Russia does not actually need to extract oil and gas from Crimea, although it would be a nice bonus for them if they could. The main imperative is to prevent Ukraine from having it.
Once Russia captured Crimea, the "land corridor" to Crimea became a necessary next domino. Crimea's only source of fresh water is overland via Ukraine. Obviously the first thing Ukraine did when Russia annexed Crimea was turn off the taps. (Would you keep sending freshwater into enemy territory?) In order to maintain power in Crimea in the long run, it is necessary for Russia to invade enough of Ukraine to take over the freshwater canals that supply Crimea.