HARRISBURG -- State environmental officials are investigating a potential case of methane migration in Tioga County near gas-drilling sites operated by a Shell Oil Co. subsidiary.

Four residents within a one-mile area had been evacuated as of Thursday. A company spokeswoman said in a statement that the current methane levels indicate "a very low hazard risk to people, vegetation and fish."

Dan Spadoni, a spokesman with the Department of Environmental Protection's north-central regional office, said the agency was alerted Sunday that a drinking water well had overflowed inside a hunting cabin near the well pads in Union.

Thursday afternoon, regulators had not yet determined the source of the methane, which also was suspected to be causing bubbling in a nearby stream and "additional surface expressions" along a neighboring road.

A Wellsboro Gazette photograph identified one affected area, showing water along a grassy path spraying several feet into the air.

Methane gas shifting from an underground formation into a water supply was the cause of the often-cited contamination cases in Dimock, also in the state's northern tier. DEP officials determined in that case that poor well construction by Cabot Oil & Gas caused the migration incidents.

Mr. Spadoni said Shell -- whose subsidiary, SWEPI LP, is listed as operating the nearby drilling sites -- is fully cooperating with the response and investigation.

Reports of a potential methane migration case come as state lawmakers are considering a lucrative tax credit to persuade Shell to locate a proposed ethane-processing facility in Beaver County.

That incentive, potentially worth at least $1.65 billion over the next 25 years, is expected to be voted on during the state budget debates next week.

The DEP's list of "spud," or drilled, wells identifies 18 wells operated by SWEPI in Union, including three on the Guindon site where a spokesman confirmed that work was suspended.

Another 28 of the township's 54 well sites are listed under East Resources, which was bought by Shell two years ago. Shell's nearby well sites are in various stages of completion, according to DEP.

The cabin where the water well overflowed is about 4,000 feet from one of Shell's well pads, Mr. Spadoni said. State regulators have collected water samples, and Shell is screening private drinking wells within a one-mile radius.

While a Shell representative listed several precautions that the company is taking to manage the situation, she also noted methane gas has historically been found in groundwater in the area.

"These pre-existing conditions make it difficult to identify a specific source of the methane detected," wrote Shell spokeswoman Kelly op de Weegh in a statement. "We are evaluating the timing of the incident, reviewing other pre-drill data from nearby residences, determining the methane signature [specific characteristics], and evaluating overall water quality changes."

Neither local supervisors in Union nor a coordinator with Tioga County's emergency services department could be reached Thursday.

By Harrisburg Bureau Chief Laura Olson: