LONDON—Royal Dutch Shell PLC is selling nearly all of its Canadian oil-sands developments in deals worth $7.25 billion, deserting a region that has come to symbolize the risks for energy companies in high-cost, carbon-intensive sources of oil.

The deal marks a another milestone in an ambitious plan by the British-Dutch oil producer to sell off $30 billion of assets by next year to help pay down debt and streamline the company following its roughly $50 billion acquisition of BG Group PLC in 2016.

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