* Blow out may be sealed off on Thursday: EPA * Leaking gas, drilling mud being contained: EPA NEW YORK, April 26 A blow out at a Chesapeake Energy Corp. (CHK.N) well in Wyoming doesn't pose a threat to water supply in the region, and leaking natural gas and drilling mud are being effectively contained, the U.S. Environmental Protection Agency said on Thursday. "Our understanding is that attempts will be made to seal the actual blow out today," EPA Region 8 spokesman Richard Mylott said in an email. The loss of well control at a Chesapeake-operated well on Tuesday triggered a leak of unspecified quantities of natural gas and drilling mud, the company said on We dnesday. The company said it has a crew in place and plans to seal the well as soon as safety conditions permit. A spokesman did not provide further details on Thursday. Dozens of residents near the well, which is located about 10 miles from Douglas, Wyoming, were evacuated from their homes after the incident. No injuries, explosion or fires have been reported, and air quality readings near the well were "normal" on Wednesday, Chesapeake said earlier. The EPA has visited the site and determined the incident poses "no current threat to waters in the United States," the EPA's Mylott said. Chesapeake is "effectively containing the drilling mud and controlling the area for exposure to natural gas," he added in a statement. An EPA official had told Reuters on Wednesday that a "sheen" had been reported on an irrigation channel and pond near the well. The cause of the well incident was under investigation. Oklahoma City-based Chesapeake is the No. 2 U.S. natural gas driller. Wyoming is a key area of drilling expansion for Chesapeake, which holds leases across in the oil and gas-rich Niobrara Shale play that straddles Wyoming and Colorado. Chesapeake shares fell 0.5 percent to $18.03 on the New York Stock Exchange in Thursday morning trading.