Reported oil and gas reserves at ExxonMobil dropped by 19 per cent last year as it revised away 3.5bn barrels of heavy bitumen at an oil sands project in Canada, the largest drop to be reported by one of the big international oil companies for at least a decade. In its 10-K annual report , filed to the Securities and Exchange Commission on Wednesday evening, Exxon said low oil and gas prices during 2016 meant that some of its assets no longer qualified as proved reserves.