Oil markets slumped nearly 6% on Wednesday after weekly data from the U.S. government showed drillers in America’s shale fields adding to production for the first time since March as crude prices averaged $40 a barrel.

U.S. crude output was estimated at 11 million barrels per day for the week ended June 19, versus the estimate of 10.5 million bpd the previous week, data from the Energy Information Administration showed.

New York-traded West Texas Intermediate, the benchmark for U.S. crude futures, settled down $2.36, or 5.8%, at $38.01 per barrel.