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Biofuels and Biomass
Geochemical Characteristics and Geological Significance of the Anaerobic Biodegradation Products of Crude Oil Changhai Gao, Yunyin Zhang, Xingmou Wang, Junzhang Lin, and Yuyuan Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00632 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 17, 2019
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Energy & Fuels
Geochemical Characteristics and Geological Significance of the Anaerobic Biodegradation Products of Crude Oil Changhai Gao,* ,† Yunyin Zhang,‡ Xingmou Wang,‡ Junzhang Lin,§and Yuyuan Li† †
Key Laboratory of Deep Oil and Gas, China University of Petroleum (East China), Qingdao 266580, China
‡
Geophysical Research Institute, Shengli Oilfield Company, SINOPEC, Dongying, 257000, China
§Research
Institute of Petroleum Engineering, Shengli Oilfield Company, SINOPEC, Dongying, 257000, China
ABSTRACT: 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). Taking the Linfanjia area in the Jiyang Depression of the Bohai Bay Basin in eastern China as an example, we conducted a biodegradation simulation experiment on the conventional crude oil of well Y141-1 by 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 Pr/Ph and ∑C21-/∑C22+ are significantly reduced, while the values of Pr/nC17, Ph/nC18, the carbon-preference index (CPI), the odd-over-even predominance (OEP) 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. 1
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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.
1. INTRODUCTION Biodegradation of crude oil is an important secondary alteration process in oil reservoirs that can change many physical and chemical properties of crude oil; thus, it is the most important mechanism for the thickening of crude oil.1-2 The geochemical characteristics and formation mechanisms of biodegraded crude oil have been hot research topics in the field of petroleum geology since biodegradation was used to explain the reason for the changes in crude oil composition.3-7 At present, the methods used to study the biodegradation of crude oil mainly include chromatography-mass spectrometry of saturated and aromatic hydrocarbons, carbon isotopes, hydrocarbons entrapped in petroleum asphaltene,8 ruthenium-ion-catalyzed oxidation of asphaltenes,9 and oxygenated compounds.10 However, most of these methods are aimed at crude oil that has been biodegraded rather than at conventional crude oil. Simulation experiments are currently a widely used technique for studying the biodegradation process and degradation mechanism of conventional crude oil.11-15 The biodegradation experiments of crude oil can be divided into aerobic and anaerobic experiments. Although Kartsev proposed the idea of anaerobic biodegradation of crude oil in 1959, scholars generally believed that the biodegradation of underground crude oil is mainly caused by the action of aerobic bacteria before the 1990s.16-18 2
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Researchers found that hydrocarbons degrade rapidly under aerobic conditions, whereas the microbial degradation of crude oil under anaerobic conditions is very slow. Because fully simulating underground anaerobic biodegradation under laboratory conditions is impossible, scholars generally conduct aerobic biodegradation experiments.19-20 However, Zengler et al.11 first simulated methanogenic biodegradation of n-hexadecane (as a representative of oil alkanes) by highly enriched microorganisms under strict anaerobic conditions, and they concluded that such alkane degradation may alter oil in situ and produce methane by anaerobic microorganisms. Larter2 proposed that 50% of the oil on Earth has been consumed as a result of biodegradation and most degradation has occurred under anaerobic conditions. Currently, scholars believe that biodegradation of underground crude oil mainly occurs under the influence of anaerobic bacteria,2, 5, 13, 21 which leads to the thickening of crude oil and generates gases (e.g., methane and carbon dioxide).13, 22-26 Compared with aerobic biodegradation crude oil experiments, experimental research on the anaerobic biodegradation of crude oil needs to be strengthened. Current anaerobic biodegradation experiments of crude oil focus on the geochemical characteristics of crude oil, whereas few experiments focus on the geochemical characteristics and gas production scale of the natural gas generated during biodegradation of crude oil. Moreover, correlation research on the genesis relation between heavy oil and crude oil degradation gas is still especially rare. Therefore, we collected an oil sample from well Y141-1 in the Linfanjia area and conducted biodegradation simulation experiments of crude oil under anaerobic conditions. An analysis of the changes in the components and geochemical characteristics of the crude oil and degradation gas during the anaerobic biodegradation process can examine the mechanism underlying the anaerobic biodegradation of crude oil and thus can provide evidence for the origin of crude oil degradation gas and theoretical guidance for the joint exploration of heavy oil and degradation gas.
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2. GEOLOGICAL SETTING The Linfanjia area is located in the western Dongying Sag of the Jiyang Depression in the Bohai Bay Basin, which is a large-scale draping structure developed on the base of the low Middle and Paleozoic uplift. The study area mainly contains the Paleogene Kongdian Formation and the Neogene Guantao Formation and Minghuazhen Formation. The missing Paleogene Shahejie Formation and Dongying Formation due to uplift and erosion of the strata result in the angular unconformity between the Kongdian Formation and the overlying Guantao Formation. Exploration practice shows that the oil and gas resources in Linfanjia area are mainly shallow heavy oil and natural gas, of which the heavy oil is distributed in the lower part of the Guantao Formation while the natural gas is distributed in the upper and middle parts of the Guantao Formation and Minghuazhen Formation, respectively. Previous studies have shown that the shallow layer of the Linfanjia area presents geological conditions suitable for microbial survival, such as a suitable formation temperature (47 - 51°C) and lower formation water salinity (1572 - 3018 mg/l). Additionally, we analyzed the endogenous microbial community structure of well L12-11 of the Guantao Formation reservoir in the Linfanjia area through modern molecular biology techniques that target DNA (analysis methods for the microbial community structure of the endogenous archaea are as follows: centrifuge bacteria in the oil and water samples, break up cells to obtain DNA, perform specific primer PCR amplification, perform DNA sequencing to obtain sequence information, determine the alignment database sequence, and determine the cell types and proportions). The results show that a large number of methanogenic microorganisms are associated with crude oil biodegradation in the oil reservoir, mainly Methanoarchaea, including Methanobacterium (38%), Methanoculleus (7%), and Methanosaeta (10%), which account for 55% of the total number of colonies (Figure 1). Anaerobic microorganisms are mainly fermentative bacteria, nitrate-reducing bacteria, iron-reducing bacteria, sulfate-reducing bacteria, and methanogenic archaea.27 Among these microorganisms, methanogenic archaea have received extensive attention 4
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Energy & Fuels
due to their enormous role in the natural gas field.28-29 The large amounts of methanogenic archaea in the oil of the reservoirs indicate that the shallow oil reservoirs (buried depth
