Factbox: Energy ties between Venezuela and United States

CARACAS (Reuters) - The United States' revocation of the Venezuelan ambassador's visa has put a spotlight on tense ties between the ideologically opposed nations.

President Hugo Chavez has threatened to cut off oil supplies to the United States during previous diplomatic flare-ups, but he has never taken that step.

The latest dispute is unlikely to impact oil exports to the United States, which are important to both economies, analysts and diplomats believe. But energy ties have become increasingly tense in recent years. Here is a history:

FALLING EXPORTS

* The United States is the Latin American OPEC member's top customer, and Venezuela is one of the top five suppliers of oil to the United States along with Canada, Saudi Arabia, Mexico and Nigeria. The volume of Venezuelan crude and oil products shipped to the United States has fallen 48 percent to 930,000 barrels per day (bpd) in October from a peak of 1.78 million bpd in 1997, the U.S. Energy Information Administration says.

* The decline is partly due to increasing sales by Caracas to customers such as Cuba, China and Iran, as well as stagnating production by Venezuelan state oil company PDVSA.

* U.S. President Barack Obama's administration, for its part, wants to develop an energy policy that reduces dependence on foreign oil.

THREATS TO SUPPLIES

* Chavez first warned in February 2004 that he would shut off oil supplies to the United States if it tried to invade the Andean nation or impose a trade blockade against his country -- after he accused former U.S. President George W. Bush's administration of backing a short-lived coup attempt against him in 2002. The following month, he backed away from the threat and said he did not have any intention of damaging relations with Washington.

* In early 2006, Chavez warned he could shutter his government's U.S.-based refineries and sell oil to nations other than the United States if Washington decided to cut ties after a round of tit-for-tat diplomatic expulsions.

* Then in 2008, Venezuela's president again threatened to stop sending oil to the United States unless it halted an "economic war," following an Exxon Mobil Corp lawsuit that sought to freeze PDVSA assets in a nationalization dispute. Later that year, he also threatened to stop selling oil to European countries if they applied a new ruling on illegal immigrants that had been criticized by rights groups.

* In July 2010, Chavez again threatened to cut off oil supplies to the United States in case of any military attack by Colombia, amid a dispute over charges that Venezuela harbored leftist Colombian guerrillas.

NATIONALIZATIONS

* Nationalizations by the Chavez government in 2007 hit U.S. majors working in Venezuela hard. Both Exxon Mobil Corp and ConocoPhillips had assets worth billions of dollars taken over by the state, and have gone to the World Bank's International Center for Settlement of Investment Disputes (ICSID) to seek compensation.

* Supplies of Venezuelan crude to the Chalmette refinery, part-owned by Exxon, and ConocoPhillips' Sweeny refinery were disrupted because of the disputes over the nationalizations.

* U.S.-based oil service provider Tidewater Inc has also petitioned ICSID for some $45 million in compensation after the Caracas government expropriated the assets of 76 smaller service companies in May 2009.

* Then in June this year, the Venezuelan authorities nationalized 11 oil rigs owned by U.S. firm Helmerich and Payne.

PRESSURES ON CITGO

* Venezuela has been reducing the size of PDVSA's U.S. subsidiary, Citgo Petroleum, with the sale of several of its refinery assets and service stations in the United States.

* At the same time, the Chavez government has been increasing the financial demands it makes of Citgo, which buys just over a third of the Venezuelan crude that is shipped to the United States, calling for weightier dividends and for the company to hire more Venezuelans.

DISPUTE OVER STATISTICS

* Since Chavez fired half of PDVSA's workers in 2003 after a massive strike, hindering the company's operations, U.S. officials have been skeptical of official production and export figures produced by the Venezuelan government.

* There is currently a disparity of nearly 900,000 bpd in production estimates: PDVSA says it produces about 2.96 million bpd, while the U.S. government puts the figure at 2.09 million bpd.

SOME UPBEAT NOTES FOR U.S.

* It's not all bad news for U.S. companies. Another U.S. major, Chevron, has a 34 percent stake in Carabobo Project 3, in Venezuela's vast extra-heavy Orinoco crude belt. Reserves in that project are seen at around 66 billion barrels, making it a huge prospect for PDVSA, which has a 60 percent stake, and its U.S. partner.

* In another positive sign, Venezuelan Oil Minister Rafael Ramirez made his first visit to Washington for six years in April and said the Latin American nation welcomed investment by U.S. oil companies.

* In May, PDVSA signaled the importance of the U.S. market when it resumed exports of reformulated gasoline to the United States for the first time in five years.

(Reporting by Marianna Parraga and Daniel Wallis in Caracas; Editing by Frances Kerry)