Nearly a third of Russia’s current seaborne oil export potential is now stuck in tankers as the U.S. sanctions upend crude flows and Russia’s top buyers, China and India, are still struggling to assess the implications of the sanctions, according to JPMorgan. “Russia’s oil exports are entering a new phase of disruption as sanctions targeting Rosneft and Lukoil are set to take effect, prompting its two largest customers — India and China — to sharply reduce their December purchases,” the Wall…