Russia’s largest oil field, so far past its prime that it now pumps almost 20 times more water than crude, could be on the verge of gushing profits again for Rosneft PJSC . Samotlor, the 25 billion-barrel giant that bankrolled the Soviet Union for decades, would be the biggest beneficiary of proposals to encourage investment in some of Russia’s oldest and largest reservoirs, where output is plunging. One idea being debated — cutting the extraction tax in half for fields producing a lot of water with the oil — could add as much as 90 billion rubles ($1.57 billion) a year to Rosneft’s earnings, said Alexander Kornilov, an analyst at Aton LLC in Moscow. “It is a super giant field even today after almost 50 years of production, the elephant of elephant fields,” said Ildar Davletshin, an analyst at Renaissance Capital. “There is still a lot of oil left,” […]