* Spill in Delta Wildlife Refuge near Mississippi mouth
* No wildlife been found covered with oil
* Coast Guard says contractor may have caused damage
(Updates with possible cause, adds no wildlife been affected)
HOUSTON, April 6 (Reuters) - An oil pipeline operated by Chevron Pipe Line Co CVX.N leaked at least 18,000 gallons (68,100 liters) of crude oil into the Delta National Wildlife Refuge in Louisiana, the U.S. Coast Guard said on Tuesday.
No wildlife covered with, or affected by oil had been found Tuesday afternoon, but the exact environmental impact remained undetermined, a spokesman for the Louisiana Department of Wildlife said.
The leak, discovered by a contractor shortly after midnight, discharged into a canal 10 miles (16 km) southeast of Venice, Louisiana, and investigative and cleanup teams were on scene, the Coast Guard said in a news release.
A barge working for Exxon Mobil Corp XOM.N was driving long pipes called "spuds" to anchor the barge in place, and one may have hit the pipeline operated by Chevron, a Coast Guard spokesman said.
Chevron shut down the affected section of pipeline and started efforts to minimize environmental impact, said a news release issued by Chevron, the Coast Guard and state of Louisiana.
Coast Guard and state environmental investigators were assessing the impact and monitoring cleanup efforts from aircraft and boats, the release said.
The refuge is marshland near the mouth of the Mississippi River southeast of New Orleans and provides habitat for alligator, brown pelican and migrating birds.
The pipeline is owned by Cypress Pipe Line Co, a joint venture between Chevron and BP Plc BP.L. (Reporting by Bruce Nichols)
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