Gazprom, Alexei Miller. He said the three parties decided to discuss the cartel further at the next meeting of their foreign ministers. Many experts say a natural gas cartel that resembles the Organization of Petroleum Exporting Countries group would be tough to achieve. Unlike oil, which is traded on an exchange that constantly updates the market price based on supply and demand, most gas is sold under tight contracts that allow buyers to lock in prices for up to 25 years. As a result, a cartel would likely have little influence. The formation of a gas exchange also would be difficult because most natural gas is delivered via pipelines and is far less easy than oil to ship around the world to different buyers. Pipeline infrastructure also requires significant investment that often makes long-term contracts necessary. Former Russian President Vladimir Putin and Qatar's emir, Sheik Hamad bin Khalifa Al Thani, announced in April, 2007, they would explore the creation of a cartel to represent the interests of producer countries in controlling the global market. Russia and Qatar are two of the world's largest producers of natural gas, and Qatar has the world's single largest gasfield. A leaked confidential study by NATO economic experts in 2006 had warned that Russia may be seeking to build a gas cartel including Algeria, Qatar, Libya, the countries of Central Asia, and perhaps Iran, and cautioned that an OPEC-like near monopoly would strengthen Moscow's leverage over Europe. After the Russian-Qatari announcement, US Energy Secretary Samuel Bodman spoke out against the formation of a cartel. Appearing at an energy conference in Houston in February, 2007, he said initiatives that seek to control the flow of energy supplies and circumvent the role of the market to set prices "are contrary to the long-term interests of both producers and consumers."