The Chatham House report suggested that high oil prices have contributed to the incentive to steal. But it was inconclusive on the question of how much of the stolen crude was exported, and how much was processed in operations in the Nigerian swamps. “The bush refining business is highly decentralized and secretive,” it said, “which makes its size hard to estimate.”

Mr. Ribadu’s report gave a higher possible figure for the scale of daily theft — 250,000 barrels — but the Chatham House document emphasized the unreliability of figures from government and oil companies. Mr. Ribadu suggested, in stronger terms than the more recent document, that the problem is growing, noting that Shell claimed a fivefold increase in losses between 2009 and 2012, from 10,000 barrels per day to 50,000 in March of last year.

Chatham House, by contrast, said that “outsiders should look closely at claims that Nigeria is losing oil at unheard-of rates,” adding that “the high divergence in industrywide estimates makes it hard to gauge trends reliably.”