VIENNA (Reuters) - OPEC on Thursday warned of growing uncertainty over demand for its oil in the years ahead, raising doubts whether multi-billion-dollar investments in new supply will be needed.

OPEC, in its 2008 World Oil Outlook, said demand for its oil could fall to 31 million barrels per day (bpd) in 2012, below current production, as additions to supply excluding OPEC crude outpace growth in demand.

The outlook underscores OPEC’s concern about moves in Europe and the United States to cut oil dependence and promote alternatives. Any investment shortfall would worry consumer nations facing record oil prices.

“Our member countries, if they see the demand is low, they will not invest,” OPEC Secretary-General Abdullah al-Badri told Reuters, referring to the supply and demand outlook to 2020.

“They have education problems, they have housing problems, they have health problems, they have roads problems. They have a lot of activities they can invest their money on.”

Concern that supply will fall short of future demand due to rising consumption in Asia and the Middle East and limited investment in new oilfields has helped to send oil prices to a record near $146 a barrel.

The Organization of the Petroleum Exporting Countries, source of two in every five barrels of oil, expects world demand to rise by 1.3 million bpd annually to 2012 and ease to 1.2 million bpd in the longer term.

In the period to 2012, the group expects additions to total supply excluding OPEC crude -- such as non-OPEC oil, biofuels and OPEC’s natural gas liquids -- to average 1.4 million bpd, more than demand growth.

The 13-member OPEC, which holds about three-quarters of the world’s proven crude reserves, said current plans to develop oilfields should increase supply capacity by more than 5 million bpd by 2012.

“These projects are already under way, some are currently under construction and some are close to completion,” OPEC said.

“Yet there is a real prospect of wasting resources on capacity that is not needed.”

ENOUGH OIL

Demand would reach 113 million bpd by 2030 in OPEC’s reference case -- more than 4 million bpd less than expected in last year’s outlook -- from 85 million bpd in 2006.

OPEC said the slowdown reflects efficiency improvements based on a higher price assumption of $70 to $90 a barrel for its index of crudes, up from $50 to $60 in 2007.

Its outlook is in contrast to that of the International Energy Agency, an adviser to 27 oil consumers which has been urging OPEC to raise output and to boost investment.

The IEA on July 1 forecast consumption would rise by 1.5 million bpd to 2013, faster than the average rate of supply growth, and signaling little relief from high prices.

Badri in the report reiterated OPEC’s view that there is “more than enough” crude on the market now and blamed factors other than supply such as the weak U.S. dollar for rising prices.

Further ahead, OPEC warned that policy changes made investment decisions more difficult, saying demand for its crude could vary between 29 million and 38 million bpd by 2020.

“The range of uncertainty for OPEC oil is considerable,” it said. “This translates into an uncertainty gap for upstream investment needs in OPEC member-countries of over $300 billion.”

Such policy moves include the European Union’s plan to get 10 percent of road fuels from renewable sources such as biofuels by 2020 and a U.S. law boosting the fuel efficiency of cars and trucks.

OPEC also said rising construction costs and a shortage of skilled labor posed threats to projects to build refineries to satisfy the world’s growing demand.

It also warned the gap between supply and demand for middle distillates would grow, unless there was more investment in diesel-oriented projects.