In addition to the Black Sea deposits in Crimean waters, Ukraine has massive gas reserves in the east of the country, in particular the huge Yuzivske field. With these deposits, Ukraine holds 5.4 trillion cubic metres of gas, the third largest reserves in Europe.
When Russia occupied Crimea in 2014 Ukraine lost, and Russia gained, the huge gas deposits under the Scythian section of the Black Sea Shelf.
The Black Sea Shelf deposits had been under exploration for several years and Ukraine finally awarded the licences for their exploration in August 2012. Ukraine did not possess the technology or expertise to exploit the deposits itself (they are very deep), and so the contracts were inevitably going to go abroad.
Given the very close association between then Ukrainian President Viktor Yanukovych and President Putin, the Kremlin expected the contracts would be issued to the Russian energy giant, Gazprom. But the Kremlin should have foreseen a problem. Until the Russian occupation of Crimea and the Donbas in eastern Ukraine, Ukraine imported almost all its gas from Gazprom. Gazprom had, in turn, imposed numerous price hikes on Ukraine and Ukraine’s gas distributor Naftogaz, had struggled to pay the bills. By 2014, Naftogaz owed Gazprom US$4.5 billion. Because of the repeated price hikes and resulting tensions between Gazprom and Naftogaz, Ukraine had been trying to establish other, non-Russian supplies. It turned to Europe for them, and when the contract licences were issued for exploitation of Ukraine’s on gas they went not to Gazprom and Russia, but to a group led by American ExxonMobil but including Dutch-British Royal Dutch Shell and Romanian OMV Petrom, working with the Ukrainian state company Nadra.