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Crude oil is a complex mixture containing tens of thousands of compounds mixed together and the major part are hydrocarbons. Hydrocarbons (HCs) are defined as compounds that only contain carbon and hydrogen without any functional groups.  This gives them stability and low reactivity. Under the right conditions, living organisms such as bacteria, algae or fungi are able to consume hydrocarbons as an energy source.  The process is called hydrocarbon biodegradation. Hydrocarbon biodegradation by living organisms is a well-known alteration process and the majority of crude oils in the world are biodegraded to different degrees<ref name="Roadifer">Roadifer, R. E., 1987, [https://archives.datapages.com/data/specpubs/methodo2/data/a081/a081/0001/0000/0003.htm Size distributions of the world’s largest known oil and tar accumulations], ''in'' R. F. Meyer, ed., [https://archives.datapages.com/data/alt-browse/aapg-special-volumes/sg25.htm Exploration for heavy crude oil and natural bitumen]: AAPG Studies in Geology 25, p. 3–23.</ref>. The degree of hydrocarbon biodegradation is dependent on many factors including salinity, hydrocarbon composition, temperature, pH, pressure and moisture.   

Crude oil is a complex mixture containing tens of thousands of compounds mixed together and the major part are hydrocarbons. Hydrocarbons (HCs) are defined as compounds that only contain carbon and hydrogen without any functional groups.  This gives them stability and low reactivity. Under the right conditions, living organisms such as bacteria, algae or fungi are able to consume hydrocarbons as an energy source.  The process is called hydrocarbon biodegradation. Hydrocarbon biodegradation by living organisms is a well-known alteration process and the majority of crude oils in the world are biodegraded to different degrees<ref name="1Roadifer">Roadifer, R. E., 1987, [https://archives.datapages.com/data/specpubs/methodo2/data/a081/a081/0001/0000/0003.htm Size distributions of the world’s largest known oil and tar accumulations], ''in'' R. F. Meyer, ed., [https://archives.datapages.com/data/alt-browse/aapg-special-volumes/sg25.htm Exploration for heavy crude oil and natural bitumen]: AAPG Studies in Geology 25, p. 3–23.</ref>. The degree of hydrocarbon biodegradation is dependent on many factors including salinity, hydrocarbon composition, temperature, pH, pressure and moisture.   

==Biodegradation of hydrocarbons definition==

==Biodegradation of hydrocarbons definition==

Biodegradation of hydrocarbons is an alteration process by living organisms either aerobically and anaerobically (i.e. with or without the presence of oxygen) affecting organic matter. This process can occur in petroleum reservoirs or during migration at shallow depths where water contact is possible and subsurface temperatures are below 80 oC because this temperature is considered as the cutoff temperature where micro-organisms are absent beyond it.<ref name="Petersetal">Peters, K. E., D. J. Curry, and M. Kacewicz, 2012, [https://archives.datapages.com/data/specpubs/hedberg4/INTRODUCTION/INTRODUCTION.HTM Introduction: An overview of basin and petroleum system modeling: Definitions and concepts], ''in'' K. E. Peters, D. J. Curry, and M. Kacewicz, eds., [https://archives.datapages.com/data/alt-browse/aapg-special-volumes/hed4.htm Basin modeling: New horizons in research and applications]: AAPG Hedberg Series 4, p. 1–16.</ref>

Biodegradation of hydrocarbons is an alteration process by living organisms either aerobically and anaerobically (i.e. with or without the presence of oxygen) affecting organic matter. This process can occur in petroleum reservoirs or during migration at shallow depths where water contact is possible and subsurface temperatures are below 80 oC because this temperature is considered as the cutoff temperature where micro-organisms are absent beyond it.<ref name="2Petersetal">Peters, K. E., D. J. Curry, and M. Kacewicz, 2012, [https://archives.datapages.com/data/specpubs/hedberg4/INTRODUCTION/INTRODUCTION.HTM Introduction: An overview of basin and petroleum system modeling: Definitions and concepts], ''in'' K. E. Peters, D. J. Curry, and M. Kacewicz, eds., [https://archives.datapages.com/data/alt-browse/aapg-special-volumes/hed4.htm Basin modeling: New horizons in research and applications]: AAPG Hedberg Series 4, p. 1–16.</ref>

==Crude oil Composition==

==Crude oil Composition==

Crude oil consists of four classes (SARA):

Crude oil consists of four classes (SARA):

Saturates: Linear or branched chains of hydrocarbons with only single bonds connecting the atoms (also called alkanes). (e.g. Propane and Iso-butane). [3]

Saturates: Linear or branched chains of hydrocarbons with only single bonds connecting the atoms (also called alkanes). (e.g. Propane and Iso-butane).<ref name="3Paraffins">Eser, S., [https://www.e-education.psu.edu/fsc432/content/paraffins Paraffins]: Penn State course FSC432, [https://www.e-education.psu.edu/fsc432/ Petroleum processing].</ref>

Aromatics: Chemical geometry contains of at least one unsaturated hydrocarbon cyclic with single and double bonds in alternating order (e.g. Benzene and Toluene). [4]

Aromatics: Chemical geometry contains of at least one unsaturated hydrocarbon cyclic with single and double bonds in alternating order (e.g. Benzene and Toluene).<ref name="4Aromatic Hydrocarbons">Eser, S., [https://www.e-education.psu.edu/fsc432/content/aromatic-hydrocarbons Aromatic hydrocarbons]: Penn State course FSC432, [https://www.e-education.psu.edu/fsc432/ Petroleum processing].</ref>

Resins: Are similar in terms of composition to asphaltenes but different in terms of percentage of presence of atoms relative to each other (i.e. H/C ratio is higher in resins than in asphaltenes). [5]

Resins: Are similar in terms of composition to asphaltenes but different in terms of percentage of presence of atoms relative to each other (i.e. H/C ratio is higher in resins than in asphaltenes).<ref name="5Speight">Speight, J. G., 2004, [https://ogst.ifpenergiesnouvelles.fr/articles/ogst/pdf/2004/05/speight1_vol59n5.pdf Petroleum asphaltenes: Part 1: Asphaltenes, resins and the structure of petroleum]: Oil & Gas Science and Technology – Rev. IFP, v. 59, no. 5, p. 467–477.</ref>

Asphaltenes: Is more complex organic compounds that are considered as the heaviest and most polar constituent in crude oil. Asphaltenes consist from Carbon, Hydrogen, Sulfur, Nitrogen, Oxygen, Vanadium and Nickle (e.g. phenols, fatty acids, esters and porphyrins). [5]

Asphaltenes: Is more complex organic compounds that are considered as the heaviest and most polar constituent in crude oil. Asphaltenes consist from Carbon, Hydrogen, Sulfur, Nitrogen, Oxygen, Vanadium and Nickle (e.g. phenols, fatty acids, esters and porphyrins).<ref name="5Speight" />

Alkanes are generally biodegraded faster than aromatics.  This is because aromatics requires more energy to be consumed than alkanes. Furthermore, high molecular weight molecules are more difficult to be biodegraded than low molecular weight molecules.  

Alkanes are generally biodegraded faster than aromatics.  This is because aromatics requires more energy to be consumed than alkanes. Furthermore, high molecular weight molecules are more difficult to be biodegraded than low molecular weight molecules.  

[2]  

[2]  

[3] https://www.e-education.psu.edu/fsc432/content/paraffins

[3]  

[4] https://www.e-education.psu.edu/fsc432/content/aromatic-hydrocarbons

[4]  

[5] https://ogst.ifpenergiesnouvelles.fr/articles/ogst/pdf/2004/05/speight1_vol59n5.pdf

[5]  

[6] https://www.hindawi.com/journals/btri/2011/941810/

[6] https://www.hindawi.com/journals/btri/2011/941810/