Royal Dutch Shell has confirmed the cause of what could be Nigeria's worst oil leak since 1998.

Images from an underwater vehicle proved the company's original fears that the leak sprung from the transfer of oil from a floating storage facility to an awaiting cargo tanker.

The underwater vehicle found an export line had failed and tore, according Shell's website. Export lines are underwater flexible pipelines used to transport crude oil from offshore rigs to the shore.

Since Tuesday, when we became aware of this regrettable leak at our Bonga offshore facility, substantial progress has been made in mitigating the consequences, said Shell's country chair in Nigeria, Mutiu Sunmonu.

To prevent further oil from flowing, the company depressurized the pipeline and shut down the Bonga well. 

The tear in the pipeline leaked 40,000 barrels into the Atlantic. Company officials as of Wednesday said 50 percent of the oil has dissipated.

Nigerian officials on Thursday were expecting the oil from the failed line to reach the country's shore, but in a Friday update, Shell maintained the oil sheen remains in the ocean.

Let me express my heartfelt thanks to industry colleagues and government agencies for their ongoing support to mitigate the impact of the leak, Sunmonu said in a company statement.

The well was the first offshore discovery in Nigeria and produced 10 percent of the country's crude. It is located roughly 75 miles from the country's coast, reported Bloomberg on Friday.