Beijing reviewing its energy needs with slowing economy, says report.
New field will enable eventual exports to China via 'western route' through Altai Mountains.
Altai route for gas to China could take priority, reported Reuters.
Agreement means Power of Siberia pipe could be delayed, it is claimed.
Construction launched on 3,968 kilometre 'Power of Siberia' gas pipeline.
Talks also underway on gas supply route to China via Altai Mountains.
Dignitaries gather on Chinese side of the pipeline to witness the formal welding of the first joint in $55billion line.
Gazprom says operations are 'in full swing' with work due to start soon on iconic pipeline to provide gas on eastern route.
After months of speculation, parliament and President rubber-stamp deal that will see gas sent to China.