LONDON, Sept 24 (Reuters) -North Sea Ekofisk crude differentials weakened after a lower offer in the Platts window on Tuesday while Forties and U.S. WTI Midland were steady.

PLATTS WINDOW

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

* No deals, a trade source said.

* Forties: Mitsui offered Oct. 16-20 at dated Brent plus $2.10 on a CIF Rotterdam basis, equal to roughly dated Brent plus $1.16 on an FOB basis according to Reuters calculations. This was above an offer on Monday at dated plus 90 cents so Forties is steady.

* Ekofisk: Conoco offered Oct. 17-22 at dated Brent plus $2.20, lower than an offer on Monday and equal to roughly dated Brent plus $1.23 after allowing for the Platts quality premium according to Reuters calculations.

* WTI Midland: No indications, last traded on Monday at dated Brent plus $2.75 CIF Rotterdam or about plus $1.88 on a FOB basis according to Reuters calculations.

* Forties remains the cheapest grade in the dated Brent basket, and therefore set the benchmark price.

Reporting by Reuters; Editing by Tomasz Janowski

 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