus reducing imports.

But if Carter let U.S. oil prices jump all at once to world levels next spring, he would drain some purchasing power from consumers and businesses and might cause the economy to falter just at a time when administration economists think it will need stimulus.

Thus aides say the president is most 'likely to propose another tax to drive up the price of crude oil next year, with the proceeds to be rebated to consumers through a reduction in income taxes. Then as controls were phased out and prices rose over the next three years, the tax would decline.

This is also what Carter proposed - and Congress rejected - as central element in his original energy plan.

To make Congress take it this time, administration strategists say the president may threaten to abolish controls if the tax is killed.

That way he would be telling Congress, "The price is going to rise no matter what you do. The only question is whether you want the money to go to the companies or the Treasury."

Whatever happens on a tax, Carter is also thought likely to move seprately in the next few months to relax the present system of controls. The administration has already promised oil-state members of Congress it will do this.

The present controls system sets up two main categories of oil - old, from wells drilled before 1972, and new, which is mainly oil from newer wells or increased daily volumes of oil from old one.

Old oils is controlled at about $5.90 a barrel now. New oil is allowed to sell for about twice that.

The administration is expected to redefine some old oil into new by changing what is known as the "decline curve" for old wells. That curve tells how much oil a well could normally be expected to produce in each year of its life.

Since anything over normal production is classified as new, a judicious change in the decline curve can benefit producers greatly.

In any case, the amount of old oil in the mix is declining each year and the amount of new oil rising, simply as old wells give out.

Currently, old oil makes up about [WORD ILLEGIBLE] percent of U.S. production and 20 percent of U.S. consumption, counting [WORD ILLEGIBLE]. Those percentages are expected to decline in the next few [WORD ILLEGIBLE]

The average price of oil produced in this country is now about $9 a barrel under controls. The "uncontrolled" world price of oil delivered to U.S. refineries is about $13.50.

If Carter succeeded in lifting U.S. prices to the world level next year, the added cost to consumers would be more than $10 billion. The president promised at last summer's economic summit in Bonn to move U.S. prices to world levels by 1980.

1 There is one great irony as Carter decides what to do on oil prices next year. Some of the strongest opposition to the abandonment of controls is coming from within the oil industry.