Biogenic gas: Controls, habitats, and resource potential
- Geological Survey, Denver, CO (United States)
As much as 20 percent of the world's natural-gas resource is estimated to have been generated by the decomposition of organic matter by anaerobic microbes at low temperatures. This gas is commonly referred to as biogenic gas. Most biogenic gas was generated early in the burial history of sediments. Some biogenic gas was also generated in relatively recent geologic time and is associated with groundwater flow. The factors that favor significant generation of biogenic gas are anoxic conditions, low sulfate content, low temperature, abundant organic matter, and sufficient pore space for the microbes to thrive. Conditions beneficial for the accumulation of biogenic gas include stratigraphic or early structural traps, adequate seals, low permeability, low pressure, early dissolution of the gas, and formation of gas hydrates. Rapid sediment deposition is critical to both the generation and the accumulation of biogenic gas generated during the early stage. Biogenic gas is distinguished by its molecular and isotopic composition. The hydrocarbon fraction is generally more than 99 percent methane, and the diagnostic isotopic composition of the methane component is as follows: [delta][sup 13]C values are generally lighter than -55 parts per thousand (permil), and [delta]D values are usually in the range of -150 to -250 permil. This isotopic composition indicates that the methane generally resulted from CO[sub 2] reduction. Significant accumulations of ancient biogenic gas have been discovered in Africa, Asia, Europe, North America, and South America. These accumulations occur in Mississippian and younger rocks, at burial depths as much as 4,600 m. They are associated with a variety of rock types (carbonate, clastic, and coal), and occur in a variety of marine and nonmarine depositional settings generally characterized by rapid deposition. 111 refs., 13 figs., 3 tabs.
- OSTI ID:
- 6814498
- Journal Information:
- United States Geological Survey, Professional Paper; (United States), Vol. 1570
- Country of Publication:
- United States
- Language:
- English
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