The Role of Microorganisms in the Oil Generation (I): The degradation of macromolecular organic matter and the elimination of oxygen-containing groups

Key words: 
• biogeological processes /
• microorganism /
• oil and gas /
• thermophilic bacterium

Abstract: Thermophilic microbiology has achieved rapid progress in the past 20 to 30 years and about 70 genera (140 species) of thermophilic bacteria have been found. The optimum growth temperature range for these bacteria is generally above 60℃ or 80℃, and the maximum temperature is 110℃~113℃. Anaerobic microorganisms, including a variety of bacteria like decomposition microorganisms, hydrogenogens and methanogens, are widely distributed from low temperature to high temperature in both shallow and deep deposits. They can be distributed in the deep water or on the surface of rock. The temperature at which these species live is close to the main temperature of oil formation (60℃~100℃).
Microorganisms are unicellular organisms, which have small individuals and simple structures. Each of their cells can directly feel the environmental stimuli when the environment changes. Then, they are more capable of adaptation and easier to generate genetic variations. A hypersaline environment of high temperature, high pressure is suitable for the thermophilic bacteria. The large number discoveries about the thermophilic bacterium provide a solid theoretical basis for understanding the origin of life and the formation of the oil and gas reservoirs. The conversion of sedimentary organic matter into petroleum is related to the process of degradation of macromolecular organic matter (molecular weight can reach tens of thousands to hundreds of thousands) into middle and low molecular compounds and the process of elimination of oxygen-containing (and other heteroatoms-containing) groups. These processes are mainly carried out by the microorganisms. Carbon is the core atom that constitutes life. Microorganisms need to draw the carbon source from the organic matter to form the cellular material, such as cell wall, cell membrane, cytoplasm, and cell nucleus. Macromolecular organic matter can be absorbed by microorganisms when they are gradually broken down into simple organic matter, such as dipeptides from the decomposition of protein and simple sugars from the hydrolysis of carbohydrates. Anaerobic microorganisms will constantly obtain carbons and can help to turn the organic matter into simple compounds gradually. Anaerobic respiration of microorganisms leads to a reduction of the oxygen-containing compounds, forming the compounds that are more reduced than the original ones. As the final electron acceptor material in the anaerobic respiration of sediments, the organic matter containing groups such as hydroxyl, carboxyl, etc. then form the hydrocarbons by elimination of the oxygen-containing groups.