Methane Sources

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There are three main categories of methane sources to the atmosphere: (1) biogenic methane of natural and techno-genic origins; (2) abiogenic methane from lithosphere sources; and (3) anthropogenic methane from industry and agriculture.

Estimations of the total methane flux vary widely from 500 million tons yr~ to not less than 3000 million tons yr~ in others. The latter are based on observable annual increases of its contents, known results of measurements of distribution of its concentration at different altitudes, and conditions of balance between destruction of methane in photochemical processes and its upward transport from the Earth's surface.

Such large discrepancies can be explained by the fact that the earlier studies were limited to abiogenic methane flow from lithospheric sources, both diffuse (spreading zones, COX hydrothermal systems, etc.) and local (deposits of gas hydrates, mud volcanoes, the areas of gas, oil, and coal deposits). Latter studies included biogenic sources such as natural ecosystems - wetlands (105-300 million tons yr-1), soils (10-80 million tons yr-1), tundra (1.3-13 million tons yr-1), termites (37-88 million tons yr~ ), also for agricultural activity, such as rice fields (100-350 million tons yr~ ), ruminants and manure (70-200 million tons yr~ ), as well as for a number of technogenic processes, such as biomass combustion

(50-160 million tons yr-1) and landfills (30-70 million tonsyr-1). Variations among methane emissions from biogenic sources, achieving 2-3 times, are quite satisfactory since areas of these sources are huge, and their sizes and intensities change depending on climatic conditions.

Methane leaks during extraction of fossil fuels are an important source of anthropogenic methane. More than 2000 billion m3 of gas, over 3000 million tons of oil, and 4500 million t of coal are annually extracted in the world at present. Methane leaks in oil and gas mining occur during prospecting, extraction, transportation, storage, processing, and refilling of oil and gas products. Losses ofmethane in technological processes are estimated to be in a range of 30-300 million tonsyr-1. More precise estimation can be made through monitoring oftechnological processes, in particular oil and gas pipeline accidents. Estimations of methane emissions in underground and open-cast mining of coal deposits change from 35-40 million to 60-80 million tonsyr-1.

Special attention should be given to the lithosphere sources of abiogenic methane, which are of two main categories. The first includes deposits of fossil fuels in the Earth's crust (gas and gas condensates, oil and oil-and-gas, coal, gas hydrates). Possible pathways of natural methane into the atmosphere from fossil fuels are mainly known.

The second is due to lithospheric-scale processes (metamorphism, zones of spreading, COX systems, etc.). The amount of natural methane emissions from lithosphere sources are less known due to the extensive inaccessibility, instability, and geographic dispersion necessitating territorial measurements in different regions of the world. Difficulties are also due to measurement problems, such as registration of small concentrations, passage of emissions through thick layers of water, probe poisoning, etc. At the same time, one should stress that stocks of methane in lithosphere sources and land and oceanic deposits of fossil fuels are quite large (Table 1).

Table 1 Methane content in geospheres, deposits of natural fuels, and gas hydrate deposits

The contents of methane

Table 1 Methane content in geospheres, deposits of natural fuels, and gas hydrate deposits

The contents of methane

Source

(xi06t )

(m3)

Lithosphere

109

1.4 x 1018

Hydrosphere

108

1.4 x 1017

Gas fields

105-106

(1.4-14) x 1014

Methane in oil fields

1-2 x 104

(1.4-2.8) x 1013

Methane in coal deposits

104-105

(1.4-14) x 1013

Sea gas hydrates

(1.3-50) x 105

(1.8-70) x 1014

Gas hydrates in perma-frost

(3-6) x 104

(4.2-8.4) x 1013

Atmosphere

5x 103

7 x 1012

Khalil MAKand Rasmussen RA (1983) Sources, sinks and seasonal cycles of atmospheric methane. Journal of Geophysical Research 88(C9): 5131 — 5144; Rasmussen RA and Khalil MAK (1991) Atmospheric methane (CH4): Trends and seasonal cycles. Journal of Geophysical Research 86(C10): 9825-9832; Sokolov BA (1996) Oil-gas capacity of deposits on the Earth. Science in Russia (no. 6): 16-20 (in Russian).

Khalil MAKand Rasmussen RA (1983) Sources, sinks and seasonal cycles of atmospheric methane. Journal of Geophysical Research 88(C9): 5131 — 5144; Rasmussen RA and Khalil MAK (1991) Atmospheric methane (CH4): Trends and seasonal cycles. Journal of Geophysical Research 86(C10): 9825-9832; Sokolov BA (1996) Oil-gas capacity of deposits on the Earth. Science in Russia (no. 6): 16-20 (in Russian).

Processes of natural methane migration and other hydrocarbons from oil and gas deposits as well as their 'catastrophic' destructions are known. Increased concentration of methane is noted in near-ground troposphere of many oilfield structures. Essential influence on methane uptake is made by seismic-tectonical processes in Earth's crust. For example, the amount of methane in near-ground troposphere exceeded its average values by several orders of magnitude during the Gasli earthquakes (1977). Various mechanisms of methane release and its huge stocks in lithosphere sources, deposits of fuels, and gas hydrate deposits are capable of providing flows of abiogenic methane over 1000 million tons yr-1.

Data from literature on methane flows from the Earth's surface into the atmosphere are presented in Table 2, including minimal and maximal values from different publications. These estimates are coarse enough and show only orders of magnitude. We assumed that the total flow of methane into the atmosphere is not less than 3000 million tons yr-1, which reflects the current growth rate of methane in the atmosphere, dynamics of its transformations in physical and chemical reactions, and distribution of its concentration on height.

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