A barrel of benchmark North American oil is currently changing hands for about $90 a barrel, but the heavy crude, sourced from Canada’s tar sands, is fetching $30 less due to a perfect storm of supply-demand imbalances.

The oil blend known as West Texas Intermediate (WTI) was trading about $87 a barrel on Thursday. That’s down from recent highs, but still far more than the price being offered for each barrel of crude oil from Canada’s oil sands, a thick, tarry and “sour”…

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