Data from the state show that many counties don't have any gas drilling wells tapping into the gas-rich Marcellus Shale formation and may never have any despite the proliferation of them elsewhere.

U.S. Geological Survey maps indicate the Marcellus Shale, a rock formation thousands of feet underground in large parts of Pennsylvania, New York, Ohio and West Virginia, doesn't exist in the heavily populated southeastern portion of Pennsylvania.

Almost 90 percent of the 5,000 shale wells drilled in the last six years are located in just 11 of the state's 67 counties, according to state data compiled by the Powell Shale Digest, a specialty trade newsletter. Bradford County, in the northeast part of the state, has the most, at 1,008, followed by Tioga, also in the northern region, with 693.

In the southwestern part of the state, Washington County has the most wells, with 560.
Most of the wells were drilled in the last two years: 2,046 last year, and 1,644 the year before. But the Shale Digest notes the number may drop this year.

"Given that Marcellus drilling activity in Pennsylvania is slowing in 2012 in response to depressed natural gas prices, it will be most interesting to see how this year stacks up compared to the others," the March 12 issue noted.

Travis Windle, a spokesman for the Marcellus Shale Coalition, an industry group, said people all over the state and the region benefit from the lower energy costs that have accompanied the boom.

Over the last five years, advances in drilling technology have made the shale accessible, leading to a boom in production, jobs and profits and a drop in natural gas prices.

The drilling process, hydraulic fracturing, blasts a mix of sand, water and chemicals deep underground to break open shale rocks and stimulate oil and gas production in previously inaccessible areas.

That produces huge volumes of polluted wastewater, and environmentalists say the process, also called fracking, can taint groundwater. Energy companies deny that.