Methane biogenic/thermogenic

Of the two sources of methane (biogenic and thermogenic) in natural hydrates, most of the gas is biogenic, that is, from bacterially generated methane, with anecdotal exceptions. 

Methane formed by biogenic processes ranges in < 13C from about — 60%c to —85%c, while methane from thermogenic processes ranges from —25%c to —55%c (Hunt 1979, p. 25). 

Phase fractionation is responsible for development of the saturated gas-condensates at Magnolia and has resulted from the mixture of biogenic methane and thermogenic fluids. The relatively low pressure reservoirs that contain a large biogenic gas component are saturated. Fractionation in these cases probably occurred in situ, as undersaturated thermogenic fluids mixed with previously-emplaced biogenic gas (of unknown but probably low saturations). Two phase systems formed when, as a result, saturation pressures exceeded reservoir pressures. 

The He content of waters has rarely been investigated for hydrocarbon exploration. However, Dyck and Dunn (1987) report that broad, coincident anomalies of >2.4 pL/L He and >0.4 pL/L methane in wells and springs in the Cypass Hills district, Saskatchewan, Canada, correlate with commercial oil and gas fields and/or tectonic features. High He concentrations can distinguish thermogenic methane from biogenic methane (marsh gas). 

The most complete set of published observaTions on the sources, sinks, and distributions of methane in coastal waters is that by researchers at Texas A M University. Without detailing an exhaustive list of the observations, many of which deal with the thermogenic as well as biogenic sources of methane, the following papers are most relevant to our own discussion of coastal distributions of methane (7, 10-14). 

Biogenic and thermogenic gases can usually be distinguished on the basis of chemical and isotopic composition. Biological gas is dominantly composed of methane, which is depleted in relative to thermogenic methane (Whiticar... 

Fig. 14.8 Discrimination of biogenic and thermogenic methane sources based on A. The carbon and hydrogen isotopic composition of the methane (after Schoell 1988), and B. The ratio of methane (Cj) to higher hydrocarbons (C + C ) plotted against the carbon isotopic composition of methane (from Claypool and Kvenvolden 1983).

If methane, either from biogenic or thermogenic sources, is present in high enough concentration to stabilize the hydrate structure at thermodynamically favourable conditions (Figs. 14.3 and... 

Discuss different methane sources in sediments. Is there a difference in hydrate formation whether the gas is biogenic or thermogenic in origin ... 

In principle there is no difference which methane molecule is used to stabilize the clathrate structure. Hydrates formed from biogenic methane are thought to be more widespread in marine sediments, and these can be formed from methane gas produced in situ. In contrast, the presence of thermogenic methane requires gas migration form deeper sediment sources (e.g. in mud volcanoes). Thermogenic methane is also accompanied by the presence of higher hydrocarbons, which may result in the formation of gas hydrate structure II, whereas methane hydrate from biogenic methane is restricted to structure I. 

Magnolia fluid property variations are principally controlled by differences in the maturity of emplaced thermogenic hydrocarbons, variable biogenic methane content and phase fractionation processes. 

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