A fleet of tankers full of Saudi oil is slowly making its way to the U.S. Gulf Coast, threatening to worsen an already historic oversupply of crude. The Saudi crude, about seven times as much as the Gulf Coast took from the country in a typical month last year, will fill rapidly dwindling places to store oil, depress already low prices in key shale regions and increase pressure on drillers from Texas to North Dakota to shut off their wells. …