The last time Harold Hamm was profiled, the billionaire founder of Continental Resources, it was early 2009 and oil prices had slumped to $40 a barrel. This was bad news for Hamm's foray into the vast Bakken oil play that spreads under North Dakota and Montana. Tight rocks and tricky drilling there neces sitate $50 crude to break even. Now with oil back near $100, Hamm is sitting pretty. He's set to invest $1 billion in the Bakken this year in his quest to prove that it is the biggest oilfield in the U.S. and among the top five in the world. "Out of all the oil plays in the U.S., there's just one Bakken," Hamm says. "It towers above everything else."

The 65-year-old Hamm, worth more than $8 billion, has never lacked for confidence, but this has to be one of his boldest predictions yet. If he's right, that $8 billion could look like pocket change. If he's wrong, well, he still has himself one big oilfield. Indeed, he's already prospered from his Bakken bet--Continental shares (of which he owns 72%) are up 250% since early 2009.

The field in question stretches 8 million acres over North Dakota and Montana. Hamm started leasing land in 2003 and is now the single biggest holder, with the rights to 900,000 acres. Geologists have known since the 1970s that the formation was brimming with oil, but getting it out was technically tricky and unjustified by low oil prices. But in 2008 the U.S. Geological Survey announced to great fanfare that the formation likely held 4.3 billion barrels of recoverable oil and gas equivalents. That's big by any standard--except Hamm's.

He's convinced that the recoverable bounty is more like 24 billion barrels (20 billion of oil plus the equivalent of 4 billion barrels' worth of natural gas). This is oil on an outlandish scale--if true this would effectively double America's current total proved oil reserves. It's approaching ExxonMobil's total proved reserves of 28 billion bbl. It's enough oil to supply the entirety of U.S. demand (roughly 20 million bbl per day) for three years.

Can this longtime wildcatter possibly be right? Early signs indicate the field is bigger than the original estimate but well below Hamm's outsize appraisal. Geologists from the USGS met with Hamm recently to hear him out; they won't be done with a reassessment for two years.

Wood Mackenzie, a respected global compiler of oil and gas data, estimates the Bakken reserves are more like 9 billion bbl. The North Dakota Department of Mineral Resources figures that state's portion of the basin has 11 billion recoverable barrels--up from the 2 billion the department estimated in 2008.

So reserve estimates have been moving in Hamm's direction, if not to his 24 billion projection. What's changed? Operators like Continental, EOG Resources, Hess Corp., Occidental Petroleum and Marathon Oil have drilled some 3,000 wells there since 2008 and learn more from each one. Turns out that just 100 feet below the primary Bakken formation (itself 10,000 feet down) is a whole other layer of oil-bearing rock called the Three Forks.

Hamm's number is aggressive because his drilling technique is aggressive. Continental has developed a new drilling concept it calls Eco-Pad to exploit both reservoirs. One rig can develop 2 square miles by drilling eight wells--four into the Bakken layer and four into the Three Forks. Each well goes down 2 miles, then horizontally 2 miles through the reservoir. The drillers will blow hundreds of small holes (called "perforations") in the pipe of each well. Then comes the hydraulic fracturing in which the well is blasted with 1.8 million gallons of water mixed with sand that props open fractures in the dolomite rock to let out the oil. The "eco" in this Eco-Pad concept? All this work on eight giant wells gets done from one spot, causing less surface impact.

From there it's simple arithmetic. Hamm figures there's room for 48,000 wells. If each one delivers the 500,000-barrel average that drillers have seen so far, you get 24 billion barrels. Continental's take would be near 3 billion barrels in that scenario.

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