An offshore drilling and production platform. Gary Tramontina | Corbis | Getty Images

A deal Shell and Eni brokered with the Nigerian government in 2011 is set to cost the African nation $6 billion in lost revenue, a report claimed Monday. The report, published by campaign group Global Witness, said the allegedly corrupt deal's terms would stop Nigeria accessing its share of the profits from oil extracted from its offshore block. Shell and Eni were accused of bribery last year over a $1.3 billion payment that secured an exploration licence for the block, known as OPL 245, in 2011. It was alleged that although the funds were paid to the Nigerian government, the money actually went to Malabu Oil and Gas — a company linked to former oil minister Dan Etete. Global Witness claimed that a term granting Nigeria a share of the oil production profits was negotiated out of the 2011 deal by former Nigerian ministers, who it alleges took bribes from the oil giants. It said the clause was replaced with a back-in option that required Nigeria to pay more towards the exploration costs than it could afford. Shell and Eni — along with some of their former employees — are facing charges relating to the payment, with Italian prosecutors alleging there was awareness the funds would be pocketed by individuals. All of the defendants have denied any wrongdoing.

'No case against Shell'

In a statement emailed to CNBC, a Shell spokesperson said: "Since this matter is before the Tribunal of Milan it would not be appropriate for us to comment in detail. We maintain that the settlement was a fully legal transaction with the federal government of Nigeria and Eni and, based on our review of the prosecutor of Milan's file and all of the information and facts available to us, we do not believe that there is a basis to convict Shell or any of its former employees."



Shell's Executive Vice President Frans Everts wrote to Global Witness ahead of the report's publication to reiterate the company's position. In the letter, seen by CNBC, Everts questioned the methods used in Global Witness' research. "We dispute your characterization of the facts, your allegations of criminality and the legal conclusions you reach," he said. "The statements in your letter are based on faulty methodologies which do not meet adequate qualitative standards. They fail to take into account elements that are typically used by the industry… misconstrue the terms of the 2011 settlement and even reference legislation which has not been passed."

Nigeria's back-in right