Biogas is an important and end product in the biochemical process of microbial remineralization of organic matter. The formation pathways are mainly acetic acid fermentation and carbon dioxide reduction. The nature of the oxidant in the sediment determines the reaction process. If free oxygen is present, it is mainly aerobic decomposition, after which nitrate reduction plays a major role, then the metal oxides (MnO2 and Fe2O3) become the main oxidants, and then enter the sulfate. The reduction zone is finally introduced into the methanogen reduction zone, and the monomolecular compound decomposed by various microorganisms is reduced by the methanogen to form methane. Due to the limitations of oxidants in sediments, the main reactions of organic matter decomposition are sulfate reduction and methane formation.
In recent years, deep microbiology has received great attention, especially the detection of deep autotrophic anaerobic microbiota, which has greatly expanded the understanding of deep biodiversity and biogeochemistry. Microorganisms can inhabit from the surface to 4000m or deeper. Living microorganisms are found in hot springs and deep oil fields in the ocean floor. Their habits and geochemical behaviors are different from those found on the surface. Therefore, the importance of microbial methane formation below the sulfate reduction zone needs to be reconsidered.
The process from polymerizing organic matter to biomethane is very complicated. First, organic matter is decomposed by bacteria and other microorganisms into substrates that can be utilized by methanogens, such as hydrogen and carbon dioxide, formate, etc., and then methanogens form methane by carbon dioxide reduction and hydrogenation.
Temperature, organic substrate properties and depositional environment control the two methane formation pathways. It is generally believed that acetic acid fermentation mainly occurs in fresh water environment, and carbon dioxide reduction is mainly in marine environment. The methane produced by acetic acid fermentation in a moderately temperatureed continental environment accounts for 70%, and carbon dioxide reduction accounts for 30%. This ratio changes as the ambient temperature changes. Acetic acid fermentation and carbon dioxide reduction can occur simultaneously in the depth profile, but their importance is different. The deep part is mainly the cause of carbon dioxide reduction. This is a good explanation for why many commercial biogas accumulations are the cause of carbon dioxide reduction.
The carbon required for methane formation is generally derived from acetate or carbon dioxide and does not require the supply of additional oxidants. The main source of carbon dioxide is the diagenesis of organic matter, which is related to the evolution of early kerogen and is controlled by heat. There is also a portion of carbon dioxide that can come from deep abiotic reactions, and the contribution of these carbon dioxide to the formation of biomethane in shallow sediments is difficult to quantify.
Biogas is a metabolite of special bio-methanogens in anaerobic environments, usually found in shallow, immature sediments, which play an important role in the carbon cycle during geochronology, about 80% to 90 in the atmosphere. % of methane is biogas and is the main source of greenhouse effect. The economic significance of biogenetic gas is also quite obvious, and estimates from 20 years ago show that it accounts for 20% of global natural gas resources. In recent years, with the deepening of the research level of the deep biosphere, it has been found that the depth of microbial activity can be large, the microbial activity lasts for a long time, and the biogas reserves are higher than previously expected. In addition, in addition to modern sediments, coal, organic-rich shale and crude oil biodegradation can form a large amount of biogas, which can also make a great contribution to natural gas production. Economic development and the environment require natural gas for clean energy demand. With the gradual depletion of conventional oil and gas resources, future natural gas exploration relies heavily on the discovery of biogenetic gas.