A lack of transparency can lead to corruption
The world's largest energy group, Exxon-Mobil, is as secretive as its Russian and Chinese rivals, new research has suggested.
Transparency International evaluated the reporting practices of 42 oil and gas firms including payments made to resource-rich countries.
Exxon was the least transparent along with China's CNOOC and Russia's Lukoil.
Transparency International, which targets corruption, is a global network across more than 90 countries.
It said the lack of transparency can cause corruption and hurt the poor.
In response to the report, Exxon says it was committed to honest and ethical behaviour and opposed corruption.
The firm said it, along with other members of the International Association of Oil & Gas Producers, disagreed with the report's methodology.
The report ranked BP, the UK's largest firm, as "medium" for revenue transparency.
HIGH: BG Group, BHP Billiton, Petro-Canada, Shell, StatoilHydro, Nexen, Petrobras, Talisman Energy
MEDIUM: BP, Chevron, Conoco-Philips, Eni, Hess, Marathon Oil, Repsol, Total, Woodside
LOW: CNOOC, CNPC, Devon Energy, Exxon-Mobil, Kuwait Petroleum Corp, Lukoil, ONGC, Petronas
Source: Transparency International
Anglo-Dutch firm Shell was classed as "high" for transparency along with Brazilian firm Petrobras.
"The tragic paradox, that many resource-rich countries remain poor, stems from a lack of data on oil and gas revenues and how they are managed," Huguette Labelle, chairperson of Transparency International.
"Companies must do more to increase transparency."
The research recommended that oil companies should report the revenues paid to the governments of individual countries for extraction rights.
If more detail was given on how these firms operate, it would be easier to target corruption, said Transparency International.
The research was based on data from publicly available company sources, such as websites, annual reports and public documents.