The source said he believed the explosion was within the section of the plant that houses the hydrogen unit.
Residents described feeling an explosion shortly after 10 a.m.
Irving's Saint John refinery is the largest in Canada.
He says there's a lot of public concern in this country about how oil is transported, but not much attention is paid to the safety issues around refineries themselves.
The Irving facility in Saint John was shut down on Monday while investigators looked for the cause of the blast.
Most of the minor injuries were to contractors, not Irving staff, he said.
While he said Monday's blast "pales" to the deadly explosion on BP's Deepwater Horizon oil rig off the Louisiana coast in April 2010, Bhalloo added: "We can draw from that experience what could happen". It produces 300,000 barrels a day of gasoline, heating oil and jet fuel, exporting half of it to the Northeast through terminals that include Searsport, Portland and Portsmouth, New Hampshire.
Flame and smoke erupts from the Irving Oil refinery in Saint John, N.B., on Monday, September 8, 2018.
"We're sitting here and watching it burn right now", he said. Nearby streets have also been closed.
"There was thick, pitch black smoke mixed with white smoke", said Guimond. "At times we were letting that just safely extinguish itself", he said.
He chose to drive by the refinery, and said he was nearby when he felt the vibrations of what he assumed must be a second explosion.
Michael Steeves said he was driving about a kilometre away when he saw flames reach as high as 100 feet in the air.
Steeves said the incident reminded him of a similar event in the late 90s, when the same refinery suffered a similar explosion which left one person dead.
Hynes says they are now on a stand-down while the investigation continues, and that they are waiting to see if they can get back in to the plant to work tonight.