The Bayelsa State Oil and Environmental Commission said in a report that it started an investigation in 2019 on the impact of spills and looked at evidence from forensic scientists, blood samples from people in affected areas and company data.
"The report finds failures of strategy, prevention, response and remediation by oil companies," it said.
The commission adopted a United Nations model used to calculate the clean-up cost of spills in Niger Delta's Ogoniland more than a decade ago and found that "the clean-up will cost US$12 billion over 12 years" in Bayelsa.
The investigation discovered, among other findings, that toxic pollutants from spills and gas flaring were many times higher than the safe limits in samples of soil, water, air and in the blood of local residents, the commission said.
Toxins that cause burns, lung problems and risk of cancer were widespread while oil company-led clean-ups were often poorly executed and could further contaminate soil and groundwater, the commission's report said.
Bayelsa is one of the major oil producing states in the Niger Delta, a region blighted by pollution, conflict and corruption related to the oil and gas industry.
Pollution in Ogoniland, which was part of a landmark $1 billion clean-up effort involving a U.N. agency, could be worse than it previously estimated, a group monitoring the project said last year.
Oil majors in Nigeria have long faced legal challenges over Niger Delta spills, which they mostly blame on sabotage and vandalism of pipelines and illegal refining.
A spokesperson for Shell Petroleum Development Company of Nigeria Limited said it was not privy to the final report and could not comment.
An Eni spokesperson said the oil spills were due to theft to feed illegal refineries as well as illegal exports and sabotage but the company undertook to remedy all spills.