NEW YORK, Aug 26 (Reuters) - WTI Midland crude prices at Plains All American Pipeline LP's Midland terminal spiked this month compared with barrels traded at Enterprise Products Partners LP's Midland terminal as Plains' Cactus II pipeline began service, oil traders familiar with the matter said.
The 670,000 barrel-per-day (bpd) Cactus II pipeline began commercial deliveries earlier this month, sending WTI Midland prices at Plains terminal as much as 50 cents per barrel above those at Enterprise's terminal, they said.
Price reporting agency Argus Media said on Monday it would change the way it assesses WTI Midland to exclusively reflect prices at Enterprise's terminal "to more closely align its assessment" with customer expectations and most spot trades.
The proposed change is open for comments until the end of the month, Argus said in a notice.
The move could draw criticism from Plains All American and shippers on the Cactus II pipeline, the market sources said.
Commodities merchant Trafigura AG is the biggest shipper on Plains' Cactus II pipeline.
Plains did not immediately respond to a request for comment.
About 90% of Argus' WTI Midland prices typically reflect trades at Enterprise's terminal, sources said.
WTI Midland at Plains was last seen about 20-25 cents per barrel above Enterprise, dealers said.
Midland prices are seen as a proxy for prices in the nearby Permian basin, the biggest shale patch in the United States. (Reporting by Devika Krishna Kumar in New York Editing by Matthew Lewis)