LONDON - Saudi state oil company Aramco has started trading a U.S. crude oil grade that underpins the global Brent benchmark in a process run by oil-index publisher S&P Global Commodity Insights, the publisher said on Monday.

Aramco, the world's largest oil company, has been expanding its trading activity. The company could not immediately be reached for comment on Monday after Saudi business hours.

On Monday, Aramco sold a cargo of U.S. WTI Midland crude to Total in the Platts Market on Close process, known as the Platts window, trade sources said.

Aramco had last Thursday sold another cargo of the U.S. crude. These were Aramco's first WTI Midland trades in the MoC, Joel Hanley, S&P global director of crude and fuel oil markets, said via email.

In December, Aramco bought a cargo of North Sea crude DUC, its first deal since joining the process, he said. U.S. WTI Midland is one of six crude oil grades assessed by Platts that can set the value of dated Brent, part of the wider Brent complex used to price over three-quarters of the world's traded oil.

More players have become involved in trading crude that can set the Brent price in the Platts window since Platts added WTI to the benchmark last year.

(Reporting by Alex Lawler in London Additional reporting by Maha El Dahan Editing by Matthew Lewis)