Unconventional oil deposits containing extremely viscous, extra-heavy oil. Oil sands are generally found near the surface. Because of this, the conventional oil is degraded as a result of exposure to oxygen and bacteria. The oxidation and degradation caused by bacteria quickly eliminate lighter molecules such as gases and alkanes (paraffins). Oil sands are contained in conventional sand and clay reservoirs and should not be confused with oil shale, from which shale oil is produced by heating the source rock, a process known as retorting.