Upon request of the VPR and the thesis advisor this item has been made administrative access only until further notice. / Biodegradation of petroleum has been observed to be one of the most important factors that can alter reservoir chemistry. Biodegradation of petroleum has been connected to the generation of heavy oil at the expense of light hydrocarbon components. Generally, heavy oil is associated with the increasing in metal and sulfur content as well as viscosity. In addition, petroleum biodegradation will result in the production of certain metabolites that are implicated in forming emulsions and corrosion problems in the producing and refining facilities. However, identifying the microrganisms that catalyse this biodegradation is crucial to understanding their role in the hydrocarbons alteration. In this thesis, I addressed the connection between the petroleum biodegradation and the formation of light hydrocarbon components at the expense of heavy hydrocarbon components, and the increase in gas/oil ratio. A comparison between light, extra light, and medium sour crudes lends support to the hypothesis of light hydrocarbons formation through biodegradation of long chain oil components. The results suggested that there was no direct relationship between the relative density of oil and the level of biodegradation, but, there was a positive correlation between the level of biodegradation, the formation of light hydrocarbons, and an increase in the gas/oil ratio. As a first step in investigating this correlation, a metagenomics approach was used to identify and characterize the biodiversity in a European oil field. Extrapolation of the oilfield microbiome data based on an analysis of 200 species generated a hypothetical metabolic map that suggests a new model for petroleum formation and degradation that challenges the accepted dogma in which aerobic and anaerobic petroleum degradation is taking place in the hydrocarbons reservoir, as it is a matter of rate; where the aerobic petroleum degradation targets the short-chain hydrocarbons specifically methane and result in heavy oil generation; whereas the anaerobic petroleum degradation leads to form the gaseous components such as methane, carbon dioxide and hydrogen sulfide. Hence, the gaseous components have a direct impact on the oil density when they represent the majority of the oil field composition by making it more gaseous than liquid.