How did conventional oil deposits become tar sand deposits?

Tar sand deposits are primarily considered to be the degraded remnants of conventional light-to-medium oil accumulations. This transformation occurs through a natural concentration process that takes place when oil migrates toward the surface and encounters descending meteoric water (rainwater) containing oxygen and bacteria at temperatures below 93 °C (200 °F).12

The degradation process involves two key mechanisms:
* Water washing: This process removes the more water-soluble light hydrocarbons, particularly aromatics.2
* Biodegradation: Bacteria preferentially consume normal paraffins, leaving behind a tarlike substance at the oil-water contact that eventually invades the entire oil pool.12

As a result of this degradation, the original oil loses its lighter fractions, leading to an increase in density and the concentration of remaining components such as asphaltenes, resins, sulfur, and metals like vanadium and nickel. The resulting heavy, immobile bitumen remains trapped within the sand or sandstone matrix, forming what is known as tar sand.213