The company said it closed the well, which is operated by Shell Petroleum Development Company of Nigeria (SPDC), roughly 15 hours after the leak was detected.
The spill impacted 300,000 square metres of water surface, an area roughly the size of 42 football pitches, in Bayelsa State.
Shell said it had deployed booms to prevent further spread of the spill.
“A follow-up joint investigation visit is being planned to lift the line from underwater and determine the cause of the leak,” Shell said in a statement.
The Seibou field produced 1.3 million barrels of crude oil, an average of around 3,500 barrels per day, in 2013, according to the Nigerian National Petroleum Company, as well as 29 million cubic metres of gas.