It may add Norwegian light sweet Troll crude to the basket of oil flows it uses to set the price of dated Brent, it said on Friday.

Adding another crude should help liquidity as supply from traditionally larger, older North Sea fields that produce Brent, Forties, Oseberg and Ekofisk (BFOE), declines.

Troll is a light sweet crude and typically 10 to 15 cargoes of 600,000 barrels each are produced each month, compared with an average of about six or seven cargoes of the same size for Brent or Oseberg.

Oil production in the North Sea was in decline for the best part of the last few years, raising questions about the validity of using dated Brent as a benchmark, which is backed by around 2 million barrels a day of supply, or roughly 2 percent of global output.

But the discovery of large fields such as Buzzard, which feeds into the Forties stream, or the newly commissioned Edvard Grieg, which feeds into Grane supply, along with a rise in supply from late-life specialist producers such as Enquest, has seen a resurgence in North Sea output.

"Ultimately Brent is a totemic benchmark that is an absolute beacon for the oil markets and the industry expects it to be here for not just two or 10 years, but 20 or 30 years," S&P Global Platts global head of energy pricing David Ernsberger said.

Platts said it would invite comment until the end of January on adding Troll to the basket.

Supply of Troll in January is expected to average around 232,000 bpd, based on monthly loading schedules.

The last addition Platts made was Ekofisk, a light, sweet crude produced jointly from British- and Norwegian-owned fields, in 2007.

Platts added Oseberg and Forties, a slightly heavier, more sour grade, to the basket in 2002.

"Dated Brent is a benchmark that finds itself at a crossroads, but not in a crisis," Platts editorial director for European and African oil Joel Hanley said.

"We're still confident with the volumes we have for the time being and we're confident of the volume of deliverable crude oil into the benchmark. But further grades certainly won't hurt and will help the benchmark maintain its fob basis."

FOB, or free on board, terms involve shipping goods to the buyer's named destination, free of charge, in contrast to CIF or charge, insurance and freight, under which the seller undertakes to ship the goods to their destination and provide the documents for the buyer to take delivery from the carrier.

Most North Sea grades are shipped on a FOB basis, but competing grades such as Russian Urals are shipped on a CIF basis.

"We feel there is a lot of support for keeping FOB stronger for longer," Ernsberger said.

Platts also said it would assess Norwegian grades Asgard, a naphtha-rich ultralight condensate, and Alvheim, which is rich in distillates, as is Forties.

(Reporting by Amanda Cooper; editing by Susan Thomas)

By Amanda Cooper