Chesapeake ordered to inspect 171 Marcellus wells

* Six wells may have leaked methane

* Stray gas reported in river, water wells

HOUSTON, Sept 17 (Reuters) - Chesapeake Energy Corp (CHK.N) must inspect 171 natural gas wells in Pennsylvania's Marcellus Shale after reports gas leaked into a river and nearby water wells, a state regulator said on Friday.

Earlier this month, Pennsylvania's Department of Environmental Protection (DEP) received reports of bubbling water on the Susquehanna River in northern Pennsylvania.

The regulator and Chesapeake believes the culprit is methane gas migrating from six wells located about two to three miles northwest of the river in Wilmot Township.

"Ventilation systems have been installed at six private water wells. Water has been provided to the three affected homes, and Chesapeake is evaluating and remediating each of its well bores within a four-and-a-half-mile radius of the gas migration, which is essential," DEP Secretary John Hanger said in a statement.

DEP sampled six private water wells and their analysis found methane levels in the water wells that fluctuated between undetectable and 4.4 percent, possibly as a result of barometric pressure in the atmosphere, it said.

A spokesman for Oklahoma City, Oklahoma-based Chesapeake was not immediately available to comment.

To help prevent against future migration issues the state ordered Chesapeake to evaluate each of its 171 wells that used the same well casing procedures used in the six Wilmot Township wells.

Well casings are installed in a well bore to act as a barrier to the rock formations and maintain the well's integrity. (Reporting by Anna Driver; editing by Andre Grenon)