The United States Geological Survey announced on Thursday that there may be 3.0-4.3 billion barrels of "undiscovered, technically recoverable oil" in the Bakken shale formation, discussed briefly here at How the World Works yesterday. The USGS said it was the largest "continuous oil accumulation" it had ever assessed. For comparison purposes, the Alberta oil sands are thought to have 175 billion to 300 billion barrels of recoverable oil, and Saudi Arabia claims upward of 260 billion. So while 4.3 billion might seem like a big number, it's not quite the big leagues.

According to the USGS press release announcing the results of the assessment, "a 'continuous' oil accumulation means that the oil resource is dispersed throughout a geologic formation rather than existing as discrete, localized occurrences."

Translation: technically recoverable, but it ain't gonna be easy. Here's how the Associated Press characterized the task ahead:

About two-thirds of the acreage is in western North Dakota, where the oil is trapped in a thin layer of dense rock nearly two miles beneath the surface. Companies use pressurized fluid and sand to break pores in the rock and prop them open to recover the oil ...

Oil companies began sharing technology about two years ago on how to recover the oil. The technology involves drilling vertically to about 10,000 feet, then "kicking out" for as many feet horizontally, while fracturing the rock to release the oil trapped in microscopic pores in the area known as the "middle" Bakken.

This process, known as "horizontal drilling" may not involve the same kind of wholesale rock pulverization usually associated with shale oil recovery, but it still sounds like a hugely energy-intensive, expensive mess.