Shares of Canadian Natural Resources Ltd. (NYSE: CNQ) are dropping this morning after an explosion and fire at one of its refineries in Alberta has halted production of oil sand.

As of 9:35 a.m. (EDT), CNQ shares are down 5.99 percent.

Four workers suffered serious burns in the explosion, which occurred at the company’s Horizon facility north of Fort McMurray.

Canadian Natural Resources said that oil production at the facility is suspended and it isn't yet known when production will resume.