Output at Iraq's Rumaila oil field has been cut to 530,000 barrels per day - down from about one and a quarter million - after two bombs blew up pipelines.
As a result of the blasts some production has been completely halted at the Basra site.
The twin explosions hit Rumaila's pipeline network late Friday.
Police say they found evidence the explosives were planted beneath the pipes.
Basra, which handles the bulk of Iraq's oil exports, has generally had fewer attacks than other Iraqi cities.
Rumaila, the workhorse of Iraq's oil industry, has estimated reserves of around 17 billion barrels, and produces the bulk of Iraq's total output of 2.9 million barrels per day.
Travis Brecher, Reuters