By Cara Lombardo, Miriam Gottfried and Rebecca Elliott

Marathon Petroleum Corp. is in discussions with potential buyers of its sprawling Speedway gas-station unit, reviving a sale that fell apart in the early stages of the coronavirus crisis.

Possible buyers include Canada's Alimentation Couche-Tard Inc., people familiar with the matter said. It couldn't be determined how much the business might fetch, but Marathon indicated last fall that it could be worth between $15 billion and $18 billion.

Any deal is likely weeks away, and Marathon might opt to stick with a plan to spin off the unit instead.

Marathon said last fall that it planned to separate the unit as part of an effort to appease activist investors including Elliott Management Corp. The fuel producer then held talks to sell the division and was close to a deal with Seven & I Holdings Co., the Japanese parent of 7-Eleven convenience stores, for more than $20 billion. But the deal fell through in March as the coronavirus pandemic began to roil markets and crimp economic activity, and the focus returned to a spinoff.

In a sign of the renewed possibility of a sale, Marathon recently pushed the expected timetable for a spinoff to the first quarter of 2021 from the end of this year.

Findlay, Ohio-based Marathon owns and operates roughly 4,000 convenience stores in the U.S., largely under the Speedway brand, making it one of the nation's biggest such chains. It also has long-term fuel-supply contracts with other gas stations.

Marathon and other refiners have had a difficult spring because of coronavirus lockdowns that eroded demand for the fuels they produce and squeezed margins. The company's market capitalization has fallen by more than a third this year to around $25 billion, roughly tracking the slide in the price of U.S. benchmark crude, which fetched about $39 a barrel Thursday.

Typically, cheap oil is good for refiners because it stimulates demand at the pump, but that hasn't been the case this time around. Marathon's $9.2 billion first-quarter loss was its largest quarterly loss on record.

Michael Hennigan took over as Marathon's chief executive officer in mid-March after former CEO Gary Heminger stepped down under pressure from the activists.

As of earlier this year, Couche-Tard, which has a market value of roughly $35 billion, operated about 14,800 convenience stores around the world, including nearly 10,000 in North America. The Quebec company in April ended a nearly $6 billion pursuit of Caltex Australia Ltd. due to uncertainty caused by the pandemic, though it left the door open to reviving the effort.

Write to Cara Lombardo at cara.lombardo@wsj.com, Miriam Gottfried at Miriam.Gottfried@wsj.com and Rebecca Elliott at rebecca.elliott@wsj.com