Brent crude's premium to US crude ended at $10.75 a barrel on Thursday based on settlements, slipping to $10.59 during the session, the lowest since June 21.
It ended on Wednesday at $11.15 a barrel.
The premium was as high as $14.66 last week during the April 2 session.
Usually the wider the arbitrage, currently Brent's premium to US crude, the more supportive for US cash crude differentials. This holds especially for sweet grades that are priced in line with other global waterborne crudes such as Brent.
In the cash crude market, Light Louisiana sweet, for May delivery, traded on Thursday from $13.70 to $14.35 above the US May crude futures, also known as West Texas Intermediate (WTI).
WTI is the US light, sweet crude futures contract's benchmark grade deliverable at Cushing, Oklahoma.
Those Thursday differentials were weaker after LLS traded on Wednesday at $14.75 and $15.00 above the benchmark.
A Gulf of Mexico-produced grade, Mars sour crude, traded at $8.30 and $8.50 over the benchmark WTI, only 20 cents weaker after Wednesday's trade at $8.70 over.
Southern Green Canyon traded at $7.75 over the benchmark futures, but for a small, 1,000 barrels per day (bpd). SCG traded on Wednesday at $7.10 above, while it traded on Tuesday at $8.00 above.
West Texas Intermediate crude at Midland traded at 20 cents under the benchmark, steady to Wednesday's trade.