A mechanical failure and worker error was likely responsible for the Chesapake Energy Corp. natural gas well blowout in eastern Wyoming on April 27, a preliminary state investigation has found.

The Wyoming Oil and Gas Conservation Commission found that workers at the rig didn't properly engage wellhead lockdown pins and didn't see and respond to an increase of drilling mud coming back out of the well, which indicates that gas was forcing its way to the top of the well.

The well near Douglas vented up to 2 million cubic feet of gas and 31,500 gallons of oil-based drilling "mud" over nearly three days, as Chesapeake and its contractors worked to regain control the well, the commission reported.

What's known as the B section wellhead and lockdown pin -- part of the wellhead apparatus -- have been transported to a Louisiana lab by Chesapeake for closer examination, the WOGCC said.

The blowout impacted a 52.2-acre area around the well site, according to the investigation. The commission isn't planning to issue any citations over the incident pending final site cleanup, sampling and final inspection, it said.

Both Oklahoma City-based Chesapeake and Calgary-based Trinidad Drilling Ltd., which owned and operated the drilling rig at the site, said they're cooperating with the ongoing investigation and have no other comments.

The well into the Niobrara Shale formation blew out about 4 p.m. April 24, roaring natural gas into the air. While nobody was hurt by the blowout, several dozen nearby residents decided to evacuate when given the option by Chesapeake and local law enforcement officials.

Gas continued to vent as Chesapeake and its contractors worked to bring the well under control. The workers were stymied by shifting and variable winds, but finally plugged the well 66 hours later on April 27.

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