* Proven oil reserves up 12.1 pct to 1.19 trln barrels
* Venezuela again leads increase in reserves
* Analysts sceptical of national reserves figures (Amends quote in paragraph 5, adds Iranian reaction paragraphs 8-9)
By Alex Lawler
LONDON, July 19 (Reuters) - OPEC's proven crude oil reserves rose 12.1 percent in 2010 to 1.19 trillion barrels led by Venezuela, which has surpassed Saudi Arabia as the group's largest reserves holder, OPEC said in its Annual Statistical Bulletin.
The latest figures are unlikely to quell scepticism about reserves estimates from the Organization of the Petroleum Exporting Countries, some of which analysts say may be exaggerated although the producers deny doing so.
OPEC's growth in oil reserves was mainly due to Venezuela, whose holdings climbed to 296.5 billion barrels from 211.2 billion in 2009, the report said. Top OPEC exporter Saudi Arabia's reserves were steady at 264.5 billion barrels.
Iran and Iraq also boosted their reserves last year. In October, Iran increased its reserves to 150 billion barrels within a week of an upward revision by Iraq, ensuring that Tehran continued to rank above Baghdad.
"OPEC has a fantastic history of competitive reserves upgrades," said Bill Farren-Price, analyst at Petroleum Policy Intelligence.
Reserves are one of the criteria OPEC has used in setting output targets. Iran and Iraq were rivals in the past over OPEC quotas and OPEC in the next few years is expected to tackle the issue of bringing Iraq back into the quota system. Iraq is exempt at present.
Iraq boosted its reserves to 143 billion barrels last year, up 24 percent, the report said. Iraq has said its reserves increased as work by international oil companies started to yield results.
Venezuela's move to the No. 1 reserves spot bumps Iran and Iraq to third and fourth place respectively. Commenting on the OPEC report, an Iranian official said the country still held its status as OPEC's second-largest producer after Saudi Arabia.
"Iran is still the second crude producer of OPEC and insists on this," an unnamed Oil Ministry official told Iran's state news agency IRNA on Tuesday.
OPEC said a year ago its reserves increased in 2009 because of Venezuela. President Hugo Chavez's government said in January it had had overtaken Saudi Arabia as the world leader.
Venezuela's new deposits were booked in the South American country's Orinoco extra heavy crude belt.
The boost in Venezuela's figures helped OPEC attain a larger share of the global total. OPEC holds 81.3 percent of the world's proven crude reserves, up from 79.6 percent in 2009, the report said.
Saudi Arabia, by far OPEC's largest exporter, holds an advantage in that its oil is mostly light, conventional, easily-pumped crude. The Orinoco oil needs to be upgraded or mixed with a lighter grade to create an exportable blend.
Some countries such as Algeria, Kuwait and the United Arab Emirates had no change in their reserves in 2010 or in any year since 2006, the report said.
This trend has also given rise to doubts about the estimates, as analysts say it is unlikely new additions to reserves will exactly match production.
OPEC's 12 members pump more than a third of the world's oil. Several producers, including Saudi Arabia and Venezuela, have denied suggestions their reserves have been exaggerated. (Additional reporting by Barbara Lewis and Ramin Mostafavi; Editing by Anthony Barker)