One of Canada’s largest independent producers of crude oil has been charged by the Alberta government after it allegedly released hydrogen sulphide gas into the atmosphere near Fort McMurray.

Hydrogen sulphide gas is produced during the bitumen upgrading process. It is considered a very poisonous, flammable gas with the characteristic odour of rotten eggs.

Canadian Natural Resources Ltd. is facing three charges, all stemming from an incident in May 2010 at the Horizon Oil Sands facility, located 70 kilometres north of Fort McMurray.

The Alberta government alleges the gas "caused a significant adverse effect" and that company officials failed to report the release.

The allegations are not sitting well with some environmental groups.

The Pembina Institute says it’s disappointed with the lack of information around environmental charges against the oilsands plant.

 "Nobody likes to see -- whether it's air quality or any other type of environmental law -- being violated and not even reported at that time," said Marc Huot, a policy analyst with Pembina Institute."That's not something that anybody wants to see and unfortunately, it's occurring too often."

The first court appearance for Canadian Natural will be on July 11 at the Fort McMurray Provincial Court.