The average "energy returned on investment," or EROI, for conventional oil is roughly 25:1. In other words, 25 units of oil-based energy are obtained for every one unit of other energy that is invested to extract it.
But tar sands oil is in a category all its own.
Tar sands retrieved by surface mining has an EROI of only about 5:1, according to research released Tuesday. Tar sands retrieved from deeper beneath the earth, through steam injection, fares even worse, with a maximum average ratio of just 2.9 to 1. That means one unit of natural gas is needed to create less than three units of oil-based energy.