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(Adds background on Midwest gasoline prices, other outages)
HOUSTON, June 9 (Reuters) - Exxon Mobil Corp saidits 238,600 barrel per day (bpd) Joliet, Illinois, refinery wasreturning to normal operations on Sunday after completing atwo-month, plantwide overhaul.
Exxon did not disclose the planned length of the overhaulwhen it began in April, or its cost.
The return to normal operations at Joliet could ease theU.S. Midwestern market for gasoline, especially in the Chicagoarea, where retail prices have spiked to near record highs dueto refinery outages.
Prices at the pump are underpinned by an intermediate marketthat moves fuels between a refiner and a retailer. The tradesare expressed as a deficit or premium to the price of futurescontracts on the New York Mercantile Exchange.
These differentials for Chicago gasoline jumped last week byas much as 45 cents a gallon to trade at a premium to gasolinefutures of 80 cents a gallon, the highest level in at least ayear. Differentials normally move by several cents a week.
Traders and industry analysts have said maintenance atJoliet - which continued longer than industry watchers hadexpected - was a big factor in the spike in differentials.
However, they warned prices will continue to be supported bya recently announced turnaround at Phillips 66' 356,000bpd Wood River refinery and the long standing upgrade of a crudedistillation unit (CDU) at BP's 405,000 bpd Whiting,Indiana, refinery.
Phillips 66 did not say whether any units at Wood River hadbeen shut down, nor how long the work will last. Some traderswere surprised the refinery was undergoing maintenance after aturnaround of at least seven weeks earlier this year.
BP has said it expected the upgraded CDU to restart by theend of this month. (Reporting by Erwin Seba in Houston and Sabina Zawadzki in NewYork; Editing by Louise Heavens)
