Oil prices will tumble to $40 a barrel if OPEC doesn’t extend its pact later this year to cut output, according to one of the most prominent producers in the shale patch. U.S. shale drillers are keeping an eye on the second half of the year to see if OPEC and non-OPEC members extend their agreement, which lasts through June, to reduce production by 1.8 million barrels a day, Scott Sheffield, chairman of Irving-based Pioneer Natural Resources Co., said Tuesday in an interview at the CERAWeek industry conference held by IHS Markit in Houston.