A fire is burning oil product tanks Friday afternoon at a Greeley well that injects wastewater from hydraulic fracturing into the ground, fire officials say.

Three homes near the well have been evacuated and fire crews are waiting until the blaze dies down before extinguishing it fully, calling their approach "conservative."

Crews are currently staged about a quarter-mile from the site — operated by NGL Energy Partners — out of safety concerns, according to Dale Lyman, division chief and fire marshal for the Greeley Fire Department. Representatives from the Colorado Oil and Gas Conservation Commission are headed to the scene, he said.

Todd Hartman, spokesman for COGCC, said the regulatory agency is monitoring whether any fracking wastewater has been released in the explosion.

"Any such release would have to be reported to the COGCC and soil cleaned to standards that have been set out by the Colorado Department of Public Health and Environment," Hartman said. "It's a bit early to gauge those impacts, if any."

The blaze was first called in at about 1:30 p.m. in the area of Weld County roads 64 and 47. Lyman likened the situation to a hazardous materials call, calling the blaze "significant."

A photo from the scene showed heavy black smoke pouring into the sky from the product tanks. One video of the fire recorded a loud explosion and fireball shooting from flaming tanks, a large metal object being hurled into the air.

Lyman said "most of our department is within a quarter-mile" of the fire on standby.

No injuries have been reported in the blaze. Officials say they don't know how the fire began.

"We're still seeing pure black smoke and every once in a while you see a flash of orange up through the middle," said Linda Belleau, who owns the Barnstormer Restaurant inside the Greeley-Weld County Airport near the fire.

Bellau said she has seen at least two trucks consumed in the blaze.

Officials say they are not concerned about any health hazards from the fire outside of the immediate area.

NGL is a Tulsa, Okla.-based oil and gas operations provider that operates at several sites in Weld County. A spokesman for NGL did not immediately return a message left seeking comment.

The Greeley Tribune reports the fire can be seen for miles.

This is a developing story that will be updated as more information becomes available.

Jesse Paul: 303-954-1733, jpaul@denverpost.com or twitter.com/JesseAPaul