Introduction to microbial and thermal methane

Journal Article · Fri Jan 01 04:00:00 EST 1993 · United States Geological Survey, Professional Paper; (United States)

OSTI ID:6814539

Wiese, K; Kvenvolden, K A ^[1]

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1. Geological Survey, Menlo Park, CA (United States)

Commercially, important accumulations of natural gas contain methane formed by two distinctly different processes. Microbial methane, produced biologically by methanogens, makes up roughly 20 percent of all known commercial gas accumulations, whereas thermal methane, generated during the thermochemical alteration of organic matter, accounts for the remaining 80 percent. Microbial and thermal methane can often be distinguished by isotopic and molecular ratios such as [delta][sup 13]C, [delta]D, and methane/(ethane+propane) [C[sub 1]/(C[sub 2]+C[sub 3])]. Microbial methane is produced in strictly anaerobic environments such as numerous aquatic environments, both marine and freshwater. Methanogens exist at surprising depths in the Earth's crust, at temperatures as high as 97[degrees]C. The primary metabolic processes by which methanogens gain energy are fermentation of acetate (CH[sub 3]COO-) and reduction of CO[sub 2]. Other processes can occur depending upon the species of methanogen and the type of available organic matter (such as conversion of formate, methanol, methylamines, and methylated reduced sulfur compounds to methane), but in most cases these processes are not significant ones in nature. CO[sub 2] reduction dominates in marine sediments, whereas acetate fermentation is most common in freshwater sediments. Thermal or thermogenic methane is formed as organic-rich sediments move through progressively higher thermal maturity regimes found during increasing depths of burial. Thermal methane results from the thermochemical decomposition of organic matter. During early thermal maturation, thermal methane is accompanied by other hydrocarbon and non-hydrocarbon gases and is often associated with crude oil. At the highest thermal maturities, methane alone is formed by cracking of carbon-carbon bonds in kerogen, bitumen, and oils. 18 refs., 5 figs., 2 tabs.

• OSTI ID: 6814539
• Journal Information: United States Geological Survey, Professional Paper; (United States), Journal Name: United States Geological Survey, Professional Paper; (United States) Vol. 1570; ISSN XGPPA9
• Country of Publication: United States
• Language: English

Related Subjects

03 NATURAL GAS
030200* -- Natural Gas-- Reserves
Geology
& Exploration
ALKANES
ANAEROBIC DIGESTION
BIOCONVERSION
CATAGENESIS
DIAGENESIS
DIGESTION
ECOLOGY
FERMENTATION
HYDROCARBONS
HYDROTHERMAL SYNTHESIS
MANAGEMENT
METAMORPHISM
METHANE
ORGANIC COMPOUNDS
PROCESSING
SYNTHESIS
THERMOCHEMICAL PROCESSES
WASTE MANAGEMENT
WASTE PROCESSING