Last August, ExxonMobil warned that it may need to remove 20 percent of its oil and gas proved reserves from its books. While that was a shocking number from the oil major, reality proved to be even more of a shock to the company. On February 24, Exxon reported that it would actually remove over 30 percent of its proved reserves from its books — essentially wiping out the value of its Canadian tar sands holdings from its books. According to the Securities and Exchange Commission (SEC), proved reservesare “the estimated quantities of crude oil, natural gas, and natural gas liquids which geological and engineering data demonstrate with reasonable certainty to be recoverable in future years from known reservoirs under existing economic and operating conditions.”