A caller alerted the state's Office of Emergency Services on Monday about the spill at Platform Harmony, located 4.6 miles off the coast in federal waters.
The agency's report says the initial sheen on the water was 10 feet wide and stretched for about a mile.
Stephanie Young, a spokeswoman for the Coast Guard, says a search is continuing Wednesday for the exact source of the spill.
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She says initial reports are that it's lubricating oil for the platform and not oil pumped from the well.
Officials with Exxon Mobil did not immediately return a call seeking comment.