Geochemical Characteristics and Geological Significance of the Anaerobic Biodegradation Products of Crude Oil

Anaerobic biodegradation of crude oil is one of the main mechanisms for the formation of heavy oil and gas (referred to as crude oil degradation gas). With the Linfanjia area in the Jiyang Depression of the Bohai Bay Basin in eastern China taken as an example, we conducted a biodegradation simulation experiment on the conventional crude oil of well Y141-1 using cultured anaerobic microbial flora. The causes and characteristics of crude oil alteration and natural gas formation were studied by combining oil and gas geochemistry and carbon isotope data. The theoretical analysis and experimental results show that the saturated hydrocarbon content of crude oil is significantly reduced with the progress of anaerobic biodegradation, while the contents of aromatic compounds, resins, and asphaltenes remain relatively high. Crude oil components, such as n -alkanes and isoparaffins, undergo different degrees of biodegradation. The values of pristane (Pr)/phytane (Ph) and ∑C 21– /∑C 22+ are significantly reduced, while the values of Pr/ n -C 17, Ph/ n -C 18, the carbon-preference index, the odd-over-even predominance, and 25-norhopane/hopane are relatively high. Biodegradation causes crude oil to continuously thicken to form heavy oil. Moreover, methane and carbon dioxide gas are produced as crude oil is degraded by anaerobic microorganisms; for example, the average production per gram of crude oil reaches 3 mmol of methane during 250 days of biodegradation. Both the carbon isotope values of methane (all less than −45‰) and carbon isotope values of carbon dioxide (all greater than 2‰) tend to become heavier, which are typical characteristics of crude oil degradation gas. Therefore, anaerobic biodegradation of crude oil can not only generate heavy oil but also produce a considerable amount of crude oil degradation gas that is mainly composed of methane. Data provide evidence for the large-scale methanogenic biodegradation of crude oil under anaerobic conditions during the geological period. Crude oil degradation gas and crude oil release gas together form the main sources of shallow gas reservoirs and at least 60% of the crude oil degradation gas in shallow gas reservoirs. The homologous symbiotic relationship and coupling distribution characteristics of heavy oil reservoirs and shallow gas reservoirs can provide a theoretical basis for the efficient joint exploration of both types of reservoirs.