There's a ton of oil and gas in the ground in the northeast and south of Ukraine. Potentially they are sitting on a significant fraction of what Russia has, and

There's a ton of oil and gas in the ground in the northeast and south of Ukraine. Potentially they are sitting on a significant fraction of what Russia has, and since it's closer to the EU, it will be cheaper to export in volume. Also I'm fairly sure that Ukraine has the right to deny Russia the right to transport oil through the existing pipelines in their territory, which IIRC is how the majority of Russian exports get to Europe.