The broad selloff in oil accelerated Friday, as investors grew increasingly concerned about a surge in supplies and fears of a slowdown in global economic growth.

Light sweet crude for January delivery fell 7.7% to $50.42 a barrel on the New York Mercantile Exchange. The U.S. benchmark notched its biggest one-day percentage decline since July 2015—and closed at the lowest level since October 2017. Brent, the global benchmark, was recently down 5.8% at $58.98 a barrel, breaking below $60 for the first time in over a year.

...