NEW YORK — Mention oil spills, and images of birds coated in black slime and a shiny slick on the ocean’s surface come to mind. But not all oil spills are the same.

About 672,000 gallons of oil spilled when a pipeline fractured about a mile below the ocean’s surface this month in the Gulf of Mexico southeast of Venice, La., which is about 65 miles south of New Orleans.

Hardly any of it was visible.

“The thing that sort of confused people about this one is that we weren’t seeing any oil,’’ Lieutenant Commander Steven Youde of the Coast Guard said.

Aside from a few areas with a light sheen on the surface of the ocean, the oil seemed to have completely disappeared, and it was not expected to affect the shoreline.

The oil spill appeared to be the largest since the Deepwater Horizon explosion in 2010, when 4 million barrels of oil leaked. This month’s episode was far smaller: 16,000 barrels in less than two days. Even so, 16,000 barrels is “a pretty substantial leak,’’ said Edward B. Overton, an emeritus professor of environmental sciences at Louisiana State University who is studying the environmental effects of Deepwater Horizon. “But it was not enough on the surface to warrant a cleanup response.’’

In this case, the oil degraded quickly, in part because of environmental forces.

The company that operates the pipeline, LLOG Exploration, believes the pipe fractured in the early morning hours of Oct. 11, said Rick Fowler, a company spokesman.

On Oct. 12, LLOG discovered that the amount of oil leaving its wells was different from the amount of oil leaving the company’s floating production system, Delta House, which is in the Gulf of Mexico, about 40 miles southeast of Venice, La.

The small crack in the pipeline has not been fixed, Fowler said, but wells were shut and the flow through the pipe was stopped. What caused the fracture was unclear. The Bureau of Safety and Environmental Enforcement is investigating.

Officials searched for the oil for several days before determining that it was unlikely to ever be seen, he said. The droplets were ingested by oil-degrading bacteria that live in the Gulf of Mexico, Overton said.

Some of the larger drops rose to the surface, where sunlight, wind, and wave action helped break them down. As a result, the spill has “some environmental impact,’’ Overton said, but it doesn’t appear to be measurable or significant.