Tapping shale, seeking sustainability. A Rare Oilman

It was not until the engineers fracked the formation with a solution of water that Mitchell made a breakthrough.

It was thought that water would cause the clay in shale to swell, locking in the gas, rather than liberating it. But Mitchell's engineers discovered that hydraulic fracturing caused the brittle shale to shatter like glass. Sand was added to keep the tiny fissures open.

Steward said using water had an added benefit: It was much cheaper than other fluids.

In 2002, Mitchell merged with Devon Energy Corp. for $3.5 billion, and Devon incorporated Mitchell's fracking technology with horizontal drilling techniques to dramatically expand the amount of gas that can be captured from one well.

Mitchell relishes the effect of a global glut of shale gas: less reliance on imported oil and coal, which generate more pollution and greenhouse-gas emissions.

But the natural gas boom he helped launch has also triggered an upwelling of fear about hydraulic fracturing.

Though Mitchell says the fracturing itself is not an environmental threat - he says the shale formations are too deep and too isolated to allow the fluids to migrate into groundwater - he agrees that some toxic additives used in fracturing are "not good," and that the industry needs to better manage handling and disposing of wastewater.

The industry should move quickly to embrace recycling of fracturing fluids, or replacing the additives with greener alternatives.

"If you can reuse it, it'll cost you a little money, but it's a good thing," Mitchell said. "Because the environmentalists will give you hell, and I think they're right, they should give you hell."

Contact staff writer Andrew Maykuth at 215-854-2947 or amaykuth@phillynews.com.