A brief supply disruption related to a zinc contamination issue in the Mars crude stream appears to have been resolved. Citing two sources familiar with the matter, Reuters reported on July 16 that ExxonMobil had purchased Mars crude for August delivery, having briefly halted purchases until the contamination had been addressed.

This came after Chevron was reported as saying in a separate Reuters report last week that the contamination had resulted from the start-up of an offshore well in the US Gulf of Mexico.

The Mars crude stream is a mix of oil produced at various platforms off the coast of Louisiana. It is a medium sour crude grade that is favoured by refiners on the US Gulf Coast owing to both the properties of the oil and the proximity of production. Zinc does not typically occur in crude and Reuters pointed to concerns among industry sources over potential for oil containing zinc to damage refining units and catalysts used in the processing of oil.

Chevron said last week that it was working to resolve the issue and did not expect any impact to its current production guidance. However, the disruption resulted in Mars trading at a $0.30 discount to US crude at the Cushing hub, compared to a $1 premium at the end of June, Reuters reported this week.

ExxonMobil appears to have been the most affected by the issue, based on what has been publicly reported. On July 11, the US Department of Energy (DoE) announced that it had authorised an exchange from the country’s Strategic Petroleum Reserve (SPR) with ExxonMobil. The DoE agreed to release up to 1mn barrels of crude from the SPR to ExxonMobil in a bid to support the restoration of operations at the supermajor’s refinery in Baton Rouge, Louisiana.

The DoE said ExxonMobil would return the borrowed crude along with additional barrels at no cost to the taxpayer, without providing a timeline. It also noted that the release would not affect or delay its efforts to refill the SPR.