Biogas is a byproduct of specialized methanogenic microbes that thrive in anaerobic environments. It is commonly found in shallow, immature sediments and plays a crucial role in the global carbon cycle. Approximately 80% to 90% of atmospheric methane originates from biogas, making it a major contributor to the greenhouse effect. Economically, biogas holds significant value—estimates from two decades ago suggested it accounts for about 20% of global natural gas reserves. Recent advancements in deep biosphere research have revealed that microbial activity can occur at great depths, persist for extended periods, and produce more biogas than previously thought. In addition to modern sediments, processes like coal, organic-rich shale, and crude oil degradation also generate substantial amounts of biogas, contributing significantly to natural gas resources. As conventional oil and gas reserves decline, the future of energy exploration increasingly depends on the discovery of biogenetic gas. Biogas is a key end product of microbial remineralization of organic matter, formed mainly through acetic acid fermentation and carbon dioxide reduction. The type of oxidant present in the sediment dictates the reaction pathway. In the presence of oxygen, aerobic decomposition dominates, followed by nitrate reduction, then metal oxide (like MnO₂ and Fe₂O₃) reduction, and eventually sulfate reduction. Finally, methanogens reduce simple organic compounds into methane. Due to limited oxidants in sediments, sulfate reduction and methane formation are typically the dominant processes. Recent advances in deep microbiology have highlighted the importance of autotrophic anaerobic microorganisms living at great depths, expanding our understanding of deep-sea biodiversity and geochemical cycles. Microbes can be found from the surface down to over 4,000 meters, even in hot springs and deep oil reservoirs. These organisms exhibit different metabolic behaviors compared to those on the surface, emphasizing the need to reevaluate the significance of microbial methane production below sulfate-reducing zones. The transformation of organic matter into biomethane involves multiple steps. First, bacteria break down complex organic material into simpler substrates such as hydrogen, carbon dioxide, and formate, which are then used by methanogens to produce methane through either carbon dioxide reduction or hydrogenation. Environmental factors like temperature, organic substrate composition, and depositional conditions influence which pathway dominates. It is generally believed that acetic acid fermentation occurs more frequently in freshwater environments, while carbon dioxide reduction is more common in marine settings. In continental environments with moderate temperatures, about 70% of methane comes from acetic acid fermentation, and 30% from carbon dioxide reduction. This ratio shifts with temperature changes. Both pathways can coexist at different depths, but carbon dioxide reduction becomes more prominent in deeper layers. This explains why many commercial biogas accumulations are primarily the result of this process. The carbon source for methane formation usually comes from acetate or carbon dioxide, without requiring additional oxidants. Carbon dioxide is mainly produced during the diagenesis of organic matter, linked to early kerogen evolution and influenced by thermal conditions. Some carbon dioxide may also originate from abiotic reactions deep within the Earth, though quantifying its contribution to shallow sediment biogas remains challenging.

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