Benchmark oil for August delivery was down 38 cents at $79.38 a barrel at late afternoon Singapore time in electronic trading on the New York Mercantile Exchange. The contract rose $1.56 to settle at $79.76 in New York on Friday.
In London, Brent crude for August delivery was down 60 cents at $90.38 per barrel on the ICE Futures exchange.
Tropical Storm Debby, with top sustained winds of about 60 mph (95 kph), is lashing Florida and Alabama with heavy rains and is expected to come ashore later this week over the Florida Panhandle. As of Sunday morning, 23 percent of oil and gas production in the Gulf region had been suspended.