Marathon Oil Co. said Wednesday it intends to sell its St. Paul Park oil refinery in a tentative $800 million-plus deal that also involves all SuperAmerica gasoline and convenience stores in Minnesota and some in South Dakota and Wisconsin.

The Houston-based company plans to sell 166 company-owned SuperAmericas and its stake in 67 franchises, along with SuperMom's Bakery, which supplies the Minnesota SA outlets and is also based in St. Paul Park. A terminal associated with the refinery and pipeline assets in Minnesota also would be part of the deal.

The buyers who signed the nonbinding letter of intent are three private equity firms: ACON Investments, NTR Partners and TPG Capital, which did not comment on the deal. Marathon anticipates it will go through in the second half of this year.

Marathon spokesman Robert Calmus said SuperAmerica customers should not see any change in how the convenience stores are run. As for the fate of the employees, Calmus said, "We fully expect that the new owners are going to need most, if not all, of our trained workforce to continue to operate these facilities."

A person familiar with the deal said the three private equity firms are creating a new company to manage the acquired assets and plan to retain employees and make more acquisitions.

The deal involves 220 SuperAmericas in Minnesota, 11 in Wisconsin and two in South Dakota, Calmus said. Marathon-branded gas stations are not affected. SuperAmerica operates about 1,600 stores in nine states, with some of those outlets branded as Speedway.

The St. Paul Park refinery handles 74,000 barrels a day of crude oil. Marathon, the fourth-largest U.S. oil company, said in February that about 20 percent of its 2010 spending would go toward refining, compared with an estimated 37 percent for 2009. The company aims to boost oil and natural-gas output by 4 percent a year.

"They probably wanted to shift the balance down a little bit toward" exploring for petroleum, said John Parry, an analyst at IHS Herold in Norwalk, Conn. Marathon may have decided its operations were too weighted to refining, he said, "and so this was an opportunity to create some value."

Marathon shares closed at $32.53, up 0.9 percent, on the New York Stock Exchange.

Staff writer David Shaffer and Bloomberg News contributed to this report. Paul Walsh • 612-673-4482