Methanogenic biodegradation of petroleum in the Western Siberia Basin and its significance for formation of giant gas pools
Secondary microbial methane is a terminal product of anaerobic petroleum biodegradation. It is possible to recognize such gas in subsurface using geological (e.g., association with biodegraded oils in the same or deeper reservoirs) and geochemical (e.g., ‘dry’ hydrocarbon composition with δ13C of methane from -40‰ to -55‰ and often positive δ13C of CO2) evidence. Occurrences of secondary microbial methane formed during petroleum biodegradation have been documented in oil, gas, and gas hydrate accumulations in >20 basins around the world (e.g., in San Joaquin, Los Angeles, Carnarvon, South Caspian, and other basins, Fig. 1).