The world's most respected investor, Warren Buffett has famously not made any investments directly in the shale oil boom. Apart from indirect exposure through ConocoPhillips and ExxonMobil, Buffett has stayed away from the domestic oil boom and indeed, the oil industry in general.

However, Buffett has made some key indirect investments in the shale oil boom. For example, two of Buffett's indirect plays on the industry are Phillips 66 (NYSE:PSX) and National Oilwell Varco (NYSE:NOV), as well as BNSF and pipeline logistics company Phillips Specialty Products. These four companies all display the famous Buffett criteria of a wide moat and industry dominance.

Key technology
Originally owned by Phillips 66, Phillips Specialty Products' business is the lubrication of oil's movement through pipelines. Planning the efficient movement of oil is an increasingly crucial process for the industry to move both tar sands crude and oil obtained via hydraulic fracturing. Without technology like this, there is no doubt that the domestic oil boom would be struggling.

Alongside Phillips Specialty Products, Buffett owns BNSF, which moves over 1 million barrels per day of Bakken crude to market, once again a key part of the domestic oil boom.

Of course, as both BNSF and Philips Specialty are owned by Buffett's Berkshire Hathaway, the only way for Foolish investors to get in on the action is to buy Berkshire shares, which in itself is never a bad idea.

However, for those seeking to invest like Buffett themselves, Phillips 66 and National Oilwell Varco are two great choices. 

All the Buffett qualities
Phillips 66 exhibits all the qualities of a Warren Buffett-style investment. The company is the largest refiner in the U.S. with assets around the world and as a result has impressive economies of scale and size that others can only dream of. Further, the company is well placed the benefit from the domestic oil boom. Specifically, the company is benefiting from low crude input costs and is divesting any high-cost refineries.

What's more, Phillips has been able to achieve sector-leading returns on capital as seen in this slide from the company's recent investor presentation. 

Source: Phillips 66 investor presentation.

Like all Buffett's investments, Phillips also looks after its shareholders. The company has returned $6 billion to investors through buybacks and dividends since the second quarter of 2012, and this is set to continue with the recently announced additional $2 billion share repurchase program -- this additional authorization brings the total authorization since the third quarter of 2012 to a $7 billion. 

In addition, Phillips has some lofty plans for growth in place. The company has earmarked $12 billion in growth capital from now until 2016 and analysts expect this to drive EPS to $6.66 this year and then $7.84 during 2015. That's an average annual growth rate of around 15%. 

Everyone needs a drill bit
Buffett's other indirect play on the shale oil boom is National Oilwell Varco.  

National Oilwell Varco is a giant of industry. With an 80% share of the market in "floater" packages, complete drilling systems for deep-sea wells, the company is essential to the global oil and gas industry. While floater packages are not directly relevant to the shale boom, National Oilwell Varco also manufactures drilling systems and provides project management for the whole oil service industry. Moreover, and here's when Buffett is likely to have found value, National Oilwell Varco is currently undervalued compared to peers.

For example, at present National Oilwell currently trades at a historic enterprise value/EBITDA ratio of 8.1 compared to Schlumberger's 12.4 and Halliburton's 10.4. Additionally, National Oilwell trades at a TTM P/E of only 14.8, compared to Schlumberger's 21.3 and Halliburton's 22.4. 

Still, National Oilwell has recently warned that business will slow over the next few months as several rig companies scale back newbuild programs. However, the rig business is cyclical, and National Oilwell is in a great position to profit when the market recovers.

Foolish summary
So all in all, Warren Buffett has avoided investing directly in the U.S. shale oil boom, but he has made indirect investments. Two of these investments are National Oilwell Varco, which will benefit from a rising demand for drilling technology and Phillips 66, which is profiting from the low cost of crude. 

If investors want to profit from the shale boom in Buffett style, Phillips 66 and National Oilwell Varco look to be two great picks.