Human rights group Amnesty International is accusing major oil companies of covering up the true cause of oil spills in Nigeria to avoid compensation claims.
Shell has disputed the accusation, saying it is working hard to ensure greater transparency.
There are hundreds of oil spills in the Niger Delta every year. In the area where Italian operator Agip operates there have been 170 spills there this year, the BBC reports.
The oil companies say the vast majority are caused by sabotage and theft from the pipelines.
But Amnesty International says in order to get out of paying compensation these oil companies including Shell sometimes site sabotage, when in fact poor maintenance and corroded pipes are the cause of the spills.
Amnesty identifies a "staggering" 474 spills in 2012 in one area alone, operated by the Nigerian Agip Oil Company - a subsidiary of Italian firm ENI.
Working with a local human rights group, Amnesty studied the oil spill investigation process in Nigeria over six months. It claims there is "no legitimate basis" for the oil companies' claims that the vast majority of spills are caused by sabotage and theft.
Members of the local community with oil company staff and government officials are supposed to investigate oil spills, but Amnesty calls this Joint Investigation Visit (JIV) process "wholly unreliable" because, it says, the companies themselves are the primary investigators and the process lacks transparency.
It says this means that both the causes and severity of oil spills may therefore be misrecorded, sometimes meaning affected communities miss out on compensation.
Shell said it "firmly rejects" the claims. "We seek to bring greater transparency and independent oversight to the issue of oil spills, and will continue to find ways to enhance this."
It said the JIV process was a federal process the company could not unilaterally change. Stolen oil, Shell said, costs Nigeria billions of dollars in lost revenue.
