Slowing US shale activity represents the key threat to the bottom line of top two oil services firms Schlumberger and Halliburton.

North America, and US shale in particular, has been the key driver of earnings growth at top two oilfield services firms Schlumberger and Halliburton since revenues bottomed out in 2016. As US drilling slows amid capacity constraints, can other regions pick up the pace?

For global number one Schlumberger the share of earnings accounted for by the firm’s...