LONDON, Sept 23 (Reuters) -North Sea Forties crude differentials weakened on Monday following keen selling interest in the Platts window, while U.S. WTI Midland traded slightly higher than previous bids.

PLATTS WINDOW

* Indications are on a free-on-board (FOB) basis unless marked as cost, insurance and freight (CIF) or delivered at place (DAP).

* WTI Midland: Petroineos bought an Oct. 18-22 cargo from Trafigura at dated Brent plus $2.75 CIF Rotterdam.

* The deal equated to around dated plus $1.88 on a FOB basis, Reuters calculations showed.

* On Friday, Petroineos bid for WTI Midland Oct. 8-12 at dated Brent plus $2.70 CIF Rotterdam.

* Forties: ExxonMobil offered an Oct. 22-24 cargo, and an Oct. 19-21 cargo, on a one-cancels-the-other basis, both at dated plus 90 cents.

* On Friday, Petroineos had bid for a Forties cargo with Oct. 16-18 dates at dated plus $1.60.

* Ekofisk: ConocoPhillips offered an Oct. 19-21 cargo at dated plus $2.55.

* Ekofisk was last in the window on Thursday, when Petroineos bid for an Oct. 11-13 cargo at dated plus $2.65.

* No deals, bids, or offers for Brent on Monday.

* On Friday, Petroineos bought a Brent cargo from BP with Oct. 20-22 dates at dated plus $1.27.

* After Monday's offers, Forties became the cheapest grade in the dated Brent basket, and therefore set the benchmark price.

Reporting by Robert Harvey; Editing by Susan Fenton

 NORTH SEA CRUDE OIL DIFFERENTIALS AND OUTRIGHT PRICES: 0#BFO-DIF 0#C-E Dated BFO BFO- Brent BFO-BRT BFO-E Forties BFO-FOT FOT-E Oseberg BFO-OSE OSE-E Ekofisk BFO-EKO EKO-E Monthly North Sea crude loading programmes O/LOAD OPEC OPEC NSEA North Sea CRU crude oil PROD oil products DRV derivatives PRO/E European products OILOIL NYMOIL, IPEOIL, OILSPD, OILARB CRDWLDPRODEUR, PRODUS, APROD ENERGY energy speed guide CRUDE/1 crude speed guide