America is in the midst of a quiet revolution. Across the country, new technologies are unlocking billions of barrels of oil and gas. And this development is so big that the IEA predicts the United States could be energy self-sufficient by 2035. 

It should be no surprise, therefore, that an opportunity this big has attracted the attention of legendary investor Warren Buffett. Over the past few years the Oracle of Omaha has been steadily increasing his energy holdings.

Unlike a test in high school, copying the answers from the smartest student in the class is actually a good practice in the investment world. So which energy stocks is Buffett adding to his portfolio? Let's take a look at three of his most recent purchases. 

Should you follow Buffett into this industry titan?
Whoever said the best things come in small packages was never an energy investor. This industry requires raw size and scale to take on the toughest energy challenges.

And it doesn't get any bigger than Exxon (XOM 0.02%). With a market capitalization over $400 billion, this is the second largest publicly traded American company. 

Why does Buffett like Exxon so much? They're the best capital allocator in the business. Over the past five years, the company has generated a return on invested capital over 25% - the highest of its peers. 

Because of these high returns, the company can return an enormous amount of cash to shareholders. Year-to-date, Exxon has paid out nearly $8.1 billion in dividends. And because of its size, Exxon hasn't missed a dividend payment in 131 years. 

Additionally, over the past decade, the company has repurchased nearly half of its outstanding shares. This has allowed investors to increase their stake in a wonderful business while deferring taxes until they sell. 

Buffett owns 17.8 million shares of this oil sands stock
This year, Buffett started looking north to Canada to find new investment opportunities. In August, Berkshire Hathaway disclosed that it had accumulated a 17.8 million share stake in Canada's largest oil producer, Suncor Energy (SU 0.31%). 

Chief Executive Steve Williams has engineered an impressive turnaround at the Alberta oil giant. Within one year after taking the helm, Williams has abandoned his predecessor's growth targets, scrapped the Voyageur Upgrader, doubled the company's dividend, and pledged to buy back 10% of outstanding shares. 

New production growth will come from Fort Hills bitumen mining project, and Williams has pledged to eke out an additional 100,000 barrels per day through low risk de-bottlenecking initiatives and refinery upgrades.  Additional pipeline and rail capacity out of Alberta should also start to reducing the discount for oil sands bitumen, thereby boosting Suncor's top line. 

Why Buffett is bullish on this oil refiner
As Buffett once said, 'In business, I look for economic castles protected by unbreachable moats.' 

Phillips 66 (PSX -0.66%) is vital to our day-to-day lives. The company owns 15,000 miles of oil and gas pipelines -- more than enough to circle the planet -- that move millions of barrels of valuable oil and gas around the country, daily. This ensures a steady demand for the commodities Phillips 66 ships and refines through its network.

Raw size and financial discipline has helped Phillips 66 achieve industry-leading returns. Year-to-date, Phillips 66 has generated a 15% return on capital employed -- the highest of its peers. This allows the company to richly reward shareholders. Today the stock yields 2.1%, and Phillips 66 has repurchased nearly 5% of its outstanding shares since going public last year. 

Meanwhile, the company is profiting handsomely thanks to a lack of energy infrastructure in the United States. Phillips 66 is making money hand over fist by buying cheap, domestic crude and selling refined products internationally at a steep mark-up. And because solutions to these problems -- such as lifting oil export bans, building new refineries, and laying new pipelines -- are politically sensitive issues, Phillips 66 is likely to continue earning excess returns for years to come. 

Foolish bottom line
Even if you aren't interested in any of these stocks, there are still something interesting themes to learn from this list. Every single one of these names a great capital allocators. Every management team is willing to skip investing in a low-return venture in favour of returning cash to investors. This is a great indication that management if working for the interests of shareholders. 

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