Russia doesn't need the Crimean oil reserves, it's more than they wanted Ukraine to not have it. Even then, energy security wasn't as much a motivator as was securing access to Sevastopol, a critical warm water port and the only place capable of housing the black sea fleet. Although control of that port, in turn, is largely to do with projecting energy control over a wider region.

Russia was leasing Sevastopol from Ukraine (til 2042). It had become increasingly important to Russia's other objectives being a staging location for supporting the incursion into Georgia, and also Russia's involvement in Syria. Both of which are key to Russia's broader goal of region control and energy security (not Ukraine per se).

It may be that Russia was far more sensitive to EU membership than NATO because EU membership travelled much faster and was already outflanking them (see map at bottom)

In the early 2000's, increasing ineffectiveness of the old Soviet style leadership in Georgia was bankrupting the country and making corruption rife. This was increasingly apparent to international businesses there and a student population that enjoyed (somewhat miraculously) the relatively free press in the form of TV stations critical of the regime and its corruption.

Subsequently, foreign NGO presence helped organise and contribute to the peaceful 2003 Rose Revolution which saw the older soviet influence brushed away in favour of new democratic parties. (Put your favourite conspiracy / neocon / deepstate analysis hat on, a major financier of the NGOs was George Soros)

The new leadership sought to put Georgia on better economic footing and in 2006 together with the EU issued a statement on the 5 year Georgia-European Union Action Plan within the European Neighbourhood Policy which was a major snub to Russia.

Russia's desire to maintain a foothold within Georgia subsequently provoked the 2008 Russia Georgian War over Georgia's northern 'South Ossetia' region. Not only because Georgia is the gateway to projecting power into the Middle East, but more immediately because in 2006 Georgia opened the Baku-Ceyhan oil pipeline which cut Iran and Russia out of the picture and connected Azerbaijan oil fields up directly with EU friendly Turkey.

Russia failed to make anyway headway with their support of South Ossetia. Then in 2013, Georgia and the EU took the next step in closer alignment, an Association Agreement. With Russia's efforts to expand influence into the Caucasus region curtailed and weakening in power to project strength over energy producing regions, Putin saw the need to permanently secure Sevastopol as becoming critical.

The Ukrainian parliament had begun legal alignment with the EU the same year.

Hence in 2014, Russia took Crimea.

(If you look at the map of EU plus Georgia, you can see how close EU alignment could be seen to have 'provoked' Russia to act. Though very much only in the sense that they are anti democratic and imperialist)

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.