AFP - Venezuela announced Thursday its proven oil reserves rose to 211.17 billion barrels at the end of 2009, a 23 percent increase thanks mostly to heavy crude oil finds that are difficult to extract.

The new aggregate reserves -- an additional 39.24 billion barrels -- came mainly from the Orinoco Heavy Oil Belt, a southeastern area where Venezuela has intensified its investments in exploration and development, and only a small part from traditional oil fields.

In March 2009, the country's proven reserves stood at 172.32 billion barrels.

Venezuela, the top crude oil exporter in South America, now holds the second-largest proven reserves in the world, behind Saudi Arabia (266 billion barrels) and ahead of Iran (138 billion barrels), according to the Organization of the Petroleum Exporting Countries.

After its proven reserves fell to 76 billion barrels of oil in 1998, Venezuela launched a "socialist project" in 2005 to boost reserves to 314 billion barrels, which would give it the world's largest reserves.

"This project would confirm that the country has the world's biggest crude reserves, and it would put them to work for humanity," state-owned oil company PDVSA said in a statement.

Venezuela is estimated to have 235 billion barrels of oil in the Orinoco basin, but the oil there is heavy, difficult to extract and more costly to refine than conventional crude.

Following oil nationalizations in 2007, US groups ExxonMobil and ConocoPhillips left Venezuela. But other oil firms have accepted to work under new partnerships where PDVSA has the controlling stake.

US-based Chevron and Spain's Repsol have indicated they stand ready to invest billions of dollars, and have already seen some payback when they were attributed projects in the Orinoco basin.