The deal was struck after at least a decade of talks—frequently hobbled by price disputes—between the countries' largest state-controlled energy companies, China National Petroleum Corp. and OAO Gazprom. Under the agreement, Russia will supply 1.4 trillion cubic feet of natural gas a year to China, equal to more than one-fifth of Chinese gas consumption last year. That is roughly equal to the annual gas consumption of the state of New York, according to U.S. government data.