Jan 12 (Reuters) - Exxon Mobil (XOM.N) on Wednesday plans to offer for sale shale oil and gas properties in Western Canada, a decision that could make oil sands its largest Canadian onshore production business. The top U.S. oil producer last year accelerated efforts to divest smaller oil and gas operations and use any proceeds to pay down debt acquired as the coronavirus pandemic triggered losses. Exxon and affiliate Imperial Oil Ltd (IMO.TO) each own 50% of XTO Energy Canada, operator of their Canadian shale business. XTO pumps about 9,000 barrels of liquids and 140 million cubic feet of natural gas per day in Canada. RBC Capital Markets has been hired to advise Imperial on the sale, the company said. The properties could fetch between $500 million and $1 billion, people familiar with energy property sales said. The shale assets were part of an impairment charge that Imperial and Exxon took in late 2020, an Imperial spokeswoman said. The companies also own petrochemical plants and Exxon operates offshore production in Eastern Canada. Past divestitures have raised more than $1.1 billion from sales of Exxon"s U.S. Gulf of Mexico and U.K. North Sea properties. It recently put several packages of U.S. natural gas properties on the market. read more XTO Energy Canada assets on offer include 568,000 acres in the Montney shale, 85,000 acres in the Duvernay shale and smaller parcels elsewhere in Alberta, Imperial said. Canada"s Montney, which straddles Alberta and British Columbia, has seen a wave of consolidation as companies buckled under when oil prices collapsed amid the COVID-19 pandemic. The oil major in 2020 took a near $20 billion writedown on properties, primarily purchased with the acquisition of XTO Energy a decade earlier. Gas assets in Appalachia, the Rocky Mountains, Oklahoma, and Texas were taken out of its development plan after the writedown.