HOUSTON - It wouldn't be a stretch to call George P. Mitchell the father of shale gas.

The billionaire tycoon is widely credited with developing the hydraulic-fracturing technique that has triggered a rush to tap into formations like Pennsylvania's Marcellus Shale. Shale-gas discoveries have added decades of supply to the nation's reserves.

"You probably have never heard of oilman George Mitchell," Washington Post columnist Robert J. Samuelson wrote recently, "but more than anyone else, he has changed the global energy outlook."

What may be more surprising - especially to those who associate hydraulic fracturing with charges of environmental degradation - is that the man who pioneered shale drilling regards his fossil-fuel discoveries as secondary to his work promoting a sustainable world.

"There's no doubt this nation is strong because of oil and gas," Mitchell, 91, said in an interview Wednesday in his downtown Houston office. "But sustainability is the most important thing I'm working on."

Mitchell is not a typical oilman.

In 2002, the National Academies called him "a renaissance American businessman and entrepreneur" when he and his wife, Cynthia, who is now deceased, gave $10 million to fund sustainability science.

"Oil and gas people are usually just prone to thinking about oil and gas," Mitchell said. "I think all of us better start thinking about sustainability. How do you make the world sustainable for your grandkids to have a chance?"

The son of Greek immigrants, Mitchell said he was introduced to environmental issues in 1964 by futurist R. Buckminster Fuller, who was preoccupied with Earth's survivability.

"If we don't start waking up, in 2050 we're in trouble, because you'll have nine billion people worldwide," Mitchell said. "And if you can't make the world work with six billion people, how can you make it work with nine?"

Mitchell's concerns about the decline of U.S. cities in the 1960s inspired him to build his own town - sort of a quest for a perfect community. He bought 15,000 acres of timberland about 25 miles north of Houston, and in 1974, he founded The Woodlands.

The Woodlands incorporated the environmental principles of Ian McHarg, the renowned University of Pennsylvania landscape architect.

Mitchell credits McHarg's design of waterways and winding streets with The Woodlands' success. The community now has an estimated population of 90,000. (It has grown to 27,000 acres, or 42 square miles, about the size of Northeast Philadelphia.)

"It's a beautiful project," Mitchell said. "This was a concept that I had: We can do better than what had been done before."

It may seem ironic that a man who got rich off fossil fuel - the quintessential unsustainable energy source - should become a champion of sustainability. Mitchell does not see it that way.

In the industry, his achievements are legendary. The Gas Technology Institute gave him a lifetime-achievement award in July, saying his work had "transformed the U.S. energy picture."

For most of the 20th century, exploration companies focused on discovering "conventional" reserves, easily recovered pockets of oil and gas.

But easy oil has become increasingly difficult to find, leading to exploration of "unconventional" geologic formations such as black shale.

Shale must be hydraulically fractured to give up its gas. "Fracking" involves the high-pressure injection of fluid into a geologic formation to shatter the rock to unlock entrapped gas molecules.

Though oil and gas wells have been fractured for 60 years - mostly to rejuvenate old, tired wells - Mitchell's persistence produced the new methods that made shale economically attractive.

The company he formed in 1946, Mitchell Energy & Development Corp., had become successful drilling shallow conventional wells in the Fort Worth basin in north Texas. But by the 1980s, the resource was in decline and nearing exhaustion.

Mitchell was not eager to pull out. His company had invested a lot to build infrastructure to gather and process natural gas and had a lucrative contract to pipe the gas to the Midwest. So he ordered his geologists to find new reserves. He directed them to explore the Barnett Shale, which lies more than a mile beneath the surface.

Many experts believed the shale gas was unrecoverable. Shale is formed from the accumulated organic matter of ancient seabeds, mostly plankton. While rich in gas, the rock is impermeable.

"People said if the Barnett was the best we had, then Mitchell was in deep doo-doo, though they used a different word for it," said Dan B. Steward, a geologist who wrote a book on Mitchell's 17-year effort to solve the shale puzzle.

"We kept trying, trying, trying," said Mitchell, a petroleum engineer and geologist, who said he spent more than $6 million drilling about 30 experimental shale wells.

Mitchell's team tried a variety of fracturing techniques - injections of nitrogen, carbon dioxide, gels, even fuel like propane. Nothing got long-lasting results.

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."