Venezuela’s supply of Russian naphtha is at risk of crumbling, threatening the South American country’s crude exports, after U.S. President Donald Trump ordered a blockade of sanctioned tankers heading to or departing from Venezuela. Russian exports of naphtha to Venezuela have soared in recent months and Venezuela relies on naphtha and other fuels to serve as diluents for its extra heavy crude. Shipments from other sources stopped and Venezuela turned to Russia for naphtha supply in the middle of this year. But Russian…