The purchases could lift in September the volume of U.S. crude loading for Asia, up from a 4-year low in the previous month when strong U.S. crude prices curbed exports, Refinitiv Eikon data showed.
Other grades such as West Texas Intermediate (WTI) and West Texas Light (WTL) that regularly head east are also being booked for Asia, the sources said.
Ongoing production restraint by Middle East producers and a delayed return of Iranian barrels to international markets have also contributed to the rise in Asian buying interest for U.S. high-sulphur crude, they said.
One South Korean refiner has bought 3 million barrels of the sour grade while Unipec, the trading arm of Asia's largest refiner Sinopec, has taken at least 2 million barrels, the sources said.
Unipec has chartered the supertanker Cosnew Lake to load sour crude from Louisiana on Sept. 23, according to two sources and Refinitiv Eikon data. The company is also looking to fix a Suezmax vessel for departure in early September, shipbrokers said. Earlier this week, Taiwanese refiner Formosa Petrochemical Corp had issued a tender to buy Mars crude but did not award it due to high offers.
There are still 2 million to 4 million barrels of Mars crude that have yet to trade and these could potentially come to Asia, said a Singapore-based trader.
"U.S. crude has become quite cheap relative to the rest of the world and U.S. exports have picked up because of it," said Scott Shelton, an energy specialist at United ICAP.
Mars Sour crude's discount to U.S. crude futures widened in late July to the most since April 2020, but has since narrowed due to the rise in export demand, traders said. Mars traded at the smallest discount to benchmark futures in 6 weeks on Tuesday.
"We have been seeing sours bid east very aggressively," one U.S.-based source at a top exporter said.
"I think there are still good refinery runs, despite the China COVID noise and SPR stuff," he said, referring to reports about China releasing crude stocks from strategic petroleum reserves (SPR) and concerns about the recent lockdowns in China to curb the coronavirus Delta variant cutting its oil demand.
Asia's purchase of Mars crude has partly depressed prices for competing supplies from the Middle East such as Iraq's Basra crude which are trading at discounts, traders said.
(Reporting by Jessica Resnick-Ault and Devika Krishna Kumar in New York, Florence Tan in Singapore and Nidhi Verma in New Delhi; Additional reporting by Heekyong Yang in Seoul; Editing by Richard Pullin and Michael Perry)
By Devika Krishna Kumar and Florence Tan