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Why are the lighter parts of crude oil removed as tar sands form?
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heavy oil and tar sand
Major oil fields of the Arabian-Iranian basin region.
The lighter parts of crude oil are removed during the formation of tar sands through a natural degradation process that occurs when conventional oil migrates toward the surface and encounters descending rainwater containing oxygen and bacteria at temperatures below 93 °C (200 °F).12
This degradation involves two primary mechanisms:
- Water washing: This process removes the more water-soluble light hydrocarbons, particularly aromatics, through solution.12
- Biodegradation: Bacteria present in the water preferentially consume and remove normal paraffins from the oil accumulation.12
As these lighter fractions are stripped away, the remaining deposit becomes increasingly concentrated with heavy molecules such as asphaltenes and resins, as well as metals like nickel and vanadium. This natural concentrating process leaves behind the highly viscous bitumen characteristic of tar sands, which represents only a small fraction of the original conventional oil.2