Crude oil production exploded in the US over the past five years as the practice of hydraulic fracturing ("fracking") was combined with horizontal drilling to open up oil resources that had previously been uneconomic to exploit, explains Robert Rapier in Personal Finance.

US oil production has risen from 5 million barrels per day (BPD) in 2008 to 7.3 million BPD in the most recent quarter. The US is now the fastest-growing oil producing region in the world.

At the center of this fracking revolution is the Williston Basin in the Bakken shale formation of North America. Below are our favorite companies operating in this area. Continental Resources (CLR) was a pioneer in the Bakken Formation, entering the Bakken in 2003 with a purchase of 300,000 acres. In 2004, Continental completed the first commercially successful well in the North Dakota Bakken that was both horizontally drilled and fractured.

Continental is presently the largest leaseholder and oil producer in the Bakken, with more than 1.1 million acres leased.

In 2012, Continental increased proved reserves in the Bakken by 92% over 2011. Overall production in 2012 was 58% higher than in 2011, and Continental's earnings were up 51%, based on EBITDAX (earnings before interest, taxes, depreciation, depletion, amortization, and exploration expenses).

Continental's Enterprise Value is $21.5 billion, and its Enterprise Value/EBITDA ratio is 9.5.

Whiting Petroleum (WLL) is one of Continental's biggest competitors in the Bakken. Whiting is the second-largest oil producer in North Dakota, averaging 82,500 barrels of oil equivalent (BOE) of production in 2012, across more than 700,000 acres of leased land.

Like Continental, Whiting has been actively engaged in pad drilling, citing a cost savings of more than $500,000 over single wells drilled in its Pronghorn field. At the end of 2012, Whiting had 10 rigs capable of pad drilling in North Dakota.

Whiting has increased oil production and oil reserves in each of the last four years. Oil reserves at the end of 2012 were 88% higher than at the end of 2008, and oil production was 104% higher.

Whiting's Enterprise Value is $8.2 billion, and its Enterprise Value/EBITDA ratio is 5.2.

Oasis Petroleum (OAS) is a pure Bakken/Three Forks play, with 335,000 leased acres in the Williston Basin.

Oasis has only been a public company since 2010, and in addition to the $400 million raised in its initial public offering, the company has funded operations with $1.2 billion in debt.

In 2012, Oasis produced 22,500 BOE per day, more than doubling its 2011 output and quadrupling production since 2010. Management expects 2013 production to be 30,000 to 34,000 BOE per day.

The company's proved reserves at the end of 2012 were 143 million BOE, up from 79 million BOE at the end of 2011.

Oasis's Enterprise Value is $4.8 billion, its Enterprise Value/EBITDA ratio is 7.6. Aggressive borrowing by Oasis to fund operations has left it with a debt to equity ratio of 131%, higher than that of Whiting (61%) or Continental (122%).

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