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" ... Does this matter? Consider that resource estimates from more than a decade ago rarely included shale oil (though many included oil shales, or kerogen, a solid substance that requires expensive processing to be converted into conventional liquids). Wells in the Bakken shale might produce a barrel of oil—if they were unfractured vertical wells, and so oil in Bakken shales would be disregarded by geologists estimating global resources. Enter George Mitchell and his team’s discovery that shales which were hydraulically fractured could, at a three or five-fold increase in cost, would produce a hundred times more oil. Voila, the prohibitively expense resource has gone from invisible to part of the supply curve. ... "