Possibly relevant: there are shale oil reserves in Ukraine. The possibility of a competing source might have been a factor in the decision to invade, although given the lack of consideration of other factors probably not.

From looking at US shale oil production, compared to ‘traditional’ wells it takes less time from initial drilling to production, but there’s less supply long-term from each well — but multiple wellheads can spread outwards from a single ‘pad’, making infrastructure less onerous. These are primarily tight shale oil, which probably has a bearing. I’m seriously out of my depth here (see what I did there?) any petro geologists care to chime in?