Warren Buffett was one of the biggest shareholders in ExxonMobil. Not anymore.

The billionaire sold his nearly $4 billion stake in Exxon (XOM) at some point during the fourth quarter when oil prices were in a seeming free fall. Crude plunged from over $100 in July to $50 by December.
Big Oil used to seem like a slam dunk investment, but the unexpected plunge in crude oil changed the game. Cheap oil is already pressuring Exxon's revenue and threatens to hurt its ability to return lots of cash to shareholders.

Time to exit? But did Buffett, who has made a career out of being a long-term investor, sell at the bottom?
While it's not known exactly when in the fourth quarter Buffett sold, Exxon's share price dropped from about $105 in June to as low as $86 near the end of the year.
Buffett also dumped his stake in ConocoPhillips (COP), an exploration and production company.

The bullish case for Big Oil: Some experts disagree with Buffett. They believe now is actually the time to buy Big Oil companies. BlackRock (BLK), the world's largest asset manager, said integrated oil companies are its favorite in the beaten-down energy sector.
"We favor the 'super majors' because of their strong balance sheets, high dividends and integrated business models," the BlackRock Investment Institute wrote in a recent report.

Oil prices have rallied since mid-January, raising hopes the worst is over. Exxon shares up 6% from their recent lows.
While oil services firms still look risky to BlackRock, the firm loves that integrated oil companies are sporting a 12-month forward price-to-earnings ratio of 10, which is below their historical levels and global equities broadly. In other words, these stocks are very cheap.
The integrated model that Exxon, Chevron (CVX) and others have means they are involved in all parts of the energy business. That helps shield them a bit from the oil crash because today they stand to benefit from higher refining margins and lower capital costs.

http://money.cnn.com/2015/02/18/investing/warren-buffett-oil-prices-exxon/