Archrivals Saudi Arabia and Iran are expected to battle it out at Friday's OPEC meeting over leadership of the oil cartel at a time when it's facing the grave challenge of the U.S. shale oil boom, an issue that's divided the group.

The gathering in Vienna of the Organization of Petroleum Exporting Countries' oil ministers is slated to elect a new secretary-general. That's a largely ceremonial post but it plays a central role in shaping policy for the 11-member oil-exporting group, which has a direct impact on global oil markets.

The issue assumed heightened importance because OPEC faces losing much of its dominance of the global energy market and its ability to influence oil prices because of the shale oil boom that has turned the energy market on its head.

OPEC is bracing for a big increase in oil supplies in the second half of the decade because of the U.S. shale boom and heightened competition from natural gas in core markets as advanced technology opens up hitherto unreachable reserves, threatening traditional production leaders like Saudi Arabia.

The importance of the OPEC post "cannot be overstated with oil prices still hovering above $100 a barrel," the Financial Times observed.

The rapid growth of U.S. shale oil and gas production is expected to push prices down, negating OPEC's traditional role as price-setter.

The International Energy Agency, the Western energy watchdog, forecast this year that oil will average $89 a barrel over the next five years.

A more recent IEA report May 15 warned that rising U.S. shale oil production will send "shock waves" through the global oil industry over that period, boosting U.S. refiners and displacing OPEC as the driver of supply growth.

None of this is good news for the cartel and it's stoking the rivalry between Saudi Arabia, for decades the world's leading oil exporter which has in the main sought to keep prices at a reasonable level, and Iran, long OPEC's second-ranked producer which invariably seeks to pump up prices.

This is particularly true now as Iran's oil exports have slumped to their lowest level in two decades because of U.S.-led international economic sanctions imposed in 2010 over Tehran's refusal to abandon its contentious nuclear program.

The political schism between Saudi Arabia, together with the other Arab petro-monarchies in the Persian Gulf, and Iran resulted in the failure at OPEC's December meeting to appoint a secretary-general.

The current cartel chief, Abdalla al-Bardi of Libya, was more or less pushed into extending his tenure because Riyadh and Tehran, respectively the standard-bearers of Islam's mainstream Sunni sect and the breakaway Shiites, couldn't agree on a successor.

Riyadh is proposing Majid Munif, a highly regarded economist who was formerly the kingdom's senior representative to OPEC. Tehran's candidate is Gholam-Hussein Nozari, a former Iranian oil minister.

A possible compromise could be reached through Iraq's Thamir Ghadban, oil minister in 2004-05 and currently an energy adviser to Prime Minister Nouri al-Maliki.

Iran and Iraq, longtime adversaries who fought a ruinous war in 1980-88, formed an alliance within OPEC -- more of convenience than conviction, since both wanted prices to be as high as possible -- in 2012.

Industry observers have for some time voiced concern that Iraq's apparent move into the hard-liners' camp and the growing politicization of the cartel challenges the Saudis' traditional dominance of the organization.

OPEC has always been divided over oil prices and how far the major consumers should be accommodated but the process of politicization, most notably between Riyadh and Tehran, has grown more acute in recent years.

The increasingly costly sanctions against Iran have deepened the political rift, making it more difficult for the cartel, which produces around one-third of the world's oil supplies, to remain neutral.

"It could prove increasingly difficult to maintain that neutrality, with the issue of sanctions reinforcing already deep divisions between hardliners Iran, Venezuela and Algeria and moderates like Saudi Arabia and its gulf allies, Kuwait and the United Arab Emirates," the Financial Times said.

Oxford Analytica observed: "While OPEC is likely to face an increasing number of challenges in the medium term, the oil market will remain susceptible to 'cartelization.'

"The economic benefits of this to OPEC members will probably outweigh the political frustrations."