From BBC News (link):
Russia and China are engaged in a complicated dance over energy – complicated enough to baffle many observers.
At the head of a Russian delegation to Beijing, President Vladimir Putin has signed a number of energy deals which – had the signatories been different – might have changed the shape of the global oil and gas market.
Moscow promised – among other deals – to build two gas pipelines from Eastern and Western Siberia to China, and to join forces with Chinese state energy company CNPC in developing gas offshore of Sakhalin, an island east of the Russia’s mainland.
Energy-hungry China is obviously interested in Russia’s vast oil and gas reserves.