I don't think the resources of Ukraine are the primary motivation for Putin. Russia is certainly not in any dire need of those with its massive land to population ratio and its own diverse resources.

The national borders drawn for Ukraine captured a great many Russian-speaking people in its eastern area and recent talk of separation from Ukraine in that region has provided Putin with an incentive to tip the balance for them. Russia has a long history of expanding its territory to provide a buffer zone against its enemies, and the threat of NATO expansion definitely spells "enemy" to Putin.