ConocoPhillips' interest in the Clyden lease, 150 kilometres south of Fort McMurray, Alta., covers more than 91,000 hectares.
The companies would use a method known as steam-assisted gravity drainage, or SAGD, to soften the oilsands bitumen and draw it to the surface.
"The Clyden oil sands lease is a high-quality addition to Imperial's portfolio of oil sands in-situ opportunities," Imperial CEO Rich Kruger said.
"This acquisition is consistent with Imperial's strategy to position ourselves for long term business growth."
Initially, Imperial will have a 27.5 per cent interest, and ExxonMobil a 72.5 per cent stake.
However, Imperial said ExxonMobil may bring in another partner to develop the lease, after which it would have a stake equal to Imperial's.
The deal is subject to Competition Bureau approval.
Houston-based ConocoPhillips has been paring down its portfolio. Including the Clyden sale, it has announced $13.5 billion in asset sales as part of its 2012-2013 disposition program.
"This transaction is a significant step toward rebalancing our world-class oil sands portfolio," said Don Wallette, the company's executive vice president of commercial, business development and corporate planning.
"We are pleased Imperial Oil and ExxonMobil Canada recognize the value of the Clyden asset."