US Cash Crude-Grades firm slightly on supply worries

Aug 30 (Reuters) - U.S. crude grades firmed slightly on Friday and markets watched for supply impact as Libya's oil exports were halted.

Libya's National Oil Corporation said recent oilfield closures have caused the loss of approximately 63% of the country's total oil production, as a conflict between rival eastern and western factions continues.

Refiners that usually import light sweet crude could look to replace Libya's Sharara with U.S. WTI Midland, among other grades, trade sources said.

WTI Midland WTC-WTM and WTI at East Houston WTC-MEH, also known as MEH, firmed 5 cents a barrel.

Meanwhile, in the U.S., the of active oil rigs was unchanged at 483 this week.

* Light Louisiana Sweet WTC-LLS for October delivery gained 10 cents at a midpoint of a $1.90 premium and was seen bid and offered between a $1.60 and $2.20 a barrel premium to U.S. crude futures CLc1

* Mars Sour WTC-MRS fell $1 at a midpoint of a $1.5 discount and was seen bid and offered between a $1.75 and $1.25 a barrel discount to U.S. crude futures CLc1

* WTI Midland WTC-WTM gained 5 cents at a midpoint of a 75-cent premium and was seen bid and offered between a 50-cent and $1 a barrel premium to U.S. crude futures CLc1

* West Texas Sour WTC-WTS was unchanged at a midpoint of a 10-cent premium and was seen bid and offered between a discount of 5 cents and 25-cent a barrel premium to U.S. crude futures CLc1

* WTI at East Houston WTC-MEH, also known as MEH, traded between a $1.20 and $1.60 a barrel premium to U.S. crude futures CLc1

* ICE Brent October futures LCOc1 fell $1.14 to settle at $78.8 a barrel on Friday .

* WTI October crude CLc1 futures fell $2.36 to settle at $73.55 a barrel on Friday .

* The Brent/WTI spread WTCLc1-LCOc1 widened 17 cents to minus $4.20, after hitting a high of minus $4.18 and a low of minus $4.20.

(Reporting by Arathy Somasekhar in Houston; Editing by Rod Nickel)

((Arathy.s@tr.com))