Marathon’s Andeavor Deal Gives Stealth Access to Cheap U.S. Oil

Inside the largest refiner merger of all time is a hidden pipeline grab.

Andeavor purchased a 25 percent stake last week in Phillips 66 Partners LP’s Gray Oak pipeline, which will haul Permian shale oil to the U.S. Gulf Coast starting in the end of next year. The integrated refining company also owns oil gathering assets in the world’s most attractive shale basin. Now Andeavor has revealed where it will be shipping the crude: To Marathon’s nearly 600,000 barrel-a-day operation in Galveston Bay, the nation’s second-largest refining complex.

“They’re long in gathering, and we’re short in Galveston Bay,” said ...