LONDON, Sept 17 (Reuters) -North Sea Ekofisk crude differentials weakened on Tuesday after a trade, while U.S. WTI Midland and Forties were supported by higher 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).

* Ekofisk: Gunvor bought from BP Oct. 11-13 at dated Brent plus $2.20, down from Monday's indications.

* Ekofisk: Allowing for the Platts quality premiums for October, the deal works out as equal to dated Brent plus $1.23 for dated Brent purposes, according to Reuters calculations.

* Forties: Mitsui bid for Oct. 4-13 at dated Brent plus $1.55, up from a bid on Monday.

* WTI Midland: Petroineos bid Oct. 7-11 at dated Brent plus $2.55 CIF Rotterdam, or around dated plus $1.68 on a FOB basis according to Reuters calculations.

* Ekofisk becomes the cheapest grade underpinning the benchmark.

Reporting by Alex Lawler; Editing by Shreya Biswas

 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