Some of the world’s largest oil companies reported sharply lower earnings on Thursday as they gave up on some ventures that no longer make sense in a world of crude prices around $50 a barrel. Royal Dutch Shell RDS.A -0.87 % PLC posted a $6.1 billion loss in the third quarter after its decision to walk away from exploring the Arctic for oil and exploiting Canada’s oil sands resulted in $7.9 billion in charges. Petro China Co., the biggest oil-and-gas producer by volume in China, said its third-quarter net profit fell by more than 80%. This unpleasant picture is presented after a third quarter in which Brent crude, the international benchmark for oil prices, traded at about $50 a barrel on average, the lowest sustained levels since the financial crisis. The companies said they were straining to change how they do business as prices are depressed by new supplies […]