Three leading petroleum ministers in the Organization of Petroleum Exporting Countries met informally here today in an effort to find ways to reconcile warring Iran and Iraq and restore the oil cartel's unity.

Because of the priority the oil ministers are planning to give to unity efforts, Venezuelan Oil Minister Humberto Calderon Berti said in an interview that he does not expect OPEC to focus very much on oil price increases during its forthcoming semiannual ministerial conference in Bali, Indonesia.

That meeting will be the first major OPEC gathering since Iraq invaded Iran Sept. 22 and later captured the new Iranian oil minister, Mohammed Javad Tondguyan.

Indonesian Oil Minister Subroto, OPEC's current chairman, organization sources reported, has appealed to Iraq to release the Iranian minister as a goodwill gesture to set the stage for the Bali meeting, which is to open Dec. 15. But, OPEC sources said, Iraq seems reluctant to do that now.

Calderon said that if Iran and Iraq were to ask OPEC to mediate their dispute, "I think we can do it. We are ready to do it, but they have to ask us."

The Venezuelan met here today with Subroto, OPEC's incoming chairman, and Algerian Oil Minister Belkacem Nabi, the outgoing chairman. As the founding member of OPEC, Venezuela has made a point over the years of maintaining good relations with the other 12 member states.

The three ministers who met here today would be the most logical group to serve as mediators. Algeria is probably the only Arab country recognized by both sides in the Persian Gulf war as being truly neutral.

The trio met on the eve of a conference in Kuwait Monday of the Organization of Arab Petroleum Exporting Countries, which groups the Arab members of OPEC plus several non-members such as Syria. Claderon said he, in an exception, had been asked to go to Kuwait to be available for consultations on the sidelines of the OAPEC meeting, which traditionally is held before every regular OPEC ministerial meeting.

The gulf war is expected to be the central issue at the Kuwait session, and pressure is expected from the other Arab states for Iraq to release the Iranian minister, the highest-ranking prisoner taken in the war.

Both Iran and Iraq have indicated that they will attend the Bali meeting, and the Iranian delegation is generally expected to be headed by Iran's former oil minister, Ali Akbar Moinfar, who has usually represented Tehran at OPEC meetings since Iran's Islamic revolution.

Calderon said that he does not expect major fireworks at Bali because an OPEC board of governors meeting held in Vienna last week had turned out peacefully between Iran and Iraq.

The last OPEC ministerial meeting in Vienna in mid-September ended just four days before the outbreak of the war and was the scene of verbal assaults by Moinfar against Iraq and its close oil ally, Saudi Arabia.

The war led to postponement of a 20th anniversary OPEC summit that was to be held in Baghdad in November and to cancellation of two other ministerial meetings to prepare for that summit.

The ministers had been working toward preparation for their heads of state of a long-term, oil-pricing formula under which prices would have risen on a regular schedule based on a number of factors, including inflation, the industrialized world's real economic growth and the fluctuations of Western currencies.

The original idea was for the formula to get the formal blessings of OPEC heads of state in Baghdad and to be put into practice at the Bali ministerial meeting.

Now, Calderon said it is difficult to see such an agreement being reached. The basis for it could be established nonetheless, he said, if Saudi Arabia, OPEC's largest producer, agreed to raise its current official price from $30 a barrel for its benchmark crude oil to the official OPEC ceiling of $32.