U.S. Gulf crude grade premiums strengthened as the difference between the U.S. benchmark West Texas Intermediate and Brent surged to a record.

The gap between WTI and its European counterpart for June delivery increased $1.60 to $21.09 a barrel as of 12:19 p.m. in New York. When Brent gains versus WTI, it strengthens the value of low-sulfur U.S. grades that compete with West African oil priced against the European benchmark.

Light Louisiana Sweet’s premium gained 20 cents to $17.80 a barrel over WTI as of 11:52 a.m. in New York, according to data compiled by Bloomberg. Heavy Louisiana Sweet added 20 cents to $17.40 over the benchmark.

Among sour, or high-sulfur, grades, the premium for Mars Blend widened $1.10 to $14.50 a barrel. Poseidon gained 85 cents to $13.75 over WTI.

Thunder Horse’s premium widened 10 cents to $16.90 a barrel. Southern Green Canyon increased $1.75 to $12.50 a barrel over WTI. West Texas Sour’s discount widened 10 cents to $1.90 a barrel.

The premium for Syncrude was unchanged at to $9.85 a barrel. Syncrude is a light, low-sulfur synthetic oil derived from the tar sands in Alberta.

The discount for Western Canada Select widened 10 cents to $18.10 a barrel.