JACKSONBURG, W.Va. - Almost all the natural gas produced in the Marcellus Shale - billed by its advocates as the clean, domestic fuel of the future - is extracted from the earth using dirty diesel fuel derived from imported oil.

Some Marcellus Shale producers are beginning to practice what they preach: switching to gas-powered drilling rigs that are cleaner, quieter, and cost less than diesels.

On a remote hilltop in northern West Virginia, EQT Corp. began operating a rig in July powered by liquefied natural gas, which is brought in by tanker trucks from Pennsylvania.

EQT operates a second rig in West Virginia powered by tapping into pipelines that carry "field gas" - unprocessed natural gas - from nearby wells.

Energy Corp. of America launched a bifuel rig last year that can run on diesel or field gas. Consol Energy has two dual-fuel systems in operation in Greene County, Pa. In northern Pennsylvania, Seneca Resources Corp. announced last month it had converted two of its rigs to operate on natural gas.

Marcellus producers are under increasing pressure from activists and environmental regulators to curtail emissions, including the intensive, short-term pollution caused by drilling, as well as the ongoing emissions from processing and transporting the fuel once wells are completed.

"We want to be a leader in reducing the environmental impacts related to drilling," said Steve Schlotterbeck, EQT's president of exploration and production.

A single drill rig can consume up to 80 gallons of diesel an hour to run its massive electrical generators, which produce enough power to light 2,000 homes.

Compared to diesel, a natural-gas rig emits 20 percent to 30 percent less carbon dioxide. It emits a small fraction of the nitrogen oxides, sulfur oxides, and particulates.

Building a rig with a natural gas generator costs more, but its operating costs are lower. Ensign Energy Services Inc., a Denver rig operator, estimates switching to LNG could knock $660,000 off the annual fuel cost of one rig. Using field gas can save more than $1 million a year in fuel costs.

Oddly, the lack of fuel supply has held back drillers from switching to natural gas. There are few suppliers of LNG, which must be supercooled to convert the gas into a liquid. Before it can be used as fuel, LNG must also pass through a heat exchanger to convert it back into gas.

"Operators are hesitant to switch to 100 percent natural gas because the availability of the fuel is questionable," said Alan Grosse, business development manager for GE Energy, which manufacturers gas generators.

Ensign, the drilling operator that has gone all in on natural gas rigs, estimates that if every land-based drilling rig in America were converted from diesel, it would reduce nitrogen oxide emissions by 250,000 tons a year and particulate emissions by 6,300 tons per year.