White House energy adviser James R. Schlesinger Jr. told Congress yesterday there is a "50 per cent to 60 per cent chance" that President Carter's standby gasoline tax would be imposed if enacted into law.

Conserving gasoline to avoid the tax "represents the type of challenge Americans have been successful in meeting in the past." Schlesinger said during Congress first hearings on the administrations energy bill.

Testifying before the Senate Committee on Energy and Natural Resources, he seemed in the face of mounting public and congressional opposition to the tax, to reinforce Carter's statement last week that it could be avoided if drivers reduce gasoline consumption.

Some senators voiced mixed support for and others outright opposition to parts of the comprehensive 103-section energy bill Carter sent Congress last Friday.

Sen. Henry F. Jackson (D-Wash.), chairman of the energy panel, said. "We'll debate it, discuss it, and we'll come up with a program . . . I except we'll report to the Senate most of those parts of the program which come to this committee."

Carter's standby gasoline tax, which would add 5 cents a year to the price of gasoline over the next decade, starting in 1979 for every year that consumption exceeds a national target, continued to draw Congressional fire.

Sen. Dale Bumpers (D-Ark) said that few economists supported the gasoline tax. He called it "a patently unfair method of trying to conserve energy."

Sen. J. Bennett Johnston (D-La) opposed the administration's proposal that newly discovered oil and gas be free of price controls if they are found at least 2.5 miles from existing wells. Johnston argued that the proposal would not provide an adequate incentive to increase production.

"We are open to suggestions on that point," Schlesinger said.

Another Carter bill that would pull all energy functions together into one Cabinet-level department is expected to be approved by House and Senate committees this week.The Senate Governmental Affairs Committee began work on the bill yesterday. The House Government Operations Committee will vote on it today or Thursday.

A major issue will be whether to accept the President's proposal to let the new Secretary of Energy, who will be Schlesinger, set the price of natural gas, or to retain that authority in an independent regulatory agency.