Marathon Petroleum has agreed to acquire rival Andeavor for $23.3 billion in cash and stock.

Why it matters: The deal would create the largest U.S. oil refiner by capacity, overtaking Valero Energy.

Marathon has six refineries with a combined 1.8 million barrels per day of capacity, while Andeavor — formerly known as Tesoro — has 10 refineries in western states with a combined 1.2 million barrels per day of capacity.

The context: Per WSJ, "Part of the rationale of the deal centers on the companies’ complementary footprints; with Marathon in the East and Andeavor in the West, regulatory approval could be easier to win."

Their sources say the merger would produce $1 billion worth of "synergies." Marathon CEO Gary Heminger would also lead the merged company, with his Andeavor counterpart Gregory Goff in a senior role, WSJ reports.