Unlikely. If oil was a question Russia would've probably invaded Azerbaijan instead. The things I've heard is that because black sea fleet is stationed there and it couldn't have been a thing in a NATO/EU country. And Putin loves his boats, like, the whole Assad regime is basically a Putin's little gas station.

Another is that Crimea is populated by majority of ethnic Russians who want to be in Russia. How it came to be is a bit different topic, and it was a referendum at gunpoint with falsifications, just as usual for Russia, but there is no doubt that even without those, it would've still passed.