First Stoichiometric Model of Oil Biodegradation in Natural Petroleum Systems: Part II*: Application of the BioClass 0D Approach to Oils from Various Sources

The purpose of the this study was to apply the biodegradation model BioClass 0D proposed by Haeseler et al. (2010) to three nonbiodegraded oils representative of the three main types of source rocks (types I, II, and III of Tissot et al., 1974; Tissot and Welte, 1978). The oils are described by seven chemical classes: (1) the gas fraction is split into H2S, CO2, C1, and C2–C4; (2, 3) the C6–C14 fraction including saturates and aromatics; and (4–6) the C14+ fraction, including n-, iso-, cyclo-alkanes, aromatics, and nitrogen, sulfur, and oxygen–containing compounds (NSOs). This 0D model reconstructs the chemical evolution of the residual oil with increasing biodegradation and the amounts of products generated during either aerobic or anaerobic processes. Results show that with increasing biodegradation, the residual oil composition depends on the initial proportion of the different chemical classes. For instance, for the type I oil enriched in paraffins, the C14+ n-alkanes are still present when 60% of the original oil has already disappeared, whereas for the type II oil, the same chemical class disappears after only 30% of total hydrocarbon loss. During methanogenesis, the gas-oil ratio of the initial fluid significantly increases with increasing biodegradation. However, the volume of methane may be reduced because of its solubility in water or it may leak through the cap rock. The production of H2S is always very low when sulfur minerals that can provide electron acceptors are absent. Results also show that the amount of water needed to provide the electron acceptors depends strongly on the biological process responsible for oil biodegradation. The water ratio between aerobic biodegradation and methanogenesis might be as high as 107. Consequently, aerobic biodegradation may be limited compared with methanogenesis in petroleum systems in which the oil-water volume ratio varies during fluid history.