Russia may or may not care for it's own wealth in the shale prospects, but it can't allow for Ukraine to have them. They've shown interest in being a western aligned democracy, and having such resource exports would make Ukraine much more wealthy. A wealthy, friendly Ukraine as an energy provider is much more attractive to Europe than Russia. They would have a much easier time being admitted to the EU and NATO, and it would be a severe blow to Russian pride and potentially economy, and the strategic position provided by being an energy exporter.