OPEC oil ministers today opened their fourth meeting this year on cutting production to halt the slide in oil prices, but delegates said chances of reaching a workable agreement were slim.

The talks began in a leisurely atmosphere that contrasted with the current economic crisis confronting most members of the Organization of Petroleum Exporting Countries. Oil prices have fallen this year to approximately one-third of their level at the end of 1985 because of a glut on world markets.

The 13 OPEC states were seeking an accord both on an overall production ceiling and on how much each member would be allowed to produce. But they remained deeply divided, as they have all year, on both issues, delegates said.

The outlook here was particularly grim because of OPEC's failure to reduce output as envisioned in an informal accord at its last meeting a month ago in Brioni, Yugoslavia. Production actually has risen since then, and is now estimated to be 20 million barrels a day instead of the planned ceiling of l7.6 million barrels a day.

United Arab Emirates Petroleum Minister Mana Said Otaiba said after today's session that the conference faced an "impossible" task. Nigerian Petroleum Minister Rilwanu Lukman, who also holds the rotating position of OPEC president, said "all positions are negotiable" but added, "I'm not dancing for joy exactly."

Failure to reach an accord here could push oil prices down from the current range of between $8 and $9 a barrel to about $6 a barrel, oil industry specialists said.

Most OPEC countries are facing severe revenue problems. The 13 countries together suffered a loss of nearly $100 million a day in oil revenues during the first six months of this year, according to a study by the New York-based industry publication Petroleum Intelligence Weekly.

The study showed, however, that the suffering was not shared evenly among the individual OPEC members. Saudi Arabia, which is by far the largest producer, is currently making more money than it was a year ago because it has raised its output so much, according to the study.

The policy of Saudi Arabia, and of its Arab allies in the Gulf, is at the heart of OPEC's current dilemma.

The Saudis, in a major reversal, decided at the end of last year to stop trying to prop up prices by limiting their production.

The Saudis' goal essentially is to force down the price enough to encourage non-OPEC producers, especially Britain, to reduce output and leave a larger share of the market for OPEC. The Saudis also hope the price slump will encourage other OPEC members to reach a solid production-sharing agreement.