The spill at Eleme in Rivers State on the eastern side of the Niger Delta was detected on 11 June and four days later, Shell Petroleum Development Company of Nigeria Limited (SPDC) confirmed it in a statement.
Environmental rights groups said the spill lasted a week before it was contained.
A team comprising SPDC, Nigerian Oil Spill Detection and Response Agency and local communities were at the site on Monday to gather information, analyse data, examine physical evidence, and assess the causes of the leak, said Youths and Environmental Advocacy Centre which monitors spills in the Niger Delta.
A Shell spokesperson confirmed Monday's visit to the site.
The investigation will determine the volume of oil spilt.
Shell has over the years faced several legal battles over oil spills in the Niger Delta, a region blighted by pollution, conflict and corruption related to the oil and gas industry.
The oil major blames most of the spills on pipeline vandalism and illegal tapping of crude.
Thandile Chinyavanhu, Greenpeace Africa climate and energy campaigner, said the latest spill compounded Shell's record in one of Africa's leading oil producers.
"Shell must be held accountable and financially responsible for this spill and for its neocolonial role in causing climate loss and damage," Chinyavanhu said.
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