KUWAIT – The days when OPEC members could all but guarantee consensus when deciding production levels for oil are long gone, according to a veteran of almost two decades of the group's meetings.

The global glut of crude, which has contributed to a 30 percent decline in prices since June 19, has left the organization disunited and dependent on nonmembers to shore up the market, said former Qatari Oil Minister Abdullah Bin Hamad Al Attiyah. The 12-member Organization of Petroleum Exporting Countries is scheduled to meet in Vienna on Thursday.

"OPEC can't balance the market alone," Al Attiyah, who participated in the group's policy meetings from 1992 to 2011, said in a phone interview. "This time, Russia, Norway and Mexico must all come to the table. OPEC can make a cut, but what will happen is that non-OPEC supply will continue to grow. Then what will the market do?"

Global demand will increase this year by the least in five years, to 92.4 million barrels a day, before picking up in 2015 as economic recovery gathers pace, the International Energy Agency said on Nov. 14. Further complicating OPEC's task is the boom in U.S. shale production, which has put the world's biggest economy on course toward energy self-reliance. U.S. output is expected to grow 12 percent next year to the highest since 1970, according to the U.S. Energy Information Administration.

OPEC has benefited from prices averaging $108 a barrel since December 2011, when the group changed its production target to the current 30 million barrels a day. Prices began sliding in June, and benchmark Brent crude dropped below $80 this month, threatening the budgets of Venezuela and Iran.

"OPEC had been enjoying easy meetings, and decisions were taken without a sweat," Al Attiyah said. "Now the situation is different."

Oil markets are oversupplied by about 2 million barrels a day, and global economic growth is below expectations, he said. "The U.S., which was a major market for OPEC, is no longer welcoming imports. It's now striving to become an oil exporter."

Led by Saudi Arabia's Ali Al-Naimi, OPEC's longest-serving minister and the voice of its biggest producer, officials flew from capital to capital for consultations ahead of this week's meeting. Al-Naimi visited Latin America, and his country's foreign minister flew to Russia.

Officials from Iraq to Libya visited Saudi King Abdullah, and Venezuela's Foreign Minister Rafael Ramirez met energy ministers from six producers. Iran's Oil Minister Bijan Zanganeh is to meet with Al-Naimi in Vienna about a possible proposal to cut OPEC output by as much as 1 million barrels a day, state-run Mehr News reported Sunday. Zanganeh also plans to hold talks with Russian Energy Minister Alexander Novak, according to Mehr.

Supplies from outside OPEC increased by a record of almost 2 million barrels a day in the first 10 months of the year, to reach 57.1 million a day in October, according to the IEA, an adviser to industrialized nations.

In Vienna, ministers and their delegations will gather in the grand hall of OPEC's secretariat building. The ministers will then meet among themselves in a separate room nearby, emerging only after they reach a unanimous decision.

"If OPEC asked me for an advice, I'd tell them, 'Look, you can't do it alone, you need to seek help from non-OPEC producers and make everyone share the responsibility,' " Al Attiyah said. "Within OPEC there are no eternal friends, nor enemies. There are only shared interests."