Gulf and Conoco -- two of the nation's largest oil companies -- reported hefty third-quarter profit increases yesterday as the Energy Department issued sharply higher estimates of the impact of President Carter's oil decontrol program on comsumer prices and industry revenues.

Gulf, the fifth-largest U.S. petroleum firm, said third-quarter profits climbed 97 percent from the same period last year while Conoco, ranked ninth in the industry, posted an increase of 134 percent.

Both firms attributed all or most of the higher profits during the recent quarter to overseas operations. For example, Gulf listed substantial gains in Europe because of "the tight supply situation, a more liberal attitude by European governments in allowing the prompt pass-through of increased costs and the weakness of the U.S. dollar in relation to European currencies," according to Gulf Chairman Jerry McAfee.

Similar steep profit increases were reported earlier by Exxon Corp., Standard Oil Co. of Indiana (Amoco), Atlantic Richfield Co. and Occidental Petroleum -- all at a time of heightened political interest in oil profitability as Congress considers the administration's proposed tax on "windfall" industry profits.

The projected windfall is expected to develop under a program of phasing out oil price controls over the next two years, but the Energy Department said yesterday that earlier estimates of the impact of decontrol on prices and oil industry revenues were too low.

By October 1981, when controls expire, prices of gasoline and other fuels will increase between 7 1/2 and 12 1/2 cents a gallon, the DOE said yesterday.

In contrast, when Carter unveiled last April his decontrol plan to phase out all price controls for domestic production, Council of Economic Advisers Chairman Charles Schultze said decontrol would raise fuel prices by 4 cents or 5 cents a gallon.

The higher estimates are due largely to the sharp increase in world crude oil prices by the Organization of Petroleum Exporting Countries. Since Jan. 1, these prices have jumped and average 60 percent -- not counting price boosts announced in recent days by several producing nations.

The DOE estimated yesterday that oil company revenues would increase cumulatively between next January and late 1981 by between $14.6 billion and $24.8 billion.

Wall Street securities analysts have been advising investors in recent months to buy oil stocks, saying that the oil decontrol program will boost profits far above earlier expectations.

These forecasts and the third-quarter profit reports were in the background yesterday as Treasury Secretary G. William Miller urged Congress to enact promptly the proposed tax on oil profits. Miller's comments came in an unusual Treasury statement.

"The windfall profits tax proposed by President Carter and passed by the House . . . left ample incentive for oil companies to explore for new oil," he said. "Furthermore, substantial increases in world oil prices since the House action mean oil company profits will be higher than previously expected . . . there is no justification for diluting the proposed [tax]."

The House has approved a tax to generate $273.4 billion of new revenues by 1990 compared with $141.7 billion in a Senate Finance Committee measure. "The third-quarter earnings reports of major U.S. oil companies dramatize the merits of our proposed windfall profits tax, which is fair both to the oil companies and to the American people," Miller asserted.

Gulf's third-quarter profits were a record $416 million ($2.13 a share) compared with $211 million ($1.08) a year ago, as revenues rose to $6.7 billion from $5 billion. Nine-month earnings increased to $956 million ($4.90) from $548 million ($2.81), and revenues rose to $18.4 billion from $14.7 billion.

McAfee said his firm's performance would fall below the 1974 level if adjusted for inflation over the past five years. He also said that Gulf's profits in the United States were up 36 percent in the first nine months and that return on capital invested in the 12 months ended Sept. 30 was 12.1 percent vs. 8.2 percent a year ago.

Conoco's profits in the recent quarter were $247 million ($2.30 a share) compared with $106 million (98 cents) a year ago, as revenues increased 41 percent to $3.5 billion. Nine-month earnings rose to $625 million ($5.81) from $296 million ($2.75). U.S. profits in the third quarter were up just $6.6 million to $72 million, Chairman Ralph Bailey reported.