TULSA, Oklahoma - The thick black smoke over west Tulsa is the result of a training exercise at HollyFrontier Refinery. The training exercise was planned in advance and was not caused by any emergency.

The smoke became visible in the skyline shortly after 9 a.m. and again just before 10 a.m.

HollyFrontier Refining Corporation is located at 1700 South Union Avenue. The switchboard confirmed that the smoke is part of the company's training process.