When Russia seized Crimea in March, it acquired not just the Crimean landmass but also a maritime zone more than three times its size with the rights to underwater resources potentially worth trillions of dollars.

Russia portrayed the takeover as reclamation of its rightful territory, drawing no attention to the oil and gas rush that had recently been heating up in the Black Sea. But the move also extended Russia’s maritime boundaries, quietly giving Russia dominion over vast oil and gas reserves while dealing a crippling blow to Ukraine’s hopes for energy independence.

Russia did so under an international accord that gives nations sovereignty over areas up to 230 miles from their shorelines. It had tried, unsuccessfully, to gain access to energy resources in the same territory in a pact with Ukraine less than two years earlier.

“It’s a big deal,” said Carol R. Saivetz, a Eurasian expert in the Security Studies Program of the Massachusetts Institute of Technology. “It deprives Ukraine of the possibility of developing these resources and gives them to Russia. It makes Ukraine more vulnerable to Russian pressure.”