The Houston Ship Channel, one of the country’s busiest seaports and the gateway for crude oil deliveries for more than 10 percent of U.S. refining capacity, was shut down for a third day on Monday as crews continued to clean up an oil spill.

The Houston Ship Channel shut down Saturday after a bulk cargo ship, Summer Wind, and a Kirby Inland Marine oil barge collided, spilling about 4,000 barrels (168,000 gallons) of residual fuel oil and injuring six crew members. The channel, which empties into the Gulf of Mexico, gives oil barges and cargo ships access from the Gulf Coast further inland to terminals and refiners.

Channels to Houston, Texas City, Galveston and the Intracoastal Waterway remain shut.

A warning issued by the Coast Guard on Sunday said the channel might not open until the end of the week or later. About 80 vessels pass through the channel on a normal day.

Exxon Mobil Corporation (NYSE:XOM) told Reuters Sunday that the closure has not affected its operations in Baytown, Texas, the second-largest refinery in the U.S. If the shutdown is prolonged, fuel prices could briefly rise.

The Coast Guard and National Transportation Safety Board are still investigating what caused the crash and spill. About 24 vessels are working on skimming the oil out of the water.

Edgar Ang, with Oil Price Information Service (OPIS) reported that about half of the stranded ships are oil tankers. About 35 ships are waiting to depart, and 40 ships are waiting to go into the Houston Ship Channel.

So far, only cruise ships are allowed to transit in that area.