LONDON—Royal Dutch Shell PLC on Wednesday offered the first glimpse of just how bad a year 2015 was for the world’s biggest oil companies, as crude prices continued their relentless fall.

The Anglo-Dutch oil giant said its adjusted earnings for 2015 likely were to fall more than 50%, to between $10.4 billion and $10.7 billion from more than $22 billion in 2014. Adjusted earnings exclude items such as impairments, divestment gains and losses and restructuring costs.

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