June 27 (Bloomberg) -- Venezuela's bolivar weakened in unregulated trading and dollar-denominated bonds tumbled after Exxon Mobil Corp. and ConocoPhillips abandoned operations in the country, raising doubts about future oil output in the world's fifth-largest crude-oil exporter.

Exxon Mobil, the world's largest oil company, agreed to give up its concession in the Ceiba oil block in western Venezuela while ConocoPhillips is ceding its interests in the Petrozuata venture. Oil accounts for about one-fifth of Venezuela's $150 billion economy.

``This sparks additional anxiety regarding the future of oil investment in Venezuela,'' said Enrique Alvarez, a Latin America economist at the research firm Ideaglobal in New York. ``Investors are going to the dollar as a safe haven.''

The bolivar weakened to 4,180 per dollar in the unofficial dollar market from 4,050 yesterday, traders said. People and businesses turn to the parallel market when they are unable to acquire the limited number of dollars the government sells at the official exchange rate of 2,150 bolivars per dollar.

ConocoPhillips, the second-largest U.S. refiner, and Exxon Mobil, the world's largest oil company, failed to reach an agreement on remaining in the country after state-owned Petroleos de Venezuela unilaterally took over at least a 60 percent stake in heavy-oil projects in the Faja del Orinoco region, where the company previously held minority ownership. Venezuelan President Hugo Chavez seeks control of the country's key industrial assets.

``There's a lot of demand for dollars today,'' said Henry Travieso, a trader with Global Capital Valores in Caracas.

Bonds Lower

Investors also sold dollar-denominated assets to obtain foreign currency.

The yield on Petroleos de Venezuela SA 5.25 percent dollar- denominated bond due April 2017 rose 9 basis points, or 0.09 percent, to 9.14 percent 3:16 p.m. New York time, according to a composite price of eight banks. The bond's price, which moves inversely to yield, fell 0.45 cent to 75.175 cents on the dollar.

The yield on Venezuela's 5.25 percent, dollar-denominated bond due March 2019, known as a TICC, rose to its highest since April 3, gaining 7 basis points to 4.97 percent, according to Banco Bilbao Vizcaya. The price declined 0.6 cents to 102.50 cents on the dollar.

Venezuelan oil output is below its peak of about 3 million barrels per day reached in 2001. Output averaged 2.34 million barrels per day in May, according to data compiled by Bloomberg. The Faja del Orinoco accounts for about a quarter of that production, averaging production of 600,000 barrels a day, according to the government. The Venezuelan government plans to double oil output by 2012 to about 5.8 million barrels a day.

To contact the reporter on this story: Andrea Jaramillo in Bogota at ajaramillo1@bloomberg.net

Last Updated: June 27, 2007 18:48 EDT