* Officials urge Wyoming residents not to drink water
* EPA reaches no conclusion on source of contamination
By Jon Hurdle
PHILADELPHIA, Sept 1 (Reuters) - U.S. government officials urged residents of a Wyoming farming community near natural gas drilling sites not to use private well water for drinking or cooking because of chemical contamination.
"Sample results indicate that the presence of petroleum hydrocarbons and other chemical compounds in groundwater represents a drinking water concern," the Environmental Protection Agency said in a statement about tests of 19 water wells around the town of Pavillion.
The Wyoming investigation precedes a national study by the EPA into the safety of the drilling technique known as hydraulic fracturing or "fracking", in response to concern in Congress and in some communities near gas rigs in many states that human health is threatened by the process.
The tests in Pavillion found that 17 of the 19 wells tested contained petroleum hydrocarbons as well as napthalene, phenols and benzene, the Environmental Protection Agency said in a report issued late on Tuesday.
The tests are part of the agency's first investigation into claims that toxic chemicals used in hydraulic fracturing are contaminating ground water.
But officials expressed no views about the source of the contamination.
"EPA has not reached any conclusions about how constituents of concern are occurring in domestic wells," the report said.
Concerns about the safety of fracking threaten to slow the development of vast shale gas reserves that may be sufficient to meet U.S. natural gas demand for a century or more, experts believe.
The EPA's latest results were analyzed by federal toxicologists who recommended that Pavillion residents find alternative sources of water for drinking and cooking.
For residents whose wells contain organic hydrocarbons, the new water supplies will be paid for by EnCana, the Canadian energy company that owns Pavillion's approximately 250 gas wells, said Richard Mylott, an EPA spokesman.
Some wells were found contain to methane, and their owners were advised to ensure proper ventilation while showering.
The new samples were collected in January and follow a less-detailed round of testing in March 2009 during which 11 of 39 wells were found to contain "contaminants of concern."
The latest tests revisited 19 of the 39 initial wells as well as four irrigation wells, two municipal wells, and some surface water.
The samples were analyzed for more than 300 substances by four laboratories, the EPA.
In coming months, scientists will continue testing, and will focus on possible sources of the contamination, Mylott said. (Editing by David Gregorio)