"In 2017, extensions and discoveries were primarily due to higher prices at Surmont"
WTF? That's not an extension or discovery
https://www.sec.gov/Archives/edgar/data/1163165/000119312518049729/d534096d10k.htm
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read the rest of the line: Extensions and discoveries: In 2017, extensions and discoveries were primarily due to higher prices at Surmont, which allowed undeveloped reserves previously de-booked due to low prices to be recognized.
Guess you've never done reserves for oil sands