Biogas is a byproduct of specialized methanogenic microorganisms that thrive in anaerobic environments. It is commonly found in shallow, immature sediments and plays a significant role in the global carbon cycle. Approximately 80% to 90% of atmospheric methane comes from biogas, making it a major contributor to the greenhouse effect. Economically, biogas holds substantial value, with estimates from two decades ago suggesting it accounts for about 20% of the world's natural gas reserves. Recent advancements in deep biosphere research have revealed that microbial activity can occur at much greater depths than previously thought, and these processes can persist over long geological timescales, leading to higher-than-expected biogas reserves. In addition to modern sediments, biogas can also be generated through the degradation of coal, organic-rich shale, and crude oil, offering considerable potential for future natural gas production. As the demand for clean energy grows, natural gas has become an essential resource. With conventional oil and gas reserves becoming increasingly scarce, the discovery and exploitation of biogas are expected to play a crucial role in future energy strategies. Biogas is a key end product of microbial remineralization of organic matter, formed primarily through two pathways: acetic acid fermentation and carbon dioxide reduction. The type of oxidant present in the sediment determines the dominant reaction pathway. In the presence of oxygen, aerobic decomposition dominates, followed by nitrate reduction, then metal oxide reduction (such as MnO₂ and Fe₂O₃), and eventually sulfate reduction. Finally, methanogens take over, converting simple organic compounds into methane. Recent advances in deep microbiology have highlighted the existence of autotrophic anaerobic microorganisms living far beneath the Earth’s surface, often at depths exceeding 4,000 meters. These microbes inhabit extreme environments such as hydrothermal vents and deep-sea oil reservoirs, exhibiting unique metabolic behaviors that differ significantly from surface-dwelling organisms. This has led to a reevaluation of the importance of microbial methane formation below the sulfate-reduction zone. The transformation of organic matter into biomethane involves multiple steps. Initially, bacteria and other microbes break down complex organic material into simpler substrates like hydrogen, carbon dioxide, and formate, which can then be utilized by methanogens. These archaea produce methane through either carbon dioxide reduction or hydrogenation. The relative contribution of each pathway depends on factors such as temperature, the nature of the organic substrate, and the depositional environment. It is generally accepted that acetic acid fermentation occurs predominantly in freshwater settings, while carbon dioxide reduction is more common in marine environments. In continental environments with moderate temperatures, approximately 70% of methane is produced via acetic acid fermentation, and 30% through carbon dioxide reduction. This ratio can shift depending on environmental conditions. Both pathways can coexist within a single depth profile, but their significance varies, with carbon dioxide reduction being more dominant at deeper levels. This explains why many commercial biogas accumulations are largely the result of this process. The carbon source for methane formation typically comes from acetate or carbon dioxide, without requiring additional oxidants. The primary source of carbon dioxide is the diagenesis of organic matter, influenced by the thermal evolution of kerogen. Some carbon dioxide may also originate from abiotic reactions deep within the Earth, though quantifying its contribution to biogas formation in shallow sediments remains challenging.

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