The process of reservoir biodegradation imposes significant effects on crude oil quality, in terms of chemical composition and physical properties. In the present work the impact of 10 months of anaerobic and aerobic laboratory biodegradation on a crude oil has been studied separately. The processes has been carried out by means of two pure, novel bacterial strains, and the two biodegraded oils have been compared to the original nonbiodegraded crude oil. It is observed that the distribution of hydrocarbons. SARA (saturated hydrocarbons, aromatic hydrocarbons, resins, asphaltenes) fractions, interfacial tension between oil and water, and oil density are affected very differently in the two different systems. Moreover, the anaerobic and aerobic processes have increased the amounts of acids of the oil. High performance liquid chromatography (HPLC) and Fourier transform infrared (FTIR) studies reveal significant differences in the composition of the acid extracts. The effects of the types of acid extracts oil gas hydrate/crude oil interfacial properties are also studied. The results suggest that the potential of an oil to form hydrate plugs may possibly be related to the acid species produced during anaerobic biodegradation.