US Cash Crude-Grades mixed on first day of volatile roll trade period

Aug 21 (Reuters) - U.S. crude grades were mixed on Thursday, on the first day of a volatile roll trading period, dealers said.

Midland and WTI at East Houston (MEH) firmed 5 cents, while Mars eased 10 cents.

Traders use the three-day roll period to adjust their crude slates, square up positions and net out exposures following the expiration of the U.S. crude futures contract.

Prices to roll U.S. crude oil futures positions from September to October traded at a midpoint of 40 cents a barrel.

Meanwhile, West Texas Intermediate crude futures' (WTI) discount to Brent WTCLc1-LCOc1 widened to as much as minus $4.24 a barrel, the most since June 13. A wider spread makes U.S. grades more attractive to foreign buyers.

Two tankers chartered by Chevron CVX.N carrying Venezuelan crude reached U.S. waters on Thursday, marking the first U.S. imports of the South American country's oil following a new license granted by Washington, vessel tracking data showed.

Increased supplies of sour crude from Venezuela could hurt demand for domestic sour grades such as Mars.

In refining news, BP's BP.L 440,000-bpd refinery in Whiting, Indiana, continued to burn materials in the flares after flooding disrupted its operations earlier this week, the company said.

* Light Louisiana Sweet WTC-LLS for September delivery firmed 15 cents to a midpoint of a $1.55 premium and was seen bid and offered between a $1.40 and $1.70 a barrel premium to U.S. crude futures CLc1

* Mars Sour WTC-MRS eased 20 cents to a midpoint of a 5-cent premium and was seen bid and offered between a discount of 20 cents and 30-cent a barrel premium to U.S. crude futures CLc1

* WTI Midland WTC-WTM strengthened 5 cents to a midpoint of a 85-cent premium and was seen bid and offered between a 70-cent and $1.00 a barrel premium to U.S. crude futures CLc1

* West Texas Sour WTC-WTS gained 15 cents to a midpoint of a 15-cent premium and was seen bid and offered between parity and 30-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 $1.50 a barrel premium to U.S. crude futures CLc1

* ICE Brent October futures LCOc1 rose 83 cents to settle at $67.67 a barrel on Thursday.

* WTI October crude CLc1 futures rose 81 cents to settle at $63.52 a barrel on Thursday.

* The Brent/WTI spread WTCLc1-LCOc1 narrowed 2 cents to last trade at minus $4.11, after hitting a high of minus $4.07 and a low of minus $4.24.