The American shale boom is slowing as innovation plateaus—and just when shale’s importance in global markets has reached new highs following an attack on the heart of Saudi Arabia’s oil infrastructure.

U.S. oil production increased by less than 1% during the first six months of the year, according to the Energy Department, down from nearly 7% growth over the same period last year.

Unlike several years ago, when shale production fell due to a global price collapse, the slowdown this year is driven partly by core operational issues, including wells producing less than expected after being drilled too close to one another, and sweet spots running out sooner than anticipated.

The challenges raise the prospect that the technological and engineering advances that have allowed shale companies to unlock record amounts of oil and gas from rock formations have begun to level off.

U.S. oil production is on track to hit an average of 12.2 million barrels a day this year, up from last year’s average of 11 million barrels a day, the Energy Department said earlier this month. But because output grew so quickly last year—from an average of 10 million barrels a day in January to 12 million barrels a day in December—that implies limited growth throughout 2019. Activity has slowed recently and employment has fallen in some of the hottest U.S. oil regions, according to a September report by the Dallas Federal Reserve.