The operating environment has certainly improved for Halliburton, as higher crude oil prices and new shale gas drilling projects have brought new opportunities for oil services firms.

FORBES: Halliburton's Valuation A Little Too Hefty

Oil shale is an idea that was tested a generation ago, then abandoned when the price of crude oil plunged.

NPR: Squeezing Oil Out of Stones in the Rocky Mountains

As traditional oil fields yield less and less, major producers are looking for crude in less conventional places, from oil sands and shale plays to ultra deepwater.

FORBES: Icahn Could Push For Dividends At Transocean, The Only Major Driller Not Paying Shareholders

For their calculations, the two assumed that where there was a gap between demand and conventional supply it would be filled with synthetic fuels, first with tar-sands oil and later with oil from coal and shale. (According to high-end estimates, coal and oil shale could together yield some ten trillion barrels of unconventional crude.) They then calculated what the impact would be on global carbon-dioxide levels.

NEWYORKER: Unconventional Crude

The big growth in oil extracted from shale rock means the US will not need to import any crude within two decades, the former boss of BP has said.

BBC: Shale will free US from oil imports, says ex-BP boss

This recent article in the New York Times describes how American oil folks have figured out how to extract crude that is trapped in rock that is largely surrounded by shale.

FORBES: US Oil Industry Leads the Way in Extracting Hard-to-Get Crude; Steel to Benefit