Saudi Arabia is preparing to lower its oil output in the fourth quarter of this year regardless of the outcome of the conference of the Organization of Petroleum Exporting Countries in Vienna later this month.

Oil, foreign and finance ministers of OPEC meet in Vienna Sept. 15 to discuss and approve a long-term strategy policy, including an automatic mechanism for price increases. Realignment of prices before the OPEC summit meeting scheduled for Oct. 25 in Baghdad is an important priority for the conference, and Saudi Arabia reportedly is prepared to cut its rate of output to achieve one.

Sheik Ahmed Zaki Yamani, Saudi minister of oil, reportedly told Lord Carrington of the decision to lower prodction when the British foreign secretary met him during his visit to the kingdom last week. But Yamani did not say how big a reduction was contemplated by the supreme petroleum council.

Apart from the coming OPEC conference, the continuing surplus on the market and the high level of stocks in themselves justify bringing down the present permitted ceiling of 9.5 million barrels a day, according to Saudi thinking. A unilateral move would also help create the right climate for compromise on a unified OPEC price.

It has become clear, meanwhile, that the amount of oil available to Exxon, Standard Oil of California, Texaco and Mobil will be drastically cut under the terms of the complete takeover by the Saudi government of the Arabian-American Oil Co. (Aramco).

Yesterday, a spokesman for Aramco in Washington confirmed that this had been substantially completed with the payment of a final installment of the $1.5 billion or so compensation in April for the companies' residual 40 percent stake in the operation.

Final terms have yet to be arranged, and the argreement has not been signed. Nevertheless, supplies to the four U.S. majors could be slashed from the present level of about 6.9 million barrels a day to as little as 4.5 million, mainly because of increased sales by the state and the expansion in its refining capacity.

Yamani has said Saudi Arabia would come down to 8.5 million barrels a day if other OPEC countries committed themselves to adopting proposals for raising prices at regular quarterly intervals according to the formula laid down in OPEC's ministerial committee on long-term strategy.

Before the OPEC conference in Vienna the six-man committee chaired by Yamani is expected to meet in London at the end of next week in an attempt to resolve the differences over the recommendations about prices accepted by the majority of members.

The committee's report proposes an index to take account of the impact of inflation in international trade based mainly on exports of industrial countries, an automatic exchange rate adjustment factor, and to give a progressive increase in real terms -- the average growth in real terms of the gross national product of Organization for Economic Cooperation and Development countries.

Initially, Iran, Algeria and Libya held out for an inflation index based on the cost of members' imports and -- far more inflationary -- a provision for increases in real terms to be based on the extraordinarily high growth rates of OPEC states.

News services reported the following.

Al Bayrak, an independent Beirut newspaper, and oil industry sources cited by Reduter also predicted that Saudi Arabia will cut back its oil production.

Al Bayrak quoted unnamed Arab oil sources on Thursday as saying Saudi Arabia is "seriously considering" the cutback before the Baghdad summit meeting of OPEC. It gave no figure for the cutback.

Reuter, in a story from London, quoted an executive of one major Western oil company as saying: "We do not see how they can stay at 9.5 million in a glut."

Market analysts told Reuter that a Saudi cut of 1 million barrels would not in itself be enough to dry up the glut and trigger a new round of panic buying by consumers anxious about Middle East supplies, such as occurred last year when Iranian exports slumped.

"If Saudi Arabia cut production 1 million barrels daily starting tomorrow, it could still take until the autumn of 1981 to erase the accumulated surplus," the trade journal Petroleum Intelligence Weekly said last Monday.

Oil industry executives said Saudi Arabia may also soon raise its $28 a barrel price, but that would still be the lowest among the OPEC countries.