Oil traders are grappling with a conundrum.

U.S. crude oil inventories have fallen for eight consecutive weeks to the lowest level since the beginning of last year. The pace at which producers have been adding drilling rigs to boost output has slowed to a crawl. And crude demand in the U.S. from motorists driving off on their summer holidays has surpassed expectations.

All of these reasons would historically be supportive for prices. But instead the benchmark West Texas Intermediate (WTI) U.S. oil contract has slumped to its biggest discount to its international rival Brent since 2015, meaning many oil producers in the U.S. are earning about $4—or 8%—less per barrel than those operating in the North Sea or west Africa.

Behind this, analysts and traders say, is seemingly unstoppable U.S. oil production. Those betting the industry will continue ramping up output are—for now—firmly winning the biggest debate in the oil sector regarding just how much production, led by the shale revolution, can grow despite prices languishing below $50 a barrel.