Here's a map of the natural gas basins in Ukraine. Note the big natural gas basin under the Sea of Azov (bottom right).
As I've said for years, Russia is an energy company with all the trappings of nation-state.
Everything Russia does militarily is aimed at expanding it's energy interests. From acquiring new oil and gas basins to exploit to forcing countries to allow it to build new pipelines or expand old ones. It's also used everything from extortion to cyberattack to pull it off.
When I wrote about this five six years ago, Gazprom (Russia's natural gas company and owner of 15% of the world's natural gas reserves) was on the way to becoming the world's first trillion dollar company. Yes, $1,000,000,000,000 a price nearly twice the price of Apple, the world's most valuable company in the world and three times that of Exxon. A massive corporate beomoth with Putin as its defacto CEO (aka, a big facile piggy bank for Putin's whims).
Today, Gazprom is worth about $100 billion (give or take a couple of billion) due to the same thing that accelerated the collapse of the Soviet Union in the 80's: cheap energy.
Russia, like the Soviet Union before it, bankrolls its programatic failures and feeds its corrupt bureaucrats (to keep them in line) through energy profits. In the 80's, there was a glut of oil on the market that drove the price per barrel down to the teens. That price destroyed the Soviet Union's ability to use it's energy sales (it's been the world's largest energy producer, off and on, for decades) to cross subsidize its bureaucratic failures and the pay for the perks necessary to keep its bureaucrats in line. That's why when Gorbachev's reforms openned a tiny door for privatization (instantly making the bureaucrats that ran these converted businesses wealthy), all of the bureaucratic nomenklatura fled for the exit (and personal wealth) by joining the privatizing game too.
Russia's and Gazprom's problem is similar. The shale gas (and now oil) revolution around the world, and particularly in the US, has depressed the value of Gazprom's shares. The price of natural gas has crashed and Europe, Gazprom's largest customer, has forced significant price concessions and it's hopes for an LNG cash cow have diminished.
Despite this massive set-back, Putin continues to acquire assets in the hope of price recovery in the future. The upside is that as a kleptocracy and not a Communist state, the amount of money he needs to spend to keep people in line is much less than it used to be.
PS: John McCain recently paraphrased me by calling Russia "a gas station masquerading as a country." Not sure he completely understands what I mean by the concept though.