BP finished pumping cement into its ruptured oil well in the Gulf of Mexico on Thursday to seal off the source of the world’s worst offshore spill, paving the way to permanently plug the blow-out later this month.

The daylong cementing operation followed earlier injections of heavy drilling mud this week that had subdued the upward pressure of oil and gas inside the deep-sea Macondo well. The crippled wellhead was provisionally capped in mid-July.

“This is not the end, but it will virtually assure us that no oil will be leaking into the environment,” retired Coast Guard Admiral Thad Allen, who oversees the U.S. oil spill response operation, said at a briefing in Washington.

“Monitoring of the well is under way in order to confirm the effectiveness of the procedure,” BP said in a statement announcing completion of the cementing work.