Which of the following processes transforms oil-saturated sandstones into tar sands?

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Which of the following processes transforms oil-saturated sandstones into tar sands? There are several processes that transform oil-saturated sandstones into tar sands¹. The two most important processes are water washing and bacterial degradation². The article will detail each of these processes further. But first, let us understand what tar sands, more accurately known as ‘oil sands’, are.

What are oil sands?

To begin with, oil sands (tar sands) are a low-quality form of oil made up of a mixture of sand, clay, water and bitumen (oil that is too thick or heavy to flow on its own)³. Bitumen is almost solid at room temperature, acting much like cold molasses^4,5. For that reason, this resource is often called “tar sands”. However, while bitumen looks and smells like tar (asphalt), they are two very different compounds^4,5. Thus, the term “oil sands” is used going forward. Moreover, the oil converted from oil sands is commonly known as synthetic crude oil and is a significant form of fossil fuel.

So, which of the following processes transforms oil-saturated sandstones into oil sands?

Oil-saturated sandstones transformed to oil sands

Firstly, the formation of oil sands, although somewhat controversial⁶, is believed to have began as living, organic material⁷. Millions of years ago, marine organisms lived and died, falling to the bottom of the oceans⁸. They decomposed with the help of bacteria, and were then covered by layers of sediment⁸. The sediment accumulating over the decaying marine life exerted pressure and temperature that enabled the organic matter to turn into oil⁸. Secondly, this oil eventually migrated to an oil trap⁵. An oil trap is an underground rock formation that essentially blocks the oil from escaping and allows it to accumulate in a reservoir that can then be exploited⁵. Eventually, if the trap moved into a more oxygen-rich zone underground, processes such as water washing and bacterial degradation could convert the oil to heavy oil – these are oil sands¹.

In short, bacterial degradation is when bacteria or microbes begin to break down the oil¹. All oil sands deposits are commonly located at or relatively close to the surface which means that near-surface fresh waters can supply the required microbes⁷ – water washing².  Thus, over millions of years, as the oil became trapped in thick layers of sand, the oil either escaped by evaporation, or was microbiologically degraded, leaving bitumen and water trapped within the sand^7,8. This eventually forms an oil sand deposit. 

The formation of Alberta’s oil sand deposits- a virtual reality tour. (Source: Canadian Association of Petroleum Producers) Available at: https://www.capp.ca/oil/oil-sands-virtual-tour/.

The Alberta oil sands

Hundreds of millions of years ago, the decomposition of microorganisms such as algae, allowed the formation of hydrocarbons in the subsurface⁴. As mentioned before, at the correct depth and temperature, the bacteria are able to breakdown that matter to form light oil⁴. In the case of Alberta’s oil sand deposits, the Rocky mountains pushed this oil east, transporting with it oxygenated water⁴. This aided the microbes to degrade the lighter hydrocarbon molecules, consequently leaving behind oil sand deposits in Alberta⁴. As a result, billions of barrels worth of oil are locked away in the form of oil sands within Alberta’s loose sandstones⁸. Following Saudi Arabia and Venezuela, Canada’s Alberta region is home to the world’s third largest oil sands reserve⁸.

References

1. Richard F. Meyer, Emil D. Attanasi, and Philip A. Freeman (USGS). (2007). Heavy Oil and Natural Bitumen Resources in Geological Basins of the World. Available at: https://pubs.usgs.gov/of/2007/1084/OF2007-1084v1.pdf 
2. Bata, T.P., Lar, U.A., Samaila, N.K., Dibal, H.U., Daspan, R.I., Isah, L.C., Fube, A.A., Ikyoive, S.Y., Elijah, E.H. and Shirputda, J.J., (2018). Effect of biodegradation and water washing on oil properties. AIMS Geosciences, 4(1), p.21.
3. Union of Concerned Scientists. (2020). What Are Tar Sands? Available at: https://www.ucsusa.org/resources/what-are-tar-sands
4. Oil Sands Magazine, Oil Sands Geology and the Properties of Bitumen (2016). Oil Sands Magazine. Available at: https://www.oilsandsmagazine.com/technical/properties.
5. CAPP. (2020). What Are the Oil Sands | Canada’s Oil Sands Facts & Information. Available at: https://www.capp.ca/oil/what-are-the-oil-sands/
6. Psu.edu. (2020). Available at: https://personal.ems.psu.edu/~pisupati/ACSOutreach/Oil_Sands.html#_Where_did_they
7. Energyeducation.ca. (2018). Oil sands – Energy Education. Available at: https://energyeducation.ca/encyclopedia/Oil_sands
8. Alberta.ca. (2020). The Formation of Oil Sands – Oil Sands – Alberta’s Energy Heritage. Available at: http://history.alberta.ca/energyheritage/sands/origins/the-geology-of-the-oil-sands/the-formation-of-oil-sands.aspx