HOUSTON, July 25 (Reuters) - Differentials for U.S. sour

crude Mars have been in a steep decline this month and hit their

widest discount to benchmark crude futures since October this

week, according to data from brokers, as refiners faced slimmer

profit margins due to weak demand.

The offshore crude grade's discount to West Texas

Intermediate crude (WTI) at the Cushing, Oklahoma, hub

widened to a $2.10 per barrel on Friday, their weakest since

Oct. 3, according to data from LSEG.

Differentials widened further by 10 cents to $2.20 on

Monday; the day of the WTI front-month expiry followed by half a

day of the roll trading period, which is typically a more

volatile session. Traders use the roll period between contracts

to square up positions.

Latest prices on Wednesday showed a slight narrowing to

$1.90.

Mars has been in decline as refiners have been making less

of a profit on narrower gasoline cracks analysts said, the

difference between gasoline futures and WTI crude ,

amid a slow start to the U.S. summer driving season, driving

down refiners' demand for sour grades.

The gasoline crack premium has peaked at around $25 per

barrel so far this summer driving season, compared with a high

last summer of just over $41 per barrel.

"Weak margins should weigh on secondary unit utilization and

drive disproportionately lower demand for sours compared with

sweets," said Energy Aspects oil markets analyst, Livia

Gallarati.

U.S. refinery crude runs fell by 521,000 barrels per day

(bpd) last week, according to the Energy information

Administration (EIA), to 16.41 million bpd, marking a 10-week

low, the lowest rate so far this driving season.

However, product supplied of gasoline, the EIA's measure

of demand, rose by 673,000 bpd, the biggest weekly increase

since November, to 9.46 million bpd, the most for this time of

the year since 2021's record high of over 10 million bpd, the

data showed.

Mars values could still dwindle further on weakened Asian

demand for U.S. grades, according to Gallarati, after Washington

gave India's Reliance Industries approval to resume

importing Venezuelan oil, a source familiar with the matter said

on Wednesday.