The leak that began Tuesday has been stopped, according to Shell, and dispersants are being deployed to clean up the crude spilled at the field some 120 kilometres off Nigeria, Africa's largest oil producer and an OPEC member.
Production has halted at Shell's Bonga field, which has a capacity of 200,000 barrels per day, due to the spill.
Shell has said "less than 40,000 barrels" leaked, while Nigerian authorities could not immediately be reached for comment.
The company claimed on Thursday that "up to 50 percent of the leaked oil has already dissipated due to natural dispersion and evaporation," but that figure was impossible to verify independently.
"To accelerate the clean-up at sea, we are deploying vessels with dispersants to break up the oil sheen at sea," Shell Nigeria head Mutiu Sunmonu said in a statement.
"We are mobilising airplanes that will support the vessels in this operation. We are deploying infra-red equipment to be able to trace the few areas where the sheen may be thicker. That allows for a targeted use of the dispersant.
Living with oil spill in Ogoniland, Nigeria
"Let me also mention that we are currently working with the Nigerian government to inform local communities and fishermen about the situation."
Shell said the leak occurred on Tuesday during a transfer of crude to a waiting tanker.
The likely source of the leak was an export line linking a production vessel to the tanker, Shell said. The line has been closed and de-pressurised, halting the flow of oil, it said.
Nigeria has been producing between 2.0 and 2.4 million barrels per day in recent months. Scores of oil spills have occurred in Nigeria, but most have been at onshore sites and their size has often been disputed.